Keep the propeller fixed or let it turn?
Flemming Torp
is best - from a pure speed point of view (no consideration
to the mechanics/oil/maintenance/gearbox etc. here ...)
whether you should let your "fixed three bladed propeller"
run/turn or keep it fixed (like put into gear) when sailing
just for the wind with your sails in a 34 feet cruiser
weighing roughly 5 T ... We have - unsuccesfully - tried to
find out using the log ... the results were not conclusive -
or one of us would not admit, that the other was right ... I
think we need a testimony from someone, that has a
'scientific based valid answer' ... or just knows for sure
...
The assumption is: There is no way to move, turn, 'collapse'
anything on the propeller - the 'blades' are fixed ... (hard
to explain in a language, that is not your own ... hope you
get my point).
Anybody in this group that can convince one of us, that he
is not right? ...
--
Flemming Torp
Roger Long
farther (up to 20%) when the nose is raised to slow them down enough
to stop the prop and best glide speed is then resumed. I can't see
any reason why this wouldn't be true of boat propellers as well. It
also agrees with just about every other reference I've seen on the
subject.
--
Roger Long
"Flemming Torp" <fletopkanelbolle2rp.danmark> wrote in message
news:42a37185$0$152$edfa...@dread11.news.tele.dk...
Leanne
"Flemming Torp" <fletopkanelbolle2rp.danmark> wrote in message
news:42a37185$0$152$edfa...@dread11.news.tele.dk...
> is best - from a pure speed point of view (no consideration
> to the mechanics/oil/maintenance/gearbox etc. here ...)
> whether you should let your "fixed three bladed propeller"
> run/turn or keep it fixed (like put into gear) when sailing
I always put the transmission in reverse when sailing. I don't
think that my Yanmar book says anything, but when I sailed on a
friend's Cape Dory, he said to always do so due the gearbox
that he had.
Leanne
Flemming Torp
"Leanne" <lea...@islc.net> skrev i en meddelelse
news:d7vse...@enews3.newsguy.com...
The question is: (If I race for instance): Will I increase
the speed of the boat by letting the propeller turn without
resistance? Or should I stop it from turning?
--
Flemming Torp
Capt. Neal®
"Flemming Torp" <fletopkanelbolle2rp.danmark> wrote in message news:42a37c43$0$159$edfa...@dread11.news.tele.dk...
It's a fact that a spinning prop creates far more drag
that a stopped prop.
CN
Jeff
for one reason: it depends on the propeller. Large ship propellers
are sometimes more efficient freewheeling, but this is not the case
for 3-blade yacht props. I suspect the the difference has to do with
the pitch, and whether or not the flow is stalled - Ship props often
high a large pitch compared to yacht props.
BTW, it worth while lining up one blade of a 3-blade with the hull
when you lock it.
Capt. Neal®
"Gary L. Burnore" <gbur...@databasix.com> wrote in message news:d80fls$ldj$1...@fluffy.databasix.com...
> On Sun, 05 Jun 2005 21:59:29 GMT, "Roger Long" <rwl...@maine.rr.com>
> top posted like a fucking moron and wrote:
>
>>It has been repeatedly demonstrated that airplanes glide significantly
>>farther (up to 20%) when the nose is raised to slow them down enough
>>to stop the prop and best glide speed is then resumed. I can't see
>>any reason why this wouldn't be true of boat propellers as well.
>
> water. Airplane props are feathered, boat props aren't.
I used to fly an ultralight aircraft - a Rotec Rally 2B. It had a
two-bladed wood prop that was driven by a Kohler air-cooled
432cc two-stroke engine via a centrifugal clutch and belt.
I could climb to about two or three thousand feet and shut off
the motor and the propeller would free-wheel because of the
centrifugal clutch being disengage. The glide ratio was terrible
with the prop free wheeling - one could feel the drag it caused.
I put a brake on the clutch hub and stopped the prop from
free-wheeling and the glide ration increased dramatically.
The same extra drag is evident on a boat propeller that free-wheels.
It's as if the two blades turn into a fixed disk.
CN
Lee308
Piper Warrior, etc. But the engine out procedure is to stop the prop
for max glide distance due to less drag as the previous post explained.
Air is a fluid like water, just lighter.
Lee
Gary L. Burnore wrote:
> On Sun, 05 Jun 2005 21:59:29 GMT, "Roger Long" <rwl...@maine.rr.com>
> top posted like a fucking moron and wrote:
>
> >It has been repeatedly demonstrated that airplanes glide significantly
> >farther (up to 20%) when the nose is raised to slow them down enough
> >to stop the prop and best glide speed is then resumed. I can't see
> >any reason why this wouldn't be true of boat propellers as well.
>
> Because aircraft are entirely in the air an boats are partially in
> water. Airplane props are feathered, boat props aren't.
>
>
>
> --
> gburnore@databasix dot com
> ---------------------------------------------------------------------------
> How you look depends on where you go.
> ---------------------------------------------------------------------------
> Gary L. Burnore | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
> | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
> DataBasix | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
> | ÝÛ³ 3 4 1 4 2 ݳ޳ 6 9 0 6 9 ÝÛ³
> Black Helicopter Repair Svcs Division | Official Proof of Purchase
> ===========================================================================
> Want one? GET one! http://signup.databasix.com
> ===========================================================================
John Cairns
"Capt. Neal®" <Capt...@Bigfoot.com> wrote in message
news:d80g3a$lt$1...@wilhelp.databasix.com...
You guys need to get a room.
John Cairns
Capt. Neal®
"John Cairns" <jgcairns@-removethis-sbcglobal.net> wrote in message news:twOoe.158$Oq7...@newssvr33.news.prodigy.com...
Did you go skiing again this winter? Did your break your leg again?
I'm telling you, KKKaty, ya gotta watch that osteoporosis since you're
post-menopausal.
CN
John Cairns
"Capt. CuNt®" <Capt...@Bigfoot.com> displaying symptoms of senile
dementia, spewed the following news:d80iv2$atb$1...@alzheimers.databasix.com...
Okay, since Steve and Gary won't help, I will. Look carefully below:
Path:
newssvr33.news.prodigy.com!newsdbm06.news.prodigy.com!newsdst02.news.prodigy.com!newsmst01b.news.prodigy.com!prodigy.com!postmaster.news.prodigy.com!newssvr33.news.prodigy.com.POSTED!e2e32dd9!not-for-mail
From: "John Cairns" <jgcairns@-removethis-sbcglobal.net>
Newsgroups: rec.boats.cruising,alt.usenet.kooks,alt.sailing.asa
References: <42a37185$0$152$edfa...@dread11.news.tele.dk>
<5BKoe.1623$HT1...@twister.nyroc.rr.com>
<d80fls$ldj$1...@fluffy.databasix.com> <d80g3a$lt$1...@wilhelp.databasix.com>
Subject: Re: Keep the propeller fixed or let it turn?
Lines: 41
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 6.00.2900.2180
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180
X-RFC2646: Format=Flowed; Response
Message-ID: <twOoe.158$Oq7...@newssvr33.news.prodigy.com>
NNTP-Posting-Host: 69.212.172.51
X-Complaints-To: ab...@prodigy.net
X-Trace: newssvr33.news.prodigy.com 1118024857 ST000 69.212.172.51 (Sun, 05
Jun 2005 22:27:37 EDT)
NNTP-Posting-Date: Sun, 05 Jun 2005 22:27:37 EDT
Organization: SBC http://yahoo.sbc.com
X-UserInfo1:
TSU[@ION_ZWURVLX\RHBNFXBWR\HPCTL@XT^OBPLAH[\RSAANVUEAE[YETZPIWWI[FCIZA^NBFXZ_D[BFNTCNVPDTNTKHWXKB@X^B_OCJLPZ@ET_O[G\XSG@E\G[ZKVLBL^CJINM@I_KVIOR\T_M_AW_M[_BWU_HFA_]@A_A^SGFAUDE_DFTMQPFWVW[QPJN
Date: Mon, 06 Jun 2005 02:27:37 GMT
Xref: newsmst01b.news.prodigy.com rec.boats.cruising:257982
alt.usenet.kooks:953006 alt.sailing.asa:396848
See the line that reads "NNTP-Posting-Host". Remember the number sequence.
From: "katysails" <katysa...@worldnet.att.net>
Newsgroups: alt.sailing.asa
References: <_4IPd.29520$wi2....@newssvr11.news.prodigy.com>
<3796l9F...@individual.net>
<1108307051.9...@c13g2000cwb.googlegroups.com>
<37993aF...@individual.net>
<1108307987.1...@f14g2000cwb.googlegroups.com>
Subject: Re: Boat built in desert a dream come true
Lines: 17
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 6.00.2900.2180
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180
X-RFC2646: Format=Flowed; Original
Message-ID: <mTPPd.35489$Th1....@bgtnsc04-news.ops.worldnet.att.net>
Date: Sun, 13 Feb 2005 21:44:50 GMT
NNTP-Posting-Host: 12.75.54.156
X-Complaints-To: ab...@worldnet.att.net
X-Trace: bgtnsc04-news.ops.worldnet.att.net 1108331090 12.75.54.156 (Sun, 13
Feb 2005 21:44:50 GMT)
NNTP-Posting-Date: Sun, 13 Feb 2005 21:44:50 GMT
Organization: AT&T Worldnet
Notice anything there? Read them twice if you're getting confused.
Now, I'll let you get some sleep, they come around early in the a.m. to
empty the dumpster behind the Winn-Dixie, wouldn't want to deprive you of
your breakfast.
John Cairns
Fred Hall
WD is still in business? They pulled out of my area two years ago.
Figured they'd be 404 by now.
>
>John Cairns
>
>
>
>
>
--
Let any real Christians reading here join me in prayer that the
Lord would reward the foul imp posting as "Fred Hall" according
to his works.
Pastor Winter
Larry W4CSC
> Anybody in this group that can convince one of us, that he
> is not right? ...
>
>
Let the boat drive the prop when sailing. That shaft produces 15-25A of DC
power on Lionheart from her shaft-driven alternator. No sense wasting good
power draggin' a stalled prop behind you. Lionheart's Perkins 4-108
transmission has been freewheeling because of her shaft alternator since
1986. Doesn't seem to have hurt it a bit.
Look over in alt.binaries.pictures.sports.ocean at the engine room on
Lionheart under the center cockpit. The shaft alternator is driven by a
flat belt from a large pulley on the shaft. At 8 knots, the special
alternator, itself, is turning about 200 RPM. To get the engine started,
you must turn off the excitation current to this shaft alternator so you
can remove the keyring from its switch to move the keyring to the engine
control panel to crank the diesel. This makes sure you have secured the
shaft alternator before starting the engine....captain proof.
Gogarty
fletopkanelbolle2rp.danmark says...
>
>
>My brother in law, and I have had a discussion of whether it
>is best - from a pure speed point of view (no consideration
>to the mechanics/oil/maintenance/gearbox etc. here ...)
>whether you should let your "fixed three bladed propeller"
>run/turn or keep it fixed (like put into gear) when sailing
>just for the wind with your sails in a 34 feet cruiser
>weighing roughly 5 T ... We have - unsuccesfully - tried to
>find out using the log ... the results were not conclusive -
>or one of us would not admit, that the other was right ... I
>think we need a testimony from someone, that has a
>'scientific based valid answer' ... or just knows for sure
>...
>
>The assumption is: There is no way to move, turn, 'collapse'
>anything on the propeller - the 'blades' are fixed ... (hard
>to explain in a language, that is not your own ... hope you
>get my point).
>
1. Many transmissions require a running engine to keep them lubricated.
Such transmissions should be locvked in reverse to prevent freewheeling.
2. A freewheeling propeller creates more drag than a locked on.. Just
consider a helicopter. Engine out and rotors freewheeling, the aircraft
will go down safely. Rotors locked and it drops like a stone.
Roger Long
> Look over in alt.binaries.pictures.sports.ocean at the engine room
> on
> shaft alternator before starting the engine....captain proof.
>
Gee, isn't this kind of "niggling and perfectionist", to quote a well
respected rec.boats contributor?
Of course, there is nothing wrong with that:)
--
Roger Long
Rosalie B.
>In article <42a37185$0$152$edfa...@dread11.news.tele.dk>,
>fletopkanelbolle2rp.danmark says...
>>
>>My brother in law, and I have had a discussion of whether it
>>is best - from a pure speed point of view (no consideration
>>to the mechanics/oil/maintenance/gearbox etc. here ...)
>>whether you should let your "fixed three bladed propeller"
>>run/turn or keep it fixed (like put into gear) when sailing
>>just for the wind with your sails in a 34 feet cruiser
>>weighing roughly 5 T ... We have - unsuccesfully - tried to
>>find out using the log ... the results were not conclusive -
>>or one of us would not admit, that the other was right ... I
>>think we need a testimony from someone, that has a
>>'scientific based valid answer' ... or just knows for sure
>>...
>>The assumption is: There is no way to move, turn, 'collapse'
>>anything on the propeller - the 'blades' are fixed ... (hard
>>to explain in a language, that is not your own ... hope you
>>get my point).
>>
>1. Many transmissions require a running engine to keep them lubricated.
>Such transmissions should be locvked in reverse to prevent freewheeling.
>
This is not what he asked. He said specifically
>from a pure speed point of view (no consideration
>to the mechanics/oil/maintenance/gearbox etc. here ...)
Plus not all transmissions can be locked in reverse to prevent
freewheeling. Ours cannot. Doesn't matter what gear you stop in, the
shaft will freewheel unless you stop it by putting a vice grip on the
shaft or something (IMHO a bad idea, but I know someone who does
this).
>2. A freewheeling propeller creates more drag than a locked on.. Just
>consider a helicopter. Engine out and rotors freewheeling, the aircraft
>will go down safely. Rotors locked and it drops like a stone.
This is probably a good analogy, although not all aircraft principles
will translate to water and v.v. (as the Wright brothers found)
My answer would be the same as yours and also that if you have a two
blade prop, it should be locked in line with the keel if it is
possible to determine where that is etc.
In our case, the freewheeling prop made so much noise that one of the
first things we did was get a feathering prop. We do have evidence
that feathering the prop increases our sailing speed, in addition to
being quieter.
grandma Rosalie
Capt. Neal®
"Dave" <Da...@nothere.com> wrote in message news:t9o8a152d6rle9aku...@news-east.newscene.com...
> On Mon, 06 Jun 2005 08:01:55 -0400, Gogarty <Gog...@Clongowes.edu> said:
>
>>A freewheeling propeller creates more drag than a locked on.. Just
>>consider a helicopter. Engine out and rotors freewheeling, the aircraft
>>will go down safely. Rotors locked and it drops like a stone.
>
> engine dies they're still rotating in a direction that generates lift.
FOOL! Rotating in the same direction, yes. Producing lift, no. They only
produce drag because of the angle of attack.
CN
Mike G
Da...@nothere.com says...
> On Mon, 06 Jun 2005 08:01:55 -0400, Gogarty <Gog...@Clongowes.edu> said:
>
> >consider a helicopter. Engine out and rotors freewheeling, the aircraft
> >will go down safely. Rotors locked and it drops like a stone.
>
> engine dies they're still rotating in a direction that generates lift.
>
I just have to ask. In what way is it a bad analogy?
Possibly you are thinking the freewheeling prop would somehow generate
some forward motion. Not likely or you would have a perpetual motion
machine. Stop the external power, in the case of a sail boat, the wind,
to a freewheeling prop and, as with the auto rotating helicopter having
it's fall slowed, the prop just slows you down faster. Neither generates
more energy, they consume it and thus slow the downward/forward motion
of what ever they are attached to. Every action has..........etc.
Personally I figure, locked or freewheeling, the difference in the
amount of drag caused by a kicker's prop is going to be so small as to
be a total non issue unless you are into times in parts of a second in
which case you pull the damn thing out of the water anyway.
--
Mike G.
Heirloom Woods
mi...@heirloom-woods.net
www.heirloom-woods.net
Brian Whatcott
>On Mon, 06 Jun 2005 08:01:55 -0400, Gogarty <Gog...@Clongowes.edu> said:
>
>>A freewheeling propeller creates more drag than a locked on.. Just
>>consider a helicopter. Engine out and rotors freewheeling, the aircraft
>>will go down safely. Rotors locked and it drops like a stone.
>
>Bad analogy. In a helicopter when the rotors are freewheeling after the
>engine dies they're still rotating in a direction that generates lift.
Hmmmm, so Dave feels that a boat prop will reverse direction if the
drive is placed in neutral from forward, while under way?
Brian Whatcott Altus, OK
Flemming Torp
"Brian Whatcott" <bet...@sbcglobal.net> skrev i en
meddelelse
news:qg09a196f486oscph...@4ax.com...
Being a sailor with no experience with aviation, I do not
understand the helicopter analogy.
In my opinion, a helicopter driven by the engine corresponds
in my understanding to a sailboat, where the propeller is
set to drive the boat *backwards* ..... When the propeller
on the boat 'gets the chance' to run without the force of
the motor, that is just sailing forward by sail, I expect
the propeller to turn in the opposite direction ... that is
'forward' ... If that happens in a helicopter, I would
expect the helicopter to hit the ground very fast ... don't
you? I do not assume, that there is a lot of inertia in a
propeller or shaft en a sailboat - may be that is the case
in a helicopter, and that could explain the smooth landing
without the assistance from the motor - or I'm I completely
wrong here? ...
--
Flemming Torp
Flemming Torp
"Larry W4CSC" <no...@home.com> skrev i en meddelelse
news:Xns966D488...@63.223.7.253...
> "Flemming Torp" <fletopkanelbolle2rp.danmark> wrote in
> news:42a37185$0$152$edfa...@dread11.news.tele.dk:
>
>> Anybody in this group that can convince one of us, that
>> he
>> is not right? ...
>>
>>
>
> Let the boat drive the prop when sailing. That shaft
> produces 15-25A of DC
> power on Lionheart from her shaft-driven alternator. No
> sense wasting good
> power draggin' a stalled prop behind you. >
>
'witty' comments in this newsgroup, and I learn a lot and
from them. But this comment does not adress my question: ...
When will my boat run fastest: 'Free' running propeller or
fixed/locked propeller? ... Simple question, complicated
answers, I understand ...
I know this is a very partial question ... but that is to
keep the issue as simple as possible ... *just speed* ... no
economy or energy considerations, no maintenance og mechanic
considerations, no ecology or noise consideration - I'm just
interested in understand the relation between the status of
the propeller and the speed og the boat - all other things
equal ...
--
Flemming Torp
Flemming Torp
"Mike G" <mi...@heirloom-woods.net> skrev i en meddelelse
news:MPG.1d0e4c862...@news.comcast.giganews.com...
> In article
> <t9o8a152d6rle9aku...@news-east.newscene.com>,
> Da...@nothere.com says...
>> On Mon, 06 Jun 2005 08:01:55 -0400, Gogarty
>> <Gog...@Clongowes.edu> said:
>>
>> >A freewheeling propeller creates more drag than a locked
>> >on.. Just
>> >consider a helicopter. Engine out and rotors
>> >freewheeling, the aircraft
>> >will go down safely. Rotors locked and it drops like a
>> >stone.
>>
>> Bad analogy. In a helicopter when the rotors are
>> freewheeling after the
>> engine dies they're still rotating in a direction that
>> generates lift.
>>
>
>
> I just have to ask. In what way is it a bad analogy?
SNIP
>
> Personally I figure, locked or freewheeling, the
> difference in the
> amount of drag caused by a kicker's prop is going to be so
> small as to
> be a total non issue unless you are into times in parts of
> a second in
> which case you pull the damn thing out of the water
> anyway.
>
>
> --
> Mike G.
> Heirloom Woods
> mi...@heirloom-woods.net
> www.heirloom-woods.net
I agree, that the practical difference might be dismissal
... but in a race every second counts per NM ... and it is
not easy to get a fixed propeller out of the water ;-) - and
I'm pretty curious of knowing the correct answer to my
question - preferably backed by solid science as I'm not a
scientist ...
When cruising, I would always lock the propeller - or follow
Larry's advice - use the energy generated ... that is
something I will consider ti implement in the future ...
--
Flemming Torp
Mike G
Torp" <fletopkanelbolle2rp.danmark> says...
> Being a sailor with no experience with aviation, I do not
> understand the helicopter analogy.
> In my opinion, a helicopter driven by the engine corresponds
> in my understanding to a sailboat, where the propeller is
> set to drive the boat *backwards* ..... When the propeller
> on the boat 'gets the chance' to run without the force of
> the motor, that is just sailing forward by sail, I expect
> the propeller to turn in the opposite direction ... that is
> 'forward' ... If that happens in a helicopter, I would
> expect the helicopter to hit the ground very fast ... don't
> you? I do not assume, that there is a lot of inertia in a
> propeller or shaft en a sailboat - may be that is the case
> in a helicopter, and that could explain the smooth landing
> without the assistance from the motor - or I'm I completely
> wrong here? ...
>
>
>
was the method of propulsion since there is no change in direction of
the water flow. In other words nothing forcing it to counter rotate.
In the case of the helicopter technically you are correct and a reversal
of direction would change the rotation of the blades. However the rotors
are connected to the engine via gears and can only rotate in the "lift"
direction. When the helicopter starts to fall due to loss of power the
rotors, slowed buy the drag of gears and engine, are forced to spin in
the lift direction causing "drag" that slows the copter to a rate where
impact with the ground is, at least, survivable.
Flemming Torp
"Dave" <Da...@nothere.com> skrev i en meddelelse
news:pe29a1hlsrfri0dku...@news-east.newscene.com...
> On Mon, 6 Jun 2005 13:06:28 -0400, Mike G
> <mi...@heirloom-woods.net> said:
>
>>I just have to ask. In what way is it a bad analogy?
>>
>>Possibly you are thinking the freewheeling prop would
>>somehow generate
>>some forward motion. Not likely or you would have a
>>perpetual motion
>>machine.
>
> understand my point. It's
> clear that a detailed explanation to you would be
> pointless, as you would
> have figured it out by now if you had the basic
> understanding.
Please give it a try anyway ... please ... ;-) ... I'm
curious, but not a physicist ...
--
Flemming Torp
Flemming Torp
"Mike G" <mi...@heirloom-woods.net> skrev i en meddelelse
news:MPG.1d0e6008d...@news.comcast.giganews.com...
I'm sorry - Now, I don't understand the argument or the
analogy from the helicopter to the boat of my brother in law
with three fixed blades ... Should he lock it or let it
'freewheel'?
--
Flemming Torp
Mike G
Torp" <fletopkanelbolle2rp.danmark> says...
> I agree, that the practical difference might be dismissal
> ... but in a race every second counts per NM ... and it is
> not easy to get a fixed propeller out of the water ;-) - and
> I'm pretty curious of knowing the correct answer to my
> question - preferably backed by solid science as I'm not a
> scientist ...
>
> When cruising, I would always lock the propeller - or follow
> Larry's advice - use the energy generated ... that is
> something I will consider ti implement in the future ...
>
> --
> Flemming Torp
>
>
>
Well, if you are racing and can't get the prop out of the water I can
see where it may be of concern but if you're looking for a solid science
answer I'd have to venture to say that there are so many variables
involved only a practical application with the boat involved would work.
Drag is drag and if you can set a straight course with a constant speed,
difficult under sail even in ideal conditions but still do able, just
locking and unlocking the prop and seeing what it does to your speed
should provide the answer.
However, I'd have to opine that there is a difference between a free
wheeling prop and one that is not only spinning but driving an
generator/alternator. There is no free ride. A prop working a generator
requires more energy then a free wheeling one and the only place that
energy can come from is the forward speed of the boat.
Again, the difference may be negligible but if you are concerned with
thousands of a second, as you would be if racing, you have now
introduced one more variable that has to be considered.
I'm afraid you can't really get a "just speed" answer and expect an
answer backed by science without considering conservation of energy,
every action has an......... and so on.
I can't prove it but my money would be on a truly free spinning prop. A
fixed prop wants to spin and is going to use energy trying to do so.
It's going to fight forward motion harder then a free spinning prop. A
prop with a generator on it will spin but not without the loss of energy
required to also turn the generator, a free spinning prop requires only
the energy required to spin it. It'll, pardon the pun, go with the flow.
Some energy penalty but less then the first two scenario's
Again, the only place the energy required can come from is the forward
motion of the boat.
Jeff
> In article <t9o8a152d6rle9aku...@news-east.newscene.com>,
> Da...@nothere.com says...
>
>>On Mon, 06 Jun 2005 08:01:55 -0400, Gogarty <Gog...@Clongowes.edu> said:
>>
>>
>>>A freewheeling propeller creates more drag than a locked on.. Just
>>>consider a helicopter. Engine out and rotors freewheeling, the aircraft
>>>will go down safely. Rotors locked and it drops like a stone.
>>
>>Bad analogy. In a helicopter when the rotors are freewheeling after the
>>engine dies they're still rotating in a direction that generates lift.
>>
>
>
>
> I just have to ask. In what way is it a bad analogy?
Perhaps you should consider an autogyro:
http://www.centennialofflight.gov/essay/Dictionary/autogyro/DI8.htm
...
>
> Personally I figure, locked or freewheeling, the difference in the
> amount of drag caused by a kicker's prop is going to be so small as to
> be a total non issue unless you are into times in parts of a second in
> which case you pull the damn thing out of the water anyway.
Well, each to his own. It is often claimed that the difference can be
up to a half of a knot. On a hundred mile passage that can be about an
hour. Given a choice, I'd rather come in before dark. My previous
boat had a two blade which I always locked aligned with the hull; my
current boat has folding props.
Roger Long
The answer will be found in conservation of energy. The energy books
have to balance in every system. The amount if drag of the water
flowing over the prop will exactly equal the energy being produced.
With the prop stopped, it is simply pull aft on the shaft. The prop
isn't very efficient that way. Try propelling a boat by putting the
prop out ahead on a long line and then pulling it in quickly. When
the prop is freewheeling, it is producing energy that is absorbed by
heating up the bearings and lube oil in the transmission, (or in
Larry's case by charging batteries).
It's slightly counterintuitive but, whatever energy is produced has to
have an exact mirror image in drag on the boat. Since the prop is
making energy more efficiently while turning, there has to be more
drag.
--
Roger Long
Capt. Neal®
"Flemming Torp" <fletopkanelbolle2rp.danmark> wrote in message news:42a4912b$0$168$edfa...@dread11.news.tele.dk...
Lock the shaft for speed. Don't allow the prop to freewheel.
It's that simple.
CN
Flemming Torp
"Roger Long" <rwl...@maine.rr.com> skrev i en meddelelse
news:nw1pe.3308$fp6....@twister.nyroc.rr.com...
doubts. In my opinion it requires quite a lot of energy just
to keep the propeller fixed when sailing. It is not
possible - for instance to hold the propeller shaft by
hand - so the shaft does not turn - as Rosalie confirmed ...
So there must be quite a constant pressure on the gear box,
when just sailing and the prop locked by the gearbox. In my
opinion, it could be compared to a kind of turbine, where
you keep the blades fixed from turning ...
My intuition tells me, however, that if you 'let the
propeller go', the total resistance/drag will be less and if
you start braking the turning propeller, the resistance/drag
gets bigger, and that resistance can be converted to
electricity - as explained by Larry -isn't that right? But I
must admit, that I don't know - or may be worse: Don't
understand - the Law of physics behind the right answer ...
--
Flemming Torp
PS - I have followed your interesting 'report' of the last
few months of projects on your boat ... good and inspiring
reading - thank you!
me
The analogys between aircraft and boat propellors do not
hold up when examined by people who understand the physics
involved. I have a very superficial understanding of the
matter, but I can see several problems with it.
An aircraft with a fixed pitch propellor will glide farther
with the propellor stopped because the propellor is bolted
directly to the engine crankshaft. If the prop is turning,
the engine is turning. If the engine is dead but still
being turned, the power to turn the engine is being
extracted from the air flowing through the propellor. The
power lost in turning the porpellor and engine shows up as
drag in the airstream which requires a steeper and shorter
glide to maintain a flyable airspeed.
If the engine can be separated from the propellor by placing
a transmission into neutral as I would expect the case to be
in a sailboat, I would guess that a free wheeling prop would
produce less drag than pulling the stalled propellor blades
through the water. It should take very little power to turn
a shaft riding in two or three bearings with no load on
them. The turbulence of the stopped propellor blades
dragging through the water at nearly right angles to their
streamline shape should put up a lot more resistance.
A helicopter rotor bears no resemblance to a boat propellor
because the rotor blades have variable pitch that can change
each blade individually. The pitch angle can be set so that
(in one exampe) the blade that is moving forward has a very
low pitch while the blade that is moving backward has a very
high pitch. This means that the individual blades are
constantly twisting and turning in their hub bearings as the
entire rotor assembly goes around. There is nothing similar
to that in any boat propellor that I have ever heard of.
Roger Long
like an airfoil at an angle of attack of nearly 90 degrees, not very
efficient. When turning, the blades are at a low angle of attack and
more efficient. A parachute is less efficient per unit area than a
wing. When the prop freewheels, it's rotational speed makes the water
flow over it faster than when it is just dragged through the water at
boat speed. Whatever force is produced has to be reflected in drag on
the boat as a whole.
This is a very crude pointer towards understanding a complex set of
relationships so, others, please don't bother jumping all over me to
point out how it isn't a rigorous and complete explanation.
There are a narrow range of conditions when a freewheeling prop may
have less drag than a fixed one but they are not commonly encountered
in actual vessels.
--
Roger Long
Flemming Torp
That's exactly, what we have tried, but the conditions were
not ideal, so we did not - unfortunately - come up with some
'conclusive empiric evidence' ...
>
> However, I'd have to opine that there is a difference
> between a free
> wheeling prop and one that is not only spinning but
> driving an
> generator/alternator. There is no free ride. A prop
> working a generator
> requires more energy then a free wheeling one and the only
> place that
> energy can come from is the forward speed of the boat.
>
That's also what my intuition tells me, but listening to
other people in this group, I get the impression, that my
intuition is not valid in this case ...
> Again, the difference may be negligible but if you are
> concerned with
> thousands of a second, as you would be if racing, you have
> now
> introduced one more variable that has to be considered.
>
I do not understand - a new variable? My question is simple:
... If all other things are equal, should I stop my
propeller or should I let i turn freely?
My problem is, that I can not see, that your above argument
is wrong ... but some other readers in this group tell me,
it is not correct ... Unfortunately, I do not
know/understand the laws of physics governing this situation
...
--
Flemming Torp
Flemming Torp
> On Mon, 06 Jun 2005 19:31:31 GMT, "Roger Long"
> <rwl...@maine.rr.com> said:
>
>>When
>>the prop is freewheeling, it is producing energy
>
> nor destroyed.
Isn't it right, that when the boat is sailing (just for sail
that is) you need some energy to keep the propeller from
turning - i.e. to keep it fixed - and that gives a certain
drag on the boat ....
My intuition tells me that when you ' loosen the grip' on
the propeller shaft and let the propeller turn freely, it
reduces the drag on the boat. Right or wrong?
--
Flemming Torp
Capt. Neal®
"Flemming Torp" <fletopkanelbolle2rp.danmark> wrote in message news:42a4b651$0$153$edfa...@dread11.news.tele.dk...
You have lousy intuition, Flemming.
CN
Flemming Torp
"me" <bi...@verizon.net> skrev i en meddelelse
news:zn2pe.7783$nr3.5795@trnddc02...
I'm not sure I get your conclusion ... ;-) Sorry, but do
you recommend me to let the propeller turn og should I stop
it from turning, if I'm only concerned with the speed of the
sailboat - when only using the sails?
--
Flemming Torp
Roger Long
to a different kind of grip. Ever notice how hot brakes get? That's
because they are turning all that energy into heat. When you let the
prop go, it is still restrained by the friction of the bearings and
oil in the transmission. They get warmer because they are turning the
energy being produced into heat. That gets reflected in drag on the
boat.
When the prop is locked, the shaft break, clutch, or whatever is
holding it doesn't get hotter. What gets hotter is the water flowing
past the blades. That energy production gets reflected in drag on the
boat as well but there is less of it at normal sailing speeds and
freewheeling shaft rpm.
--
Roger Long
"Flemming Torp" <fletopkanelbolle2rp.danmark> wrote in message
news:42a4b651$0$153$edfa...@dread11.news.tele.dk...
>
>
Peter Aitken
news:MPG.1d0e4c862...@news.comcast.giganews.com...
> In article <t9o8a152d6rle9aku...@news-east.newscene.com>,
> Da...@nothere.com says...
>> On Mon, 06 Jun 2005 08:01:55 -0400, Gogarty <Gog...@Clongowes.edu> said:
>>
>> >A freewheeling propeller creates more drag than a locked on.. Just
>> >consider a helicopter. Engine out and rotors freewheeling, the aircraft
>> >will go down safely. Rotors locked and it drops like a stone.
>>
>> Bad analogy. In a helicopter when the rotors are freewheeling after the
>> engine dies they're still rotating in a direction that generates lift.
>>
>
>
> I just have to ask. In what way is it a bad analogy?
>
It's a bad analogy because a prop and a helicopter blade operate on
different principles. A prop is simply a screw whereas a helicopter blade is
an airfoil. When it rotates it generates lift in the same way that an
airplane's wing does. THis is not the case with a prop.
--
Peter Aitken
Roger Long
OK, here is the answer you are looking for.
Unless you have a highly unusual powertrain set up and strangely
pitched prop, determine the position in which the most blade area is
shadowed by keel and hull. Mark the shaft inside. Stop the shaft in
that position. Sail the boat. It's very unlikely you'll go faster
doing anything else.
--
Roger Long
Larry W4CSC
news:deXoe.2459$g5....@twister.nyroc.rr.com:
> Gee, isn't this kind of "niggling and perfectionist", to quote a well
> respected rec.boats contributor?
>
Oh, you'd have a fit if you looked into that bilge. There's dishwater,
some oil, some rust, a few tools too deep to reach even with the magnet,
how awful! It's a boat....not house beautiful. Everything from the sinks,
shower, everything but the head, dumps in her bilge to be pumped overboard
WITHOUT those nasty thru-hull fittings rotting away...(c;
Look at the next picture. This is the NEW engine Cap bought from a guy in
NC I met on this newsgroup. It's a pullout. See? We didn't even paint
it! Looks awful...runs fantastic. The original looked worse. It had over
8000 hours on it. This "new" one is only up to 900 hours, now. Cap'n
Geoffrey couldn't even destroy it trying to start it full of seawater when
that stupid water injection line to the dripless packing he had a shipyard
install BACKED UP seawater into the exhaust ports. Try sailing up Ponce
Inlet S of Daytona Beach in the rush of an outgoing tide past the
lighthouse and seawall so the tow operator can drag you all the way to
Daytona Marina. Scary stuff that day. Cutter Doc did a great job pickling
the engine. It looked like we pumped the Exxon Valdez oil slick out of the
crankcase...(c;
These pictures are old. I need to carry my camera down, now that she's
been fitted out, and take some new pictures in her. I'm usually too busy
when Geoffrey is around working on his to-do list...(c; He finally gave up
trying to pay me. He asked what I wanted for all my work. I told him,
"That's easy. Just take me with you." It's worked out great for both of
us....(c;
Larry W4CSC
> When will my boat run fastest: 'Free' running propeller or
> fixed/locked propeller? ... Simple question, complicated
> answers, I understand ...
>
When you replace the prop with a folding prop that has nearly zero drag.
It does make a difference. I'm not crazy about a prop with so much
underwater mechanical parts, myself. Those little critters with the little
shells must be of some nuisance to a folding prop with cracks in them.
Freewheeling would seem to produce less drag than it being locked up
dragging, with no load on the prop. Ever notice how quick the dingy stops
when the outboard motor just locks up as opposed to drops to idle speed
still turning the prop slowly?
I'm also told some transmissions will not tolerate freewheeling. Case in
point is when we got this "new-used" Perkins 4-108 from another boater, the
transmission on it was much smaller than the original one in Lionheart on
her old Perkins 4-108. The mechanics said to keep this "fluid drive"
larger transmission because it was made to freewheel for the alternator.
The small transmission was not. I've never seen why, though, and don't
know why.
Oh oh...this is getting too long, again....sorry.
Flemming Torp
start to get your point - Sorry it took so long. Coming from
Denmark, where the water is around 13 degrees Celcius these
days, this is a very important message to all sailers: Get
out and sail, and lock your propeller, so we soon can come
out swimming in warmer waters! ... ;-)) ... No Roger you
don't have to give me a lecture in how many sailors we need
in order to increase the temperature in the waters around
Denmark by ten degrees ... you have been very helpful so far
... thank you!
--
Flemming Torp
"Roger Long" <rwl...@maine.rr.com> skrev i en meddelelse
news:gN2pe.2503$g5....@twister.nyroc.rr.com...
Larry W4CSC
> My intuition tells me that when you ' loosen the grip' on
> the propeller shaft and let the propeller turn freely, it
> reduces the drag on the boat. Right or wrong?
>
>
It'd be real easy to test. Get one of those plastic props for a small
outboard, put it on a shaft. Pull it loosely behind the boat,
FREEWHEELING, with no load (which produces no work) using a simple fish
weighing scale to measure its drag. Then, lock it to the shaft so it can't
turn and drag it at the same speed, yet again.
Wanna bet the locked prop draggin' through the water creates lots more drag
than the one locked that can't turn?
Take the dingy out for a spin. Run it wide open then drop the engine into
neutral, freewheeling the prop, and see how long it takes to stop dead.
Crank it back up and run it wide open again. This time, leave it in gear
and just pull the safety lanyard making it lock the prop dead still.
Retime how long it takes it to stop. More the drag, the quicker the stop.
You'll find it stops MUCH quicker pulling that STALLED prop through the
water than one FREEWHEELING.
As Lionheart's freewheeling shaft alternator is TURNED ON to produce power
(producing torque on the alternator shaft), it SLOWS the boat as the
alternator SLOWS THE PROP. The heavier it pulls (more torque produced),
the more it slows the boat and prop. Freewheeling props are producing a
minimal amount of torque...and work with no or little load...as load
increases, to the point of being stalled at maximum torque, the drag-
converted-into-torque INCREASES, not decreases!
If you were to spin the prop faster than its drag is capable of, you will
pass through a point of ZERO DRAG just at the point where increasing the
speed of the prop PRODUCES THRUST. So, as prop speed DECREASES, drag
INCREASES! As prop speed INCREASES, it passes through zero drag at the
point where it starts to produce thrust by turning it even faster!
I'm not a physicist, but I play one on Usenet....(c;
Your assignment for tomorrow's class is pages 287 through 312. Do the
workbook exercies on workbook pages 42-43 for turn in. The test will be on
Friday at 2PM.
It might also be a good time to point out that the HELICOPTER rotor CHANGES
DIRECTION as the chopper crashes. It is FORCED in the OPPOSITE direction
from freewheeling to provide DOWNDRAFT to hold the craft aloft by the
engine. As it decends freewheeling, unless you can reverse the pitch of
the rotor, the RISING air through it will make it spin in the opposite
direction.....making it not relevant to any of our discussion here as props
are fixed pitched and already going in the direction of flow caused by
thrust.
Geez....
Flemming Torp
"Roger Long" <rwl...@maine.rr.com> skrev i en meddelelse
news:b13pe.2504$g5....@twister.nyroc.rr.com...
Now we are getting close to 'basics' Roger ... but, but ...
as I wrote in the introduction, it has so far been very
difficult to get hard evidence from the log when trying to
let the propeller run and have it locked, as the speed of
the boat is a function of so many things, and I'm convinced
that there is not a big difference - so may be my question
is of a more theoretical type, as reliable data are hard to
get in the real world ... In a bassin, it might be easier
... I have seen som reports, where different kinds of
propellers - folding with two blades, folding with three
blades, fixed with three baldes etc. were compared ... and
the result indicated differences in 'thrust' and in speed
up to between ½ - 1 in worst case ... But I have not seen
any reports on the comparison between a locked and a free
wheeling propeller ... but I have certainly got a lot of
input ... also from the aviation world, that I know nothing
about ... thank you.
But your final proposal is very logical, operational and
easy to implement ... when the water gets warmer, we might
do what you have recommended ... or do as Larry - use the
energy coming from the rotating propeller - og invest in a
folding propeller ... time will show, and thank you so much
for your keen interest in learning me some physics ...
Flemming Torp
"Larry W4CSC" <no...@home.com> skrev i en meddelelse
news:Xns966DB4F...@63.223.7.253...
try to set up this experiment during the summer holidays ...
In the meantime, I must admit, I'm getting pretty much
confused ... I have just appreciated and accepted Roger
Longs 'lecture' on drag and rotation speed etc., where the
conclusion was: The locked propeller gives the maxium speed
compared to the freewheeling propeller ... And now, you
testify just to the opposite ...
My intuition was exactly as you write - I quote: Wanna bet
the locked prop draggin' through the water creates lots more
drag than the one locked that can't turn? Unquote. I think I
understand what you mean, but being a Dane, and having read
it quite a few times makes me a little uncomfortable - is
the sentence correctly phrased?
The beauty of your proposed experiment is, that it is very
operational with a little plastic propellerthing and the
weight from my wifes kitchen, a string and a piece of wood.
And you can repeat the experiment several time under
different conditions ... I will have to find out have to
keep the propeller from turning and at the same time measure
the drag on the weight, without affecting the weight
measurement ... I don't have a motor on the dinghy ...
I had serious problems understanding the helicopter analogy
... and the way you phrase the case is the way, my intuition
saw the situation - I quote: > It might also be a good time
to point out that the HELICOPTER rotor CHANGES DIRECTION as
the chopper crashes. It is FORCED in the OPPOSITE direction
from freewheeling to provide DOWNDRAFT to hold the craft
aloft by the engine. unquote. Therefore, I do not see, that
this analogy explains anything related to my question.
May be an old quotation is in place now: I'm still confused,
but - hopefully - at a higher level ??? As to the
assignment, I'm afraid I forgot my textbook in the office,
and I will be working from other places the rest of the week
... sorry!
I'm 'afraid' this debate is not over yet, but you have given
me inspiration to make some funny experiments, that might
give me and my brother in law a clear and convincing answer,
so we can settle our little dispute ... thankyou!
--
Flemming Torp
Flemming Torp
alt.binaries.pictures.sports.ocean ... Am I missing
something?
--
Flemming Torp
"Larry W4CSC" <no...@home.com> skrev i en meddelelse
news:Xns966DB09...@63.223.7.253...
Flemming Torp
just wonder whether Roger is in agreement ;-))
--
Flemming Torp
Gogarty
gmbe...@mindspring.com says...
>My answer would be the same as yours and also that if you have a two
>blade prop, it should be locked in line with the keel if it is
>possible to determine where that is etc.
Well, next time you haul you mark the shaft inside the boat to indicate
when the prop is up and down behid the keel or skeg or whatever. After
twenty years, I might yet get around to doing that.
>
>In our case, the freewheeling prop made so much noise that one of the
>first things we did was get a feathering prop. We do have evidence
>that feathering the prop increases our sailing speed, in addition to
>being quieter.
Ours doesn't freewheel in reverse. And the cost of a feathering prop really
put me off.
Gogarty
Da...@nothere.com says...
>
>
>On Mon, 06 Jun 2005 08:01:55 -0400, Gogarty <Gog...@Clongowes.edu> said:
>
>>A freewheeling propeller creates more drag than a locked on.. Just
>>consider a helicopter. Engine out and rotors freewheeling, the aircraft
>>will go down safely. Rotors locked and it drops like a stone.
>
>Bad analogy. In a helicopter when the rotors are freewheeling after the
>engine dies they're still rotating in a direction that generates lift.
Well yes. That was my point. But if somehow they get locked and don't
rotate. negligible lift. About like a stone, which has some lift
Gogarty
Da...@nothere.com says...
>
>
>On Mon, 6 Jun 2005 13:06:28 -0400, Mike G <mi...@heirloom-woods.net> said:
>
>>I just have to ask. In what way is it a bad analogy?
>>
>>some forward motion. Not likely or you would have a perpetual motion
>>machine.
>
>Those with an understanding of basic physics will understand my point. It's
>clear that a detailed explanation to you would be pointless, as you would
>have figured it out by now if you had the basic understanding.
Oh dear. Here we have the old scientist vs. engineer argument. The asymtotic
approach vs. close enough is good enough.
Gogarty
>
>
>"Flemming Torp" <fletopkanelbolle2rp.danmark> wrote in
>news:42a4b651$0$153$edfa...@dread11.news.tele.dk:
>
>> My intuition tells me that when you ' loosen the grip' on
>> the propeller shaft and let the propeller turn freely, it
>> reduces the drag on the boat. Right or wrong?
>>
>>
>
>It'd be real easy to test.
(Snip)
Nobody is interested in settling the matter. Argument is so much more fun. And
in this case, it is also trivial.
JR Gilbreath
you would not have to put it in gear to lock it, a prop out of gear
would not even freewheel it there was more drag.
Gogarty
>
>
>Mike G wrote:
>> In article <t9o8a152d6rle9aku...@news-east.newscene.com>,
>> Da...@nothere.com says...
>>
>>>
>>>
>>>>A freewheeling propeller creates more drag than a locked on.. Just
>>>>consider a helicopter. Engine out and rotors freewheeling, the aircraft
>>>>will go down safely. Rotors locked and it drops like a stone.
>>>
>>>Bad analogy. In a helicopter when the rotors are freewheeling after the
>>>engine dies they're still rotating in a direction that generates lift.
>>>
>>
>>
>>
>
>http://www.centennialofflight.gov/essay/Dictionary/autogyro/DI8.htm
>
Jeez! Are you that old? I have actually seen them.
Brian Whatcott
<fletopkanelbolle2rp.danmark> wrote:
>My intuition tells me, however, that if you 'let the
>propeller go', the total resistance/drag will be less and if
>you start braking the turning propeller, the resistance/drag
>gets bigger...
Fluid dynamics is tough on the intuition.
But if you cannot decide between two different opinions,
you *could* do what fluid dynamicists do when uncertain:
run a test.
One way:
find a spinning lure and tow it on a swivel so it can turn, behind a
boat at moderate speed (you could even do this in the bath, maybe?)
and check the drag with an ounce spring scale.
Then fix the spinner, and rerun the test.
Is the drag greater or less?
You will be surprized I expect.
[Usual disclaimer: fluid dynamics people and naval engineers know
there are scale effects as between boat sized props and little fish
size spinners.
Still, in comparing results at constant speed and constant spinner
size, you *can* get useful results.... ]
Brian Whatcott Altus, OK.
Tim
it's more geared to mounting alternators, but gives insight to prop
drag.
http://www.ybw.com/forums/showflat.php?Cat=0&Board=ym&Number=737566&page=18&fpart=1
Roger Long
system is coming to a stop and the prop is slowing as the boat is
slowing. Do it while towing the dinghy behind the big boat at a steady
speed with a scale on the tow rope and you may get a different result.
Under some circumstances, a freewheeling prop may have less drag than
a fixed one. Props on normal transmissions are not freewheeling
however. There is enough drag in the shaftline, bearings, and
transmission to upset things. Outboards, with their clutches right in
the lower unit are closer to freewheeling.
--
Roger Long
Jeff
Why?
Rosalie B.
So you don't really need it then.
>And the cost of a feathering prop really
>put me off.
The feathering prop also gave us increased speed. That alone made it
worth the money. A folding prop and a feathering prop are different
and we have the less expensive one. Plus, we found when we took the
prop off that we had a scored shaft which had to be replaced.
grandma Rosalie
Gogarty
jr...@bellsouth.net says...
>
>
>If there truly was less drag on a locked prop than a freewheeling one
>you would not have to put it in gear to lock it, a prop out of gear
>would not even freewheel it there was more drag.
I think you have it backwards.
Larry W4CSC
> I'm sory, but I can only see one picture in
> alt.binaries.pictures.sports.ocean ... Am I missing
> something?
>
>
Me, too. It's the one with the whole Perkins engine in it. The server
must have lost the one with just the shaft alternator behind it. I'll do
it again.
It should take it a while to propagate to your servers.
Larry W4CSC
> My intuition was exactly as you write - I quote: Wanna bet
> the locked prop draggin' through the water creates lots more
> drag than the one locked that can't turn? Unquote. I think I
> understand what you mean, but being a Dane, and having read
> it quite a few times makes me a little uncomfortable - is
> the sentence correctly phrased?
>
I owe you an apology. I would bet the freewheeling prop drag is LESS than
the locked prop drag. I'm sorry I made a joke of my English. I should
have read your header before putting my foot in my mouth with the joke.
Beautiful country, Danmark. We can only pray leaking, abandoned Russian
nuclear facilities don't pollute Scandanavia. I read bellona.no websites
about it. How awful.
Larry W4CSC
> I'm 'afraid' this debate is not over yet, but you have given
> me inspiration to make some funny experiments, that might
> give me and my brother in law a clear and convincing answer,
> so we can settle our little dispute ... thankyou!
>
>
The thread is wonderful. So many ideas and theories. Thank you for
starting it.
Larry W4CSC
> Under some circumstances, a freewheeling prop may have less drag than
> a fixed one. Props on normal transmissions are not freewheeling
> however. There is enough drag in the shaftline, bearings, and
> transmission to upset things. Outboards, with their clutches right in
> the lower unit are closer to freewheeling.
>
>
But the point is, as I load the shaft alternator, and therefore the
freewheeling prop, the boat slows more and more as the load increases,
terminating in worst case drag as the prop stalls and stops.
As the load decreases, and the prop turns faster, speed increases. If you
take this beyond the natural drag of the transmission and cutlass bearing
limiting prop speed, by adding slight power to it, there comes a point at
which the prop crosses zero drag trying to pull the shaft out and starts
pushing harder and harder on the shaft as speed increases, where drag goes
through zero and turns into THRUST.
I've never seen it plotted. It might not be a linear function but appears
not to have any weird peaks or valleys in the curve....
|\
D | \
R | \ It's not linear, I'm sure.
A | \ Lots more physics going on.
G | \ Somewhere it stalls.
| \
|------\--------------<"freewheeling drag"
0 PROP SPEED -> <(prop turns = boat speed = no drag here)
| \
T | \
H | \
R | \
U | \
S | \ Down here, somewhere it cavitates.
T | \
God I hate text graphing....(c;
barry lawson
in this forum for the last few days.
In the late fifties (of last century), my father was building himself a
steel ocean racing yacht. The prop drag controversy was alive and well then
There was a handicap allowance at the time for prop based on diameter, and
some silly ineffectual props that looked likesomething off a model airplane
were being tried. Sometimes two in tandem, and they were of course locked in
line with the deadwood. Safety regulations required that the boat be able to
demonstrate performance under power. In still air and calm water, of speed
in knots equal to the square root of the wateline length in feet. I remember
a 58ft 10 Meter Class yacht I was crewing on at the time being unable to
make headway into a 25 Knot breeze. She had about 50 SHP, but a small prop
that was able to transmit only a small fraction of the engine power into
thrust.
Dad didn't want any of this nonsense, if his boat needed an engine for
safety reasons, then it was going to be able to use the power of the engine
and go to windward if necessary despite the weather. But he wanted it to be
competitive.
He got a piece of steel tube about 3 inches in diameter and 18 inches long
and machined inside each end to take the outer ring of a tapered roller
bearing. The inners of the bearings were installed on the shaft with
shoulders so that the bearings were opposed and transmitted the forward and
reverse thrust to the outer rings. Outside of the bearings at each end of
the tube was a normal oil seal, installed the right way round so as to keep
oil inside the tube. The oil inside the tube was pressurised by a header
tank mounted about 3 ft above the waterline.
The tube was mounted at the trailing edge of the keel aperture, with the
prop sized to the 40 HP diesel engine.
Inside the boat between the shaft and the engine gearbox there was a dog
clutch, so that the prop and shaft could be completely disconnected from the
gearbox.
The unit had so little friction that on the slip on a windy day the prop
would revolve in the breeze.
Driven off the shaft by a small chain was an aircraft tachometer generator,
and in the cockpit the tachometer, which was calibrated to read knots. It
was about as accurate as anything I've ever used. I once ran a DR plot based
on it that was 10 nm out at the end of a 600 nm Sydney Hobart race. 1.5%?
Dad ran into the third or fourth owner of the boat in 1980, and when told
that the stern tube assemble had just required repair took delight in being
able say what a pity it was, as the guarantee had just run out.
Going back to the argument about locking or freewheeling: this subject
occupied numerous off watch race hours without as I remember any consensus.
Had we had a GPS or paddle wheel log then it would have been easy to set the
boat up with the prop locked, and then unlock it to see if the speed
increased. Though I doubt we would have done this during a race, and in
light winds (when the effect will be greatest) we never wanted to sail if it
wasn't a race. We had this beaught engine with a big prop.
Lauri Tarkkonen
>"Dave" <Da...@nothere.com> skrev i en meddelelse
>news:eic9a1tv0174qme3v...@news-east.newscene.com...
>> On Mon, 06 Jun 2005 19:31:31 GMT, "Roger Long"
>> <rwl...@maine.rr.com> said:
>>
>>>When
>>>the prop is freewheeling, it is producing energy
>>
>> A fairly basic principle: energy can be neither created
>> nor destroyed.
>Isn't it right, that when the boat is sailing (just for sail
>that is) you need some energy to keep the propeller from
>turning - i.e. to keep it fixed - and that gives a certain
>drag on the boat ....
>My intuition tells me that when you ' loosen the grip' on
>the propeller shaft and let the propeller turn freely, it
>reduces the drag on the boat. Right or wrong?
You do not create energy by letting the propellor run. In one answer
earlier it was pointed out that a stalled propellor is very different
than a rotating propellor. If there were no friction in your propeller
shaft when it is running freely the energy needed to spin the propeller
could be less than the energy needed for the water to around the
propeller blades. If you have a two blade propeller and it is in
vertical position behind your keel, the water flow behind the keel is
already somehow turbulent and the drag is fairly small. Then in most
cases it will cause less drag if it is not rotating.
There are so many facotrs involved; the sahpe of the blades, the pitcc,
the aspect ratio the shape of the rear of your boat etc. so it is
impossible for someone even very knowledgeable person to give you an
answer just by reading your description of the situation.
There are (fairly reliable) measurements of several sailors sailing and
then stopping and letting the propellor to rotate about the changes of
the speed. The problem is that in some cases it is faster to let spin
and other cases (boats) it is faster to not let to spin. In a case of a
three bladed propellor in a 40 feet Colin Archer the difference has bee
almost one knot.
So I suggest you keep on recording speed differences, there must be some
situation where you wave and weather conditions keep for more or less
constant for some 20 minutes, so you can stop and let your propeller to
roatate by putting the gear on and putting it on neutral.
You keep saying that you do not know physics. There are plenty of good
(and interesting and well written books) it would be easer to choose
from the answers of this group the relevant ones if you were not
ignorant. I can promise you that the speed of your boat is not affected
by the "votes" given in this discussion.
- Lauri Tarkkonen
Lauri Tarkkonen
>"Flemming Torp" <fletopkanelbolle2rp.danmark> wrote in
>news:42a4b651$0$153$edfa...@dread11.news.tele.dk:
>> My intuition tells me that when you ' loosen the grip' on
>> the propeller shaft and let the propeller turn freely, it
>> reduces the drag on the boat. Right or wrong?
>>
>>
>It'd be real easy to test. Get one of those plastic props for a small
>outboard, put it on a shaft. Pull it loosely behind the boat,
>FREEWHEELING, with no load (which produces no work) using a simple fish
>weighing scale to measure its drag. Then, lock it to the shaft so it can't
>turn and drag it at the same speed, yet again.
>Wanna bet the locked prop draggin' through the water creates lots more drag
>than the one locked that can't turn?
Your example has one flaw. The sailboat propeller is not loose, it is on
a shaft with some bearings and the transmission wheels and oil etc. Do
you know that some boat trhansmissions get very hot if you let the
propeller to free wheel? Some energy is needed to heat it up.
- Lauri Tarkkonen
Lauri Tarkkonen
The rotating propeller after it gets some speed is a very different
animal that the locked propeller.
- Lauri Tarkkonen
Lauri Tarkkonen
>--
>Roger Long
I will second Roger on this. The dinghy experiment would be valid on a
very naiive level of thinking and the experiment with the fishing lure
is invalid because the boat propellors is NOT freewheeling. There is
quite a bit of friction involved. In some cases the gearbox get too hot
to hold a hand on it because the transmission fluid is not pumped around
and while lubricating it is also transmitting the heat away.
- Lauri Tarkkonen
Jeff
> "Roger Long" <rwl...@maine.rr.com> skrev i en meddelelse
> news:b13pe.2504$g5....@twister.nyroc.rr.com...
>
>>OK, here is the answer you are looking for.
>>
>>Unless you have a highly unusual powertrain set up and
>>strangely pitched prop, determine the position in which
>>the most blade area is shadowed by keel and hull. Mark
>>the shaft inside. Stop the shaft in that position. Sail
>>the boat. It's very unlikely you'll go faster doing
>>anything else.
>
> Now we are getting close to 'basics' Roger ... but, but ...
> as I wrote in the introduction, it has so far been very
> difficult to get hard evidence from the log when trying to
> let the propeller run and have it locked, as the speed of
> the boat is a function of so many things, and I'm convinced
> that there is not a big difference - so may be my question
> is of a more theoretical type, as reliable data are hard to
> get in the real world ... In a bassin, it might be easier
> ... I have seen som reports, where different kinds of
> propellers - folding with two blades, folding with three
> blades, fixed with three baldes etc. were compared ... and
> the result indicated differences in 'thrust' and in speed
> up to between ½ - 1 in worst case ... But I have not seen
> any reports on the comparison between a locked and a free
> wheeling propeller ... but I have certainly got a lot of
> input ... also from the aviation world, that I know nothing
> about ... thank you.
>
> But your final proposal is very logical, operational and
> easy to implement ... when the water gets warmer, we might
> do what you have recommended ... or do as Larry - use the
> energy coming from the rotating propeller - og invest in a
> folding propeller ... time will show, and thank you so much
> for your keen interest in learning me some physics ...
>
>
Roger's advice is correct, but it does sidestep your original
question. Locking a prop in the "shadow of the hull" is generally a
winner. Even with a 3-blade prop you should have one blade lined up
with the hull. With a two blade prop the gain can easily be enough to
win a race, or come in before dark on long passage.
I've heard of two studies that addressed this. One, a publicized MIT
study showed that free-wheeling had less drag. However, this did not
directly model yacht props so it really doesn't apply. (In fact,
their prop wasn't actually free, it was powered at a speed that
minimized turbulence.) Other study, which I've heard of third hand,
"proved" that locked was less drag for yacht props. Dave Gerr, in his
"propeller Handbook," says rotating is less drag (unless you can lock
it behind the keel) but he doesn't give a reason or cite a reference.
Consider two cases: first take a hypothetical "flat prop" with zero
pitch. Obvious, it won't spin and it will have a lot of turbulence,
indicating a lot of drag. Now give it a bit a pitch. It will start
to spin, but the small pitch will mean that it has to spin very fast
fast to match the boat speed. Since it can't, there will still be a
huge amount of turbulence. Will it be less than the flat blade? Hard
to say, because there are a variety of factors - the shape of the
turbulence is different and the rotation means that a larger volume of
water is disturbed. This approximates the yacht prop where the free
rotation is very unlikely to match the boat speed and the turbulence
will be high and quite complex.
Now consider a feathering prop, where the blades are lined up with the
flow and cause no turbulence. Give them a some pitch - as long as the
flow is smooth there will be little drag. Increase the pitch
(actually a smaller number since feathering the pitch is infinite) to
the fastest spin with low turbulence. The drag will still be low at
this point. Now lock it - the turbulence (and the drag) will jump
up. Here's a case where the free rotating prop will have less drag
than the locked one, but it probably does not resemble a typical yacht
prop. It does come close to large ship props, so its not surprising
that there are some studies and anecdotal evidence from other forms of
props, that say free wheeling is less drag.
My point is (as I mentioned in my first post) that its impossible to
determine intuitively the drag for the case of a yacht prop, where the
turbulence is high whether locked or not, and analogies from other
type of props simply don't apply. The issue of the heat buildup in
the transmission is a "red herring." It is a significant side issue,
since yacht transmissions do have a lot of friction, and thus heat
problems, when free wheeling, but the dominant factor is the
turbulence. This approach tries to measure the energy extracted from
the water by the work done by the shaft, but this is only valid if the
turbulence is low and the prop is working in a relatively efficient
mode. When the turbulence is high, most of the energy goes into
heating the water, and this is rather difficult to measure.
Keith Hughes
>>a fixed one. Props on normal transmissions are not freewheeling
>>however. There is enough drag in the shaftline, bearings, and
>>transmission to upset things. Outboards, with their clutches right in
>>the lower unit are closer to freewheeling.
>
>
>
> I will second Roger on this. The dinghy experiment would be valid on a
> very naiive level of thinking and the experiment with the fishing lure
> is invalid because the boat propellors is NOT freewheeling. There is
> quite a bit of friction involved. In some cases the gearbox get too hot
> to hold a hand on it because the transmission fluid is not pumped around
> and while lubricating it is also transmitting the heat away.
>
> - Lauri Tarkkonen
>
Actually, the dingy experiment is on the right track. It seems to me
that the energy dissipated in the prop/shaft/transmission/oil train is
irrelevant. That is energy that *would* had been dissipated in
redirecting water flow. Look at it simply; you have *one* energy source,
and that's the water 'stream' past the prop (not quite accurate of
course, since the overall drag will affect the 'stream' velocity, but
it's an easier way to envision it). The amount of energy extracted from
that source will be the result of 1) frictional losses, 2) inertial
forces, i.e. energy required to redirect the mass of water striking the
prop surfaces (not straightforward to calculate), and 3) impact forces
(momemtum, if we're thinking in terms of a 'stream'). Now, the only
issue is whether the drag at the prop is more or less when fixed. So
*if* the freewheeling prop has less drag than the fixed prop, then so
will the turning prop attached to the prop/shaft/transmission/oil train.
To the extent that energy is dissipated by the drive train, the prop
will be rotating slower, and the system will behave somewhere in between
fixed and freewheeling.
Now, whether the fixed or turning has less drag, under any particular
set of parameters, I don't know. I suspect it's a wash. Clearly a
totally freewheeling prop would have less, as the impact forces are
reduced, as is the angular deflection of the water stream, leaving
mostly frictional losses. I suspect though, as appears the case on my
Catalina 30, that the prop when turning significantly slower than it's
pitch rate (i.e. dragging the shaft and trans with it) creates nearly
the same drag as the fixed prop.
Keith Hughes
Peter Aitken
news:nw1pe.3308$fp6....@twister.nyroc.rr.com...
> I'll try.
>
> The answer will be found in conservation of energy. The energy books have
> to balance in every system. The amount if drag of the water flowing over
> the prop will exactly equal the energy being produced. With the prop
> stopped, it is simply pull aft on the shaft. The prop isn't very
> efficient that way. Try propelling a boat by putting the prop out ahead
> on a long line and then pulling it in quickly. When the prop is
> freewheeling, it is producing energy that is absorbed by heating up the
> bearings and lube oil in the transmission, (or in Larry's case by charging
> batteries).
>
> It's slightly counterintuitive but, whatever energy is produced has to
> have an exact mirror image in drag on the boat. Since the prop is making
> energy more efficiently while turning, there has to be more drag.
>
> --
>
> Roger Long
>
>
Your explanation is pure nonsense. When the prop is fixed the energy created
is dissipated as heat in the water. When it is free to rotate some is
dissipated in the grease and bearings and some as heat in the water.
--
Peter Aitken
me
>
>I'm not sure I get your conclusion ... ;-) Sorry, but do
>you recommend me to let the propeller turn og should I stop
>it from turning, if I'm only concerned with the speed of the
>sailboat - when only using the sails?
>
I guess I'm too long winded.
It is my guess that a freely turning propellor would produce
less drag than a locked propellor. This is assuming that
there is no load on thepropellor other than the friction of
the bearings that support the propellor shaft.
*JimH*
Now tackle this one...how many sailors does it take to screw in a light
bulb?
Lauri Tarkkonen
>Why 65 replies on a simple question. Amazing. ;-)
Because it is not as simple as you and the majority of posters seem to
think. By the way this discussion is repeated about once in three years
and same unfounded arguments and simplifications are represented over
and over again.
>Now tackle this one...how many sailors does it take to screw in a light
>bulb?
There are not enough wise men to enlighten the ones that prefers to stay
ignorant. Looks like no lightbulbs are needed.
- Lauri Tarkkonen
Lauri Tarkkonen
>Lauri Tarkkonen wrote:
>> In <hd2dnVYrpYl...@comcast.com> "*JimH*" <m...@no-one.com> writes:
>>
>>
>>>Why 65 replies on a simple question. Amazing. ;-)
>>
>>
>> Because it is not as simple as you and the majority of posters seem to
>> think. By the way this discussion is repeated about once in three years
>> and same unfounded arguments and simplifications are represented over
>> and over again.
>>
>Jim Hertvik (JimH) doesn't actually have a boat, and, when he claimed he
>last had one, he said he spent most of his time aboard it while it was
>tied to a marina's dock in southern Lake Erie. Thus, a discussion about
>feathering or engaging a sailboat prop would not be something to which
>he could contribute, at least not from a point of knowledge.
Perhaps his point of discussion is arrogance.
- Lauri Tarkkonen
*JimH*
"Lauri Tarkkonen" <tark...@cc.helsinki.fi> wrote in message
news:d856hc$aqp$1...@oravannahka.helsinki.fi...
> In <3gmbn1F...@individual.net> HarryKrause <harry....@gmail.com>
> writes:
>
>>Lauri Tarkkonen wrote:
>>> In <hd2dnVYrpYl...@comcast.com> "*JimH*" <m...@no-one.com> writes:
>>>
>>>
>>>>Why 65 replies on a simple question. Amazing. ;-)
>>>
>>>
>>> Because it is not as simple as you and the majority of posters seem to
>>> think. By the way this discussion is repeated about once in three years
>>> and same unfounded arguments and simplifications are represented over
>>> and over again.
>>>
>>last had one, he said he spent most of his time aboard it while it was
>>tied to a marina's dock in southern Lake Erie. Thus, a discussion about
>>feathering or engaging a sailboat prop would not be something to which
>>he could contribute, at least not from a point of knowledge.
>
> Perhaps his point of discussion is arrogance.
>
> - Lauri Tarkkonen
>
Not at all Lauri.
First of all, if my original post offended you or anyone in this NG I
apologize. It was certainly a brain fart on my part as I certainly respect
the nautical and operational knowledge I have personally observed from most
of the sailors I have had the opportunity of meeting or observed in action.
As to my boating experience that Harry Krause questions: We have owned
boats for over 25 years. We had to sell our 32 footer at the end of the
2003 season due to my wife's medical condition, a condition that caused her
to take a medical retirement from her teaching career. Krause knows that
yet for some reason shows little consideration for that fact. That alone
should speak loudly of his moral character.
Yes, we are currently boatless but hope to purchase a smaller more
manageable (for me) boat in 4 years when we retire riverside (Huron River
off Lake Erie).
We live in Northern Ohio and would normally log 55-80 hours on the boat
annually....not many hours according to hard core fishermen or year round
boaters, but we did indeed spend every weekend on the boat as well as 2 full
vacation weeks during our summer vacation. We would use our boats mainly
for cruising to swimming spots or (as in the case of our 27 and 32 footers)
cruise to various ports in Canada and Michigan. Overnight trips on the hook
in protected areas were magnificent. And to the surprise of Harry Krause,
we would actually spend weekends on our boat (along with our 2 children)
even when the weather/Lake conditions were bad, spending time at the marina.
Our fishing would be limited to early season (me and my buddies) walleye
fishing and late season perch fishing.
Our children fondly recall our time on the boats (they are now 17 and 20
years old respectively) and I would not give up our experience on our boats
as a family (even though they do not meet Harry Krause's requirements).
Our children were with us on the boat since the age of 6 months.
Do not pay attention to Harry Krause. He is a bitter old man who for some
reason has to spend his time posting personal attacks on folks who disagree
with him politically. I actually feel sorry for the guy.
*JimH*
"*JimH*" <m...@no-one.com> wrote in message
news:ZYGdnUzdHdl...@comcast.com...
>
> "Lauri Tarkkonen" <tark...@cc.helsinki.fi> wrote in message
> news:d856hc$aqp$1...@oravannahka.helsinki.fi...
>> In <3gmbn1F...@individual.net> HarryKrause <harry....@gmail.com>
>> writes:
>>
>>>Lauri Tarkkonen wrote:
>>>> In <hd2dnVYrpYl...@comcast.com> "*JimH*" <m...@no-one.com>
>>>> writes:
>>>>
>>>>
>>>>>Why 65 replies on a simple question. Amazing. ;-)
>>>>
>>>>
>>>> Because it is not as simple as you and the majority of posters seem to
>>>> think. By the way this discussion is repeated about once in three years
>>>> and same unfounded arguments and simplifications are represented over
>>>> and over again.
>>>>
BTW: For your information about this person posting as Harry Krause:
1. He has stalked me and my family on the internet because my political
views did not agree with is.
2. He subsequently posted my real name (I had posted under an alias for
obvious reasons), address and wife's name to rec.boats.
3. He obtained a picture or our house via the internet.
4. He made personal threats to me and my wife.
As a result we have tracked Harry Krause's posts and continued threats to us
and forwarded them to our attorney.
He has admitted to all these things in rec.boats.
There are many active member of rec.boats.cruising who can attest to this
type of behavior by Harry Krause as similar things happened to them.
He is a sick man and I suggest you treat him with caution.
John H
>Lauri Tarkkonen wrote:
>> In <hd2dnVYrpYl...@comcast.com> "*JimH*" <m...@no-one.com> writes:
>>
>>
>>>Why 65 replies on a simple question. Amazing. ;-)
>>
>>
>> Because it is not as simple as you and the majority of posters seem to
>> think. By the way this discussion is repeated about once in three years
>> and same unfounded arguments and simplifications are represented over
>> and over again.
>>
>> - Lauri Tarkkonen
>
>
>Jim Hertvik (JimH) doesn't actually have a boat, and, when he claimed he
>last had one, he said he spent most of his time aboard it while it was
>tied to a marina's dock in southern Lake Erie. Thus, a discussion about
>feathering or engaging a sailboat prop would not be something to which
>he could contribute, at least not from a point of knowledge.
Are you not able to make enough personal attacks in rec.boats?
--
John H
"All decisions are the result of binary thinking."
*JimH*
"John H" <PocoL...@hotmail.com> wrote in message
news:jmcca1htnkp5oomt5...@4ax.com...
The simple answer is..........................................NO.
Quite sad.
I dare anyone to show a NG where Harry Karuse is both respected and shown as
credible and believable.
Not much to ask.
I dare you.
Larry W4CSC
news:d83gh9$571$1...@oravannahka.helsinki.fi:
> Do
> you know that some boat trhansmissions get very hot if you let the
> propeller to free wheel? Some energy is needed to heat it up.
>
Yes. The newer transmission that came with the replacement Perkins 4-108
was said to be one that would heat up. So, we kept the much larger
"hydraulic" transmission that was on the old engine. It doesn't even get
warm and I can spin the unloaded shaft by hand.
Larry W4CSC
> Now tackle this one...how many sailors does it take to screw in a light
> bulb?
>
>
Eight. One to hold the bulb, seven to turn the boat.
Larry W4CSC
> 2. He subsequently posted my real name (I had posted under an alias for
> obvious reasons), address and wife's name to rec.boats.
>
>
I posted a map straight to his house in MD when he pulled that shit with
me.
*JimH*
"Larry W4CSC" <no...@home.com> wrote in message
news:Xns966ECF0...@63.223.7.253...
Bzzzzt....wrong.
One to turn the bulb and .................(can we make fun of ourselves at
this point?)............
sp...@strato.net
*JimH* wrote:
> Why 65 replies on a simple question. Amazing. ;-)
Hear is number 66
Every body is getting drag mixed up with lift. The reason you should
not allow the propeller to spin freely is not because of drag it is
because of all the lift it generates opposite the direction of travel
of the boat.
Drag is always in the direction of the relative fluid flow that causes
it. The relative fluid flow that causes the drag on the spinning prop
is not at all the same (direction and speed) as the relative fluid flow
that is influencing the rest of the boat hull but every one is
determining drag as though it was the same. The more the prop rotates
the more the relative fluid flow influencing it becomes circular, and
the more the direction of the lift it generates becomes parallel to the
prop shaft. As the prop starts to spin the angle of attack becomes less
generating more lift and less drag because the prop is operating well
below the stall angle.
The best aeronautic analogy of this would be an autogyro. The rotor is
tilted slightly rearward to facilitate its rotation from on coming
relative airflow. The rotating blade not only generates enough lift to
support the weight of the aircraft but unfortunately some of the lift
has a rearward component. Igor Benson wrote one of the disadvantages of
the auto gyro is all the drag it generates that opposes thrust. He got
drag mixed up with lift also. Lift and drag are very very similar.
Aircraft have flown non stop around the world in spite of drag while
others have done it as a result of drag.
Also be aware that the water is not flowing around the prop the prop is
moving while in a fluid (rotation) as it moves linear thru a fluid. If
the fluid were moving around the prop drag would cause its motion not
oppose it. Thanks to aeronautic texts we all know that to move thru a
fluid you have to overcome drag. This represents a fraction of what
drag is all about for it is also true that to remain still while in
moving air you must also over come drag. I have about reached the
limits of my third grade education that took me four years to get.
Larry W4CSC
> (can we make fun of ourselves at
> this point?)............
>
Yes, as long as someone gets aggravated and it starts a flamewar that goes
on for months like kindergarteners in a sandbox fighting over a truck.
James Johnson
>Gogarty <Gog...@Clongowes.edu> wrote:
>
>>In article <42a37185$0$152$edfa...@dread11.news.tele.dk>,
>>fletopkanelbolle2rp.danmark says...
>>>
>>>My brother in law, and I have had a discussion of whether it
>>>is best - from a pure speed point of view (no consideration
>>>to the mechanics/oil/maintenance/gearbox etc. here ...)
>>>whether you should let your "fixed three bladed propeller"
>>>run/turn or keep it fixed (like put into gear) when sailing
>>>just for the wind with your sails in a 34 feet cruiser
>>>weighing roughly 5 T ... We have - unsuccesfully - tried to
>>>find out using the log ... the results were not conclusive -
>>>or one of us would not admit, that the other was right ... I
>>>think we need a testimony from someone, that has a
>>>'scientific based valid answer' ... or just knows for sure
>>>...
>>>The assumption is: There is no way to move, turn, 'collapse'
>>>anything on the propeller - the 'blades' are fixed ... (hard
>>>to explain in a language, that is not your own ... hope you
>>>get my point).
>>>
>>1. Many transmissions require a running engine to keep them lubricated.
>>Such transmissions should be locvked in reverse to prevent freewheeling.
>>
>
>This is not what he asked. He said specifically
>>from a pure speed point of view (no consideration
>>to the mechanics/oil/maintenance/gearbox etc. here ...)
>
>Plus not all transmissions can be locked in reverse to prevent
>freewheeling. Ours cannot. Doesn't matter what gear you stop in, the
>shaft will freewheel unless you stop it by putting a vice grip on the
>shaft or something (IMHO a bad idea, but I know someone who does
>this).
>
Not remotely applicable, autorotation (that rotating of the helicopter rotors
that you refer to in a uncontrolled descent) is caused because the pitch of the
rotor blades are deliberately and continuously adjusted to generate maximum
possible lift for that vertical airspeed. Additional differences - air is
compressible, water for all intents and purposes is not - this makes a
tremendous difference in the effect.
This is a comparing apples to oranges situation. The jury is out as to whether
it causes more drag propeller locked or rotating. Some highly respected naval
architects say one thing, others say the opposite. Until a scientific peer
reviewed study is released I will go with "I don't know and nobody else does
either" opinion.
JJ
>>2. A freewheeling propeller creates more drag than a locked on.. Just
>>consider a helicopter. Engine out and rotors freewheeling, the aircraft
>>will go down safely. Rotors locked and it drops like a stone.
>
>This is probably a good analogy, although not all aircraft principles
>will translate to water and v.v. (as the Wright brothers found)
>
>My answer would be the same as yours and also that if you have a two
>blade prop, it should be locked in line with the keel if it is
>possible to determine where that is etc.
>
>In our case, the freewheeling prop made so much noise that one of the
>first things we did was get a feathering prop. We do have evidence
>that feathering the prop increases our sailing speed, in addition to
>being quieter.
>
>grandma Rosalie
James Johnson
remove the "dot" from after sail in email address to reply
fred
Way back whenI was in school I remember something along the lines that
"a stopped propeller generates drag equal to the area of the stopped
blades,
but a spinning propeller generates drag equal to the AREA OF THE DIAMETER of
the propeller arc."
This would be a vote for stopped propeller equals less drag.
E. Currier
Roger Long
drag is really just a thought convenience and not a physical
destination. The lift in an airplane wing has a slightly backwards
component which is why adding weight slows the airplane.
The term "going ballistic" refers to pushing over the nose of a fast
plane so that the plane goes into a zero G state. Since the plane no
longer weighs anything, the wings no longer have to create lift
(therefore it is "ballistic" like a bullet), drag is dramatically
reduced, and speed increases. This requires a downward curved flight
path so can't be maintained for long.
--
Roger Long
Lee Huddleston
sometimes able to self-rescue because the pitch of the blades can be
changed. As the helicopter falls through the air, the blades are
pitched to spin the blades rapidly in one direction. Just before the
helicopter crashes, the pilot adjusts the "collective" to switch the
blade pitch. The momentum of the spinning blades keeps them going for
a short time. With the blades now pitched to create lift, the
momentum-created lift can be enough to cushion the landing. If the
pilot adjusts the collective too soon, the helicopter will come almost
to a hover and then begin to fall rapidly to the ground.
Larry's physics strike me as correct -- up to a point. If just enough
power is applied to the shaft to turn the prop to match the water flow
past it, the drag ought to be zero. If more power is applied, the
"drag" would be negative. If less power is applied, the drag will
increase right up until the prop is almost stationary. When the prop
become completely stationary, however, a new condition is created.
The blades stall. Going back to an aircraft analogy, as the wings of
a fixed-wing aircraft are flown to take a bigger and bigger bite of
the air, the drag will significantly increase. But, once you go one
step too far and the wing stalls, the plane will drop like a stone.
While Larry's idea of the increasing drag as the prop slows its
rotation makes perfectly good sense, it misses the transition to less
drag when the blades stop moving and stall.
OK, so maybe it is BS. :-D But it is pretty good sounding BS.
And its my story and I'm sticking to it.
BTW, not only is Denmark a great country, Danes are very fine people.
Lee Huddleston
s/v Truelove