Displacement Hull Speed vs Power
Margaret and Loren Block
propel displacement hulls to rated hull speed AND beyond. I've seen many
debates about the "best size" outboard motor for a given sailboat in a
given area. Some sailors use o/b's that are double the recommended size as
insurance to better cope with opposing currents and/or headwinds. It would
be nice to see some real quantitative data about this.
Thanks,
LB
Margaret & Loren Block Georgetown, TX
C22 #14903 "Perfect Harmony"
Creators of BlockBase Volunteer Center Database
http://home.att.net/~lorendi/index.htm
David Smalley
>
> In article <38987c68...@netnews.worldnet.att.net>,
> lor...@worldnet.att.net (Margaret and Loren Block) wrote:
>
> > I'd like to see computations (graphs) that show what power is required to
> > propel displacement hulls to rated hull speed AND beyond. I've seen many
> > debates about the "best size" outboard motor for a given sailboat in a
> > given area. Some sailors use o/b's that are double the recommended size as
> > insurance to better cope with opposing currents and/or headwinds. It would
> > be nice to see some real quantitative data about this.
>
> any "typical" sail or trawler pleasure boat to hull speed and somewhat
> beyond. Anything more than that, is just going to create big wakes, more
> engine noise and fuel burn considerably greater than the incremental
> speeds attained.
>
> There are some exceptions to that generalization, for unusual boats or
> conditions.
Thanks for that excellent rule of thumb.
Now lets see if I understand how to use it:
118 Gross Tons displacement
118*2,000=236,000
236/5=47.2
Does this mean that a 118 ton vessel can be pushed up to hull speed w/
50hp?
My gut feeling is that I have done something very wrong or the 5hp
should be 10.
--
DAVe
Armond Perretta
> "David Smalley" wrote ... J'h wrote ... (Margaret
> and Loren Block) wrote ...
> > >
> > > I'd like to see computations (graphs) that show what
> > > power is required
> >
> > 5 HP per 1000 lbs displacement, should be more than
> > adequate to push
> > most any "typical" sail or trawler pleasure boat to hull
>
> Thanks for that excellent rule of thumb.
> Now lets see if I understand how to use it:
>
> 118 Gross Tons displacement
>
> 118*2,000=236,000
>
> 236/5=47.2
>
> Does this mean that a 118 ton vessel can be pushed up
> to hull speed w/ 50hp?
>
> My gut feeling is that I have done something very wrong
> or the 5hp should be 10.
_My_ gut feeling is that you divided when you should have multiplied:
(118 Tons) x (2000 Lbs/Ton) x (5 HP/ 1000 Lbs) = 5 x 236 = 1180 HP
I have no idea how well this approximation works for a 118 ton vessel (1180
HP seems high). However for my 14,000 pound sailboat it appears to grossly
overstate the required horsepower:
14 x 5 = 70 HP
We get hull speed with a Volvo 18 HP engine. Comments?
Good luck and good sailing.
s/v Kerry Deare of Barnegat (remove 'BOAT')
http://kerrydeare.tripod.com
David Smalley
>
> > "David Smalley" wrote ... J'h wrote ... (Margaret
> > and Loren Block) wrote ...
> > > >
> > > > I'd like to see computations (graphs) that show what
> > > > power is required
> > > > to propel displacement hulls to rated hull speed ...
> > >
> > > 5 HP per 1000 lbs displacement, should be more than
> > > adequate to push
> > > most any "typical" sail or trawler pleasure boat to hull
> > > speed and somewhat beyond ...
> >
> > Thanks for that excellent rule of thumb.
> > Now lets see if I understand how to use it:
> >
> > 118 Gross Tons displacement
> >
> > 118*2,000=236,000
> >
> > 236/5=47.2
> >
> > Does this mean that a 118 ton vessel can be pushed up
> > to hull speed w/ 50hp?
> >
> > My gut feeling is that I have done something very wrong
> > or the 5hp should be 10.
>
> _My_ gut feeling is that you divided when you should have multiplied:
>
> (118 Tons) x (2000 Lbs/Ton) x (5 HP/ 1000 Lbs) = 5 x 236 = 1180 HP
LOL! Well it doesn't surprise me.
> I have no idea how well this approximation works for a 118 ton vessel (1180
> HP seems high).
Outrageoulsy high IMHO.
However for my 14,000 pound sailboat it appears to grossly
> overstate the required horsepower:
>
> 14 x 5 = 70 HP
>
> We get hull speed with a Volvo 18 HP engine. Comments?
Ok so you get 18hp/14,000lbs which is about 1.3hp per 1000lbs.
Lets look at another data point, I know a 70ton vessel w/ a 150hp motor:
70*2000=140000
150/140=1.07 Hmmmmmm
It's begining to sound to me like 1-2HP per 1000 lbs is more reasonable.
Anyone else got more data points?
--
DAVe
Mark Sienkiewicz
>> any "typical" sail or trawler pleasure boat to hull speed and somewhat
>Thanks for that excellent rule of thumb.
>
>Now lets see if I understand how to use it:
>
>118 Gross Tons displacement
>
>118*2,000=236,000
>
>236/5=47.2
>
>Does this mean that a 118 ton vessel can be pushed up to hull speed w/
>50hp?
You don't understand how to use it. :) Two mistakes:
First, the last computation should be
236,000 pounds * ( 5 horsepower / 1000 pounds ) = 1180 horsepower
Second, a vessel of 118 tons is hardly a typical sail or trawler
pleasure boat. You're using the approximation for something
it was never intended for.
Armond Perretta
> Ok so you get 18hp/14,000lbs which is about 1.3hp per 1000lbs.
>
> Lets look at another data point, I know a 70ton vessel w/ a 150hp motor:
>
> 70*2000=140000
>
> 150/140=1.07 Hmmmmmm
>
> It's begining to sound to me like 1-2HP per 1000 lbs is more reasonable.
>
> Anyone else got more data points?
I seem to recall about 2 HP per from one of Nigel Calder's books,
but damn if I'm gonna walk over there to the bookshelf and check <g>.
Jon V.
> <5 HP per 1000 Lbs displacement>
>
> There are some exceptions to that generalization, for unusual boats or
> conditions.
For some reason, I can't think of anything BUT exceptions off the top of
my head... I can't think of a boat in the 3000 lb range that won't make it
to hull speed on 5HP. My own boat displaces 5500 Lbs, and 5-8HP will drive
it at hull speed.
I would call 2HP/1000Lbs reasonably pessimistic, and 5HP/1000Lbs
dangerously pessimistic. In general, I would say that any rule that
didn't include the LWL was probably flawed.
On the other hand, it is probably a great rule for large vessels.
> In article <38987c68...@netnews.worldnet.att.net>,
> lor...@worldnet.att.net (Margaret and Loren Block) wrote:
>
> > I'd like to see computations (graphs) that show what power is required to
> > propel displacement hulls to rated hull speed AND beyond. I've seen many
> > debates about the "best size" outboard motor for a given sailboat in a
> > given area. Some sailors use o/b's that are double the recommended size as
> > insurance to better cope with opposing currents and/or headwinds. It would
> > be nice to see some real quantitative data about this.
Many books have those charts. The problem is that the exact curve varies
based on the hull design. The short and easy is that the power
requirements go up at the square of the speed until you reach hull speed
at which point there will be a "knee" and the power requirements will go
up at the square + something of the speed, where "something" is the energy
required to life your hull a certain distance, and depends on the design
of your hull.
If you have a long narrow hull you may not even notice the knee. If you
have a typical (~3:1 length:beam ratio) hull design the knee will be
fairly signifigant.
-Jon
-----------------------------------------------------------------------------
Everything you've learned in school as "obvious" becomes less and less
obvious as you begin to study the universe. For example, there are no
solids in the universe. There's not even a suggestion of a solid.
There are no absolute continuums. There are no surfaces. There are no
straight lines.
-- R. Buckminster Fuller
David Smalley
>
> >> 5 HP per 1000 lbs displacement, should be more than adequate to push most
> >> any "typical" sail or trawler pleasure boat to hull speed and somewhat
>
> >Thanks for that excellent rule of thumb.
> >
> >Now lets see if I understand how to use it:
> >
> >118 Gross Tons displacement
> >
> >118*2,000=236,000
> >
> >236/5=47.2
> >
> >Does this mean that a 118 ton vessel can be pushed up to hull speed w/
> >50hp?
>
> You don't understand how to use it. :) Two mistakes:
>
> First, the last computation should be
>
> 236,000 pounds * ( 5 horsepower / 1000 pounds ) = 1180 horsepower
Doh! Stupid is as stupid does. 8^)
> Second, a vessel of 118 tons is hardly a typical sail or trawler
> pleasure boat. You're using the approximation for something
> it was never intended for.
Well I am getting the feeling that the 5hp number was pretty far off. I
am currently re-reading Voyaging Under Power and there is an example in
there of a 62' vessel that required 75 HP to rach hull speed, and
another that required 58. There are other examples and calcualtions that
I am in process of trying to digest, but so far I feel that 5hp/1000lbs
is way high.
According to them 118 tons is slightly off the graph to figure F(1) but
I will extrapolate it to roughly 225.
So F(1)=225
Lets guess the LWL @ 80' on an 86' vessel, that makes hull speed about 9
knots.
Now we are loking for a S/L (speed length) ratio of 1 (the definition of
hull speed), and that makes F(2) about .525
And the formula for horsepower is simple enough F(1) x F(2).
Which in this case is 225 x 0.525 = 118 HP.
Ha! Exactly 1/10 of the original prediction.
Now we try the rule of thumb
118/236 = 0.5hp
And now it predicts 1/2hp per 1000 lbs or 1hp/ton.
Thanks for the kick in the ass to really investigate this ratio.
--
DAVe
Paul Kruse
<cdsa...@BOATmindspring.com> wrote:
>(118 Tons) x (2000 Lbs/Ton) x (5 HP/ 1000 Lbs) = 5 x 236 = 1180 HP
>
>I have no idea how well this approximation works for a 118 ton vessel (1180
>HP seems high). However for my 14,000 pound sailboat it appears to grossly
>overstate the required horsepower:
>
>14 x 5 = 70 HP
>
>We get hull speed with a Volvo 18 HP engine. Comments?
The five hp per 1000 pounds does overstate the minimum required for
hull speed in small displacement boats, and even more so in larger
ones. I've seen the four hp per 1000 hp number used more frequently,
and even that leaves a reserve for headwinds and accessory loads.
As another data point, I know a 3000 pound boat that gets hull speed
on four hp, and a 27,000 pound boat that does it at 30 hp. Many
things can cause this to vary a great deal from one boat to another.
Boatless, but building M/V Doulos I and Doulos II
http://www.trawlerworld.com/abuilding/doulos001.html
Paul Kruse
plk...@iu.net
Port Canaveral, FL, USA
David Smalley
>
> > "David Smalley" wrote ...
> > Ok so you get 18hp/14,000lbs which is about 1.3hp per 1000lbs.
> >
> > Lets look at another data point, I know a 70ton vessel w/ a 150hp motor:
> >
> > 70*2000=140000
> >
> > 150/140=1.07 Hmmmmmm
> >
> > It's begining to sound to me like 1-2HP per 1000 lbs is more reasonable.
> >
> > Anyone else got more data points?
>
> I seem to recall about 2 HP per from one of Nigel Calder's books,
> but damn if I'm gonna walk over there to the bookshelf and check <g>.
Well you intrigued me enough to look it up in Leishman's rewrite of
Beebe's Voyaging Under Power and it now seems to me that it is closer to
1/2hp per 1000lbs or 1hp per ton.
--
DAVe
Paul Kruse
wrote:
>> 5 HP per 1000 lbs displacement, should be more than adequate to push most
>> any "typical" sail or trawler pleasure boat to hull speed and somewhat
>> beyond. Anything more than that, is just going to create big wakes, more
>> engine noise and fuel burn considerably greater than the incremental
>> speeds attained.
>Thanks for that excellent rule of thumb.
>
>Now lets see if I understand how to use it:
>
>118 Gross Tons displacement
>
>118*2,000=236,000
Actually, that term would most often refer to long tons or metric
tons; but we can use your conversion for the purposes of this example
if you like.
>236/5=47.2
>My gut feeling is that I have done something very wrong or the 5hp
>should be 10.
I think you wanted to multiply instead of divide. That would give you
something over a thousand hp. That would be a bit of an over kill for
a displacement boat of that size. The general rule of thumb really
only applies to smaller recreational boats. As the boats get larger,
you will need less power per ton. You would probably be using a
factor of three or four instead of five by the time you got this big.
John Abercrombie
>For some reason, I can't think of anything BUT exceptions off the top of
>my head...
I agree. And we haven't heard any comments about pushing the boat
against wind and seas, or the influence of prop selection, etc.
Getting "hull speed" in a flat calm is easy. If you have just enough
power to do that you will have a problem when conditions get rougher.
Of course, in a sailboat you can always assume (correctly or not) that
you will be able to sail when it gets rough.
John
David Smalley
>
> On Wed, 2 Feb 2000, J'h wrote:
>
> > <5 HP per 1000 Lbs displacement>
> >
> > There are some exceptions to that generalization, for unusual boats or
> > conditions.
>
> to hull speed on 5HP. My own boat displaces 5500 Lbs, and 5-8HP will drive
> it at hull speed.
>
> I would call 2HP/1000Lbs reasonably pessimistic, and 5HP/1000Lbs
> dangerously pessimistic. In general, I would say that any rule that
> didn't include the LWL was probably flawed.
>
> On the other hand, it is probably a great rule for large vessels.
>
> > In article <38987c68...@netnews.worldnet.att.net>,
> > lor...@worldnet.att.net (Margaret and Loren Block) wrote:
> >
> > > I'd like to see computations (graphs) that show what power is required to
> > > propel displacement hulls to rated hull speed AND beyond. I've seen many
> > > debates about the "best size" outboard motor for a given sailboat in a
> > > given area. Some sailors use o/b's that are double the recommended size as
> > > insurance to better cope with opposing currents and/or headwinds. It would
> > > be nice to see some real quantitative data about this.
>
> Many books have those charts. The problem is that the exact curve varies
> based on the hull design.
This hull shape is whar Beebe refers to as the Prismatic coeffieient or
how pointy you have made the underwater part.
--
DAVe
David Smalley
>
> On Wed, 2 Feb 2000 16:09:09 -0500, "Armond Perretta"
> <cdsa...@BOATmindspring.com> wrote:
>
> >(118 Tons) x (2000 Lbs/Ton) x (5 HP/ 1000 Lbs) = 5 x 236 = 1180 HP
> >
> >I have no idea how well this approximation works for a 118 ton vessel (1180
> >HP seems high). However for my 14,000 pound sailboat it appears to grossly
> >overstate the required horsepower:
> >
> >14 x 5 = 70 HP
> >
> >We get hull speed with a Volvo 18 HP engine. Comments?
>
> The five hp per 1000 pounds does overstate the minimum required for
> hull speed in small displacement boats, and even more so in larger
> ones. I've seen the four hp per 1000 hp number used more frequently,
> and even that leaves a reserve for headwinds and accessory loads.
>
> As another data point, I know a 3000 pound boat that gets hull speed
> on four hp,
4hp/3000lbs= 1.3hp/1000lbs = 0.6hp /ton
and a 27,000 pound boat that does it at 30 hp. Many
> things can cause this to vary a great deal from one boat to another.
30hp/27000lbs = 1.1hp/1000lbs = 0.55hp/ton
Sure sounds to me like 1hp/1000lbs is being suggested in these two
smaller examples.
--
DAVe
David Smalley
>
> "Jon V." <j...@valesh.com> wrote:
>
> >For some reason, I can't think of anything BUT exceptions off the top of
> >my head...
> against wind and seas, or the influence of prop selection, etc.
> Getting "hull speed" in a flat calm is easy. If you have just enough
> power to do that you will have a problem when conditions get rougher.
> Of course, in a sailboat you can always assume (correctly or not) that
> you will be able to sail when it gets rough.
IMHO first you need to know what hp will get you to hull speed, _then_
you add your overspeed and safety factors.
--
DAVe
C. A. La Varre
>Anyone else got more data points?
Voyaging Under Power, Robert Beebe:
http://www.bluewatercharts.com/cgi-bin/SoftCart.exe/nauticalbooks/prodpages/
80190.htm?E+mystore4
has some excellent calculations in Chapter Six. These give two factors and
the resulting shaft horsepower versus speed for a specified Speed to length
ratio. I've done a regression on these for my boat and the results closely
match the data provided by the engine manufacturer (Cummins). We have a 120
HP diesel that drives a 46', 45000 lb boat to hull speed (8.6 knots) and
slightly beyond; we've had 9+ knots in flat water with a clean bottom, I
seem to recall. That comes out to about 2 2/3 HP per 1000 lbs, but as the
curves will show it isn't linear.
Andy La Varre
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David Smalley
>
> David Smalley wrote:
>
> >Anyone else got more data points?
>
> Voyaging Under Power, Robert Beebe:
> http://www.bluewatercharts.com/cgi-bin/SoftCart.exe/nauticalbooks/prodpages/
> 80190.htm?E+mystore4
>
> has some excellent calculations in Chapter Six. These give two factors and
> the resulting shaft horsepower versus speed for a specified Speed to length
> ratio. I've done a regression on these for my boat and the results closely
> match the data provided by the engine manufacturer (Cummins). We have a 120
> HP diesel that drives a 46', 45000 lb boat to hull speed (8.6 knots)
According to that same book your hull speed w/ 46' LWL would actually be
6.8kts.
--
DAVe
Marcus G Bell
> Well you intrigued me enough to look it up in Leishman's rewrite of
> Beebe's Voyaging Under Power and it now seems to me that it is
> closer to 1/2hp per 1000lbs or 1hp per ton.
It seems we are getting a few numbers in this range from a few
sources. I wonder if it's at all possible that the first poster of
the furmula, J'H, meant to write .5 hp per 1000# instead of 5, i.e. a
dropped decimal point.
--
--
Marcus. ( be...@mail.med.upenn.edu )
Marcus G Bell
<<< ... I wonder if it's at all possible that the first poster of the
**furmula**, J'H, meant to write .5 hp per 1000# instead of 5, i.e. a
dropped decimal point. >>
David Smalley (dr...@bellsouth.net) wrote:
> I think you're right.
> (as usual) 8^)
I am uverwhelmed by yuur cunfidence in my upinions. Thanks. That was a
nice egu buust.
Jon V.
> "Jon V." <j...@valesh.com> wrote:
>
>
> >For some reason, I can't think of anything BUT exceptions off the top of
> >my head...
> I agree. And we haven't heard any comments about pushing the boat
> against wind and seas, or the influence of prop selection, etc.
> Getting "hull speed" in a flat calm is easy. If you have just enough
> power to do that you will have a problem when conditions get rougher.
> Of course, in a sailboat you can always assume (correctly or not) that
> you will be able to sail when it gets rough.
Yep.
Along that line, it is important to remember that air is a lot lighter
than water...
Translated, a motor that could push a boat to 5kts in flat calm conditions
could probably push it to 4.8 kts against a 10kt headwind, 2.5kts against
a 20kt headwind. and could probably hold position against a 25kt headwind.
Arbitrary and unchecked numbers, of course, but probably not that far off.
In any case, the HP required to hold position goes up very quickly with
wind speed... just like the HP needed to approach hull speed does... but
the scale is different. You can very quickly reach wind speeds where no
reasonable engine will hold you. For less arbitrary numbers, IIRC a 42Kt
wind is is 2x the energy of a 35Kt wind? Something like that. Or, you need
twice the HP to hold yourself against a 42Kt wind as a 35kt wind.
From that, it seems pretty clear that no matter what size engine you have
aboard a sailboat, there are conditions where if you wish to make way
against the wind, you will need to sail. Sailing is the only source of
energy reasonably available to a boater which will increase automatically
to counter the energy imparted to the boat by the wind.
Which means that people who assume they will motor when conditions get
rougher will either rethink and raise sail, or be blown.
Most smallish power boats would never notice this because they carry orders
of magnitude more engine than is required for hull speed.... hence they
have more margin.
-Jon
-----------------------------------------------------------------------------
"I've seen the future, brother, it is murder"
--Leonard Cohen, "The Future"
David Smalley
>
> David Smalley (dr...@bellsouth.net) wrote:
>
> > Well you intrigued me enough to look it up in Leishman's rewrite of
> > Beebe's Voyaging Under Power and it now seems to me that it is
> > closer to 1/2hp per 1000lbs or 1hp per ton.
>
> It seems we are getting a few numbers in this range from a few
> the furmula, J'H, meant to write .5 hp per 1000# instead of 5, i.e. a
> dropped decimal point.
I think you're right.
(as usual) 8^)
--
DAVe
JAXAshby
>boat is probably just a little under prop'ed as it should be. If he
>wanted to check these numbers, he could do so by checking fuel
>consumption. He should be consuming about five gallons per hour at
>nine knots.
20 hp/gal/hr is right up near the theoretical output of a diesel engine. It
would be considered **very** good in real life. 15 hp/gal/hr seems to be more
in line with what diesel boat owners report.
C. A. La Varre
>According to that same book your hull speed w/ 46' LWL would actually be
>6.8kts.
Actually,
LOA ft LWL ft
45.93 41.2
Hull speed = 1.34 * SQRT(LWL in feet)
C. A. La Varre
>> Well you intrigued me enough to look it up in Leishman's rewrite of
>> Beebe's Voyaging Under Power and it now seems to me that it is
>> closer to 1/2hp per 1000lbs or 1hp per ton.
>
>It seems we are getting a few numbers in this range from a few
>sources. I wonder if it's at all possible that the first poster of
>the furmula, J'H, meant to write .5 hp per 1000# instead of 5, i.e. a
>dropped decimal point.
Here's the answer I'm using: power and fuel consumption for a given speed,
displacement, and LWL.
-------------------------------------------------------------------
HP =~ -2.81 +2.52 * Displacement in Long Tons * SLR
-------------------------------------------------------------------
where SLR is LESS than ONE (displacement mode) and there are a bunch of
other assumptions, see below
You can extend this with a curve of your boat engine's consumption to get
gallons per hour for the selected speed and LWL and SLR, so you can then
build his "How goes it" curve (page 65).
Enjoy,
Andy La Varre
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==============================================================================
This is based on Beebe's charts for F1 and F2, plus a simple linear
regression on F2 versus SLR in the specified (linear) region. All are
plotted out in a spreadsheet for verification.
The regressions below came from plotting his curves (page 62-63) then
fitting a curve using NLREG:
(http://www.sandh.com/sherrod/nlreg.htm).
From P 61: "The horsepower required at any S/L ratio {SLR in the equations
above} is then F1 * F2".
The regressions I got for Beebe's formulas are:
=====
F1=a + b*Tons + c*Tons^2 + d*Tons^3
where Tons are your displacement in LONG TONS (2240 lbs)
and
Parameter Final estimate
a -1.7156215
b 1.5178970
c 0.0101311
d -0.0000411
=====
F2=a + b*exp(c*SLR) + d*exp(e*SLR^2)+f*exp(g*SLR^3)
where SLR = Speed Length Ratio = Speed / SQRT (LWL in Feet)
and
Parameter Final estimate
a -0.1823652
b -1.7745472
c 0.9372606
d 3.4671516
e 0.6790214
f -0.9193136
g 0.5001897
=====
Note that, for a given boat, F1 is just a single number. so, the horsepower
for a given speed is then proportional to F2 above, which is an exponential
polynomial. This is decided NOT linear. It is quasi linear for any speed
whose SLR is LESS than one. It is quasi-linear for any speed whose SLR is
GREATER than 1.25. but in between is a very sharp knee.
Assuming you are operating at SLR less than one (displacement mode), the HP
is linear with respect to speed (F2) but then you have to look at
displacement. Looking at F1 above, the parameter b is much bigger than the
parameter c, so you can say it is *effectively linear with displacement.
And the parameter of proportionality is effectively 1.5
So,
HP *is proportional to 1.5* Displacement in Long Tons * 1.68 * SLR
=====
To get the actual approximation you need the intercept constants. So you do
a linear regression on the F2 model, and get
F2 = a + b * SLR
where
a = -1.091909919
b = 1.682572758
=====
So now we can build the formula:
HP~-1.72 - 1.09 + 1.5 * Tons *1.68 * SLR
or
HP =~ -2.81 +2.52 * Tons * SLR
=====
You can extend this with a curve of your boat engine's consumption to get
gallons per hour for the selected speed and LWL and SLR, so you can then
build his "How goes it" curve (page 65).
http://www.cummins.com/marine/specs.html has some specs on some of their
engines. I called them and they kindly sent a graph for my particular
engine, the Cummins 6B5.9-M.
So I do another regression, getting:
GPH_by_Speed = a * Exp(b * nspeed + c) + d * Sin(e * nspeed + f) - g
where nspeed = speed in knots
a = 0.0008
b = 0.414
c = 5.047
d = 0.0825
e = 233.165
f = 103.277
g = 0.4703
You now have GPH for a given speed for *your boat's displacement and LWL.
The How Goes It is then simple algebra:
distance = velocity * time
and its various alternatives. So given the remaining fuel in you tanks you
can calculate how far you can go at a given speed. if you don't like the
answer, slow down... :-)
David Smalley
>
> >> Well you intrigued me enough to look it up in Leishman's rewrite of
> >> Beebe's Voyaging Under Power and it now seems to me that it is
> >> closer to 1/2hp per 1000lbs or 1hp per ton.
> >
> >It seems we are getting a few numbers in this range from a few
> >sources. I wonder if it's at all possible that the first poster of
> >the furmula, J'H, meant to write .5 hp per 1000# instead of 5, i.e. a
> >dropped decimal point.
>
> Here's the answer I'm using: power and fuel consumption for a given speed,
> displacement, and LWL.
>
> -------------------------------------------------------------------
> HP =~ -2.81 +2.52 * Displacement in Long Tons * SLR
> -------------------------------------------------------------------
>
> where SLR is LESS than ONE (displacement mode) and there are a bunch of
> other assumptions, see below
>
> You can extend this with a curve of your boat engine's consumption to get
> gallons per hour for the selected speed and LWL and SLR, so you can then
> build his "How goes it" curve (page 65).
Well Andy,
You have done a great job of proving to me how stupid I am.
I have little to no clue as to where you were going or how you were
thinking, and I'm sitting here looking at chapter 6.
Oh well.
--
DAVe
Steven Shelikoff
>
> In article <3898a85b...@news.iu.net>, plk...@iu.net wrote:
>
> > The five hp per 1000 pounds does overstate the minimum required for
> > hull speed in small displacement boats, and even more so in larger
> > ones. I've seen the four hp per 1000 hp number used more frequently,
> > and even that leaves a reserve for headwinds and accessory loads.
>
> purchased by displacement and "semi-displacement" boat buyers?
>
> Theoretical calcs clearly show much less power is *needed* to move a
> displacement hull thru the water under ideal calm conditions, than the
> 3-5-16 hp per 1000 lbs.
>
> Here's my answer to the question.
[lot's of answers cut]
I think you answered above, and didn't need to go into all the stuff I
cut out. What are you going to do when conditions are not ideal or
calm? How much extra power does it take to push a displacement hull
into breaking waves? How much extra power does it take to push a boat
with a lot of windage into strong winds? Probably at least double what
it takes to move the same hull at the same speed in a bathtub. Not
having that reserve is dangerous.
Steve
--
/ / /
\ \ \ mailto:shel...@averstar.com
/ / / http://www.averstar.com/customers/maritime.html
Steven Shelikoff
>
> In article <38987c68...@netnews.worldnet.att.net>,
> lor...@worldnet.att.net (Margaret and Loren Block) wrote:
>
> > I'd like to see computations (graphs) that show what power is required to
> > propel displacement hulls to rated hull speed AND beyond. I've seen many
> > debates about the "best size" outboard motor for a given sailboat in a
> > given area. Some sailors use o/b's that are double the recommended size as
> > insurance to better cope with opposing currents and/or headwinds. It would
> > be nice to see some real quantitative data about this.
>
> any "typical" sail or trawler pleasure boat to hull speed and somewhat
> beyond. Anything more than that, is just going to create big wakes, more
> engine noise and fuel burn considerably greater than the incremental
> speeds attained.
>
> conditions.
5hp/1000lb seems a bit high. My 20000lb boat is adequately propelled by
an engine that has a cruise hp of 37 and a max hp of 47. It can sustain
hull speed at cruise rpm if it has a clean bottom.
Steven Shelikoff
>
> David Smalley (dr...@bellsouth.net) wrote:
>
> > Well you intrigued me enough to look it up in Leishman's rewrite of
> > Beebe's Voyaging Under Power and it now seems to me that it is
> > closer to 1/2hp per 1000lbs or 1hp per ton.
>
> It seems we are getting a few numbers in this range from a few
> sources. I wonder if it's at all possible that the first poster of
> the furmula, J'H, meant to write .5 hp per 1000# instead of 5, i.e. a
> dropped decimal point.
That's quite possible, but then I think we are on the low side. It may
move at hull speed, but only in dead calm conditions. Also, the ratings
given to most engines are max values, not cruise values. I've read
somewhere that a cruise rating of 1-2hp/1000lb should be fine.
C. A. La Varre
>I have little to no clue as to where you were going or how you were
>thinking, and I'm sitting here looking at chapter 6.
Let's see. I thought the question was,
How much horsepower is required per 1000 pounds displacement to drive a
boat to hull speed.
Or words to that effect.
Several folks wrote good stuff, and started converging on numbers of HP per
1000 pounds. When you do that it suggests that folks are assuming a linear
relationship. But they were coming up with several different constants of
proportionality. So I used the Beebe graphs (and regressions that provide a
close mathematical model of the graph, since Beebe didn't give us *his
models) to investigate the degree to which there is a linear relationship
between HP and displacement.
The formula
HP =~ -2.81 +2.52 * Displacement in Long Tons * SLR
shows that there is indeed a linear relationship. Importantly, it also
shows that the constant of proportionality is about 3.12, since hull speed
is defined to have been achieved when SLR = 1.34.
The rest was just to justify how you get to this formula.
So what else don't you get?
Marcus G Bell
> > David Smalley (dr...@bellsouth.net) wrote:
> > > ... closer to 1/2hp per 1000lbs or 1hp per ton.
> Marcus G Bell wrote:
> > It seems we are getting a few numbers in this range from a few
> > sources. I wonder if it's at all possible that the first poster of
> > the furmula, J'H, meant to write .5 hp per 1000# instead of 5, i.e. a
> > dropped decimal point.
Steven Shelikoff (shel...@averstar.com) wrote:
> That's quite possible, but then I think we are on the low side. It
> may move at hull speed, but only in dead calm conditions. Also, the
> ratings given to most engines are max values, not cruise values.
> I've read somewhere that a cruise rating of 1-2hp/1000lb should be
> fine.
You may have missed it Steve, but my postulation of a typo was
rendered moot when J'H followed up to say that "my" .5 hp/1000# was
not what he'd intended, that the higher figure was indeed what he
meant. He then provided a comprehensive survey of several types of
displacement and semi-displacement vessels whose hp/displacement
numbers varied greatly, including three sailboats with figures of 2.4,
2.6, and 4.2 hp per 1000# which he says were almost exactly predicted
by the 5 hp figure he gave.
If 1-2 hp/1000# to cruise as you've stated is right, then 2-4 hp/1000#
would be double that to fit the original poster's statement that
sailboaters tend to fit outboards with twice the required power
("required power" I take to mean power to cruise at hull speed). J'H's
sailboat numbers fit nicely in that range.
Steven Shelikoff
Yes, I think we're saying about the same thing. If you look at the hp
figures from a typical marine recreational engine, the hp at cruise rpms
is maybe only 50%-70% or so of the max hp rating, which is what is
what's usually quoted when discussing engine power. Therefore, if you
are looking at 2-4hp/1000lb power rating, you are at the same time
looking at 1-2hp/1000lb cruise power rating.
Marcus G Bell
> [snip] Most slow boaters simply don't want to have to run their
> motor at near max rpm levels to maintain hull speed, as would be the
> case if motors were sized based purely on theoretical calcs.
I'd say that theoretically, you should have about twice the power that
it takes to cruise, to deal with wind and waves and all. So there's a
theoretical cruising HP that you can calculate from some formula, or
gosh almighty take a test drive or two (except then it's not
theoretical anymore ;-), then just double that and see if it works in
the real world under more diverse conditions.
If it doesn't work, then rethink the theory to fit; it's not like
theory and practice have to be at odds. Ultimately the theory should
reflect the real world. A good first approach is to see what others
have done and how it has worked, and relate it to your situation,
which is why sharing experience and observation on these groups is
such a good thing. Observation is what led to the theories published
in our venerated tomes to which we run when these questions arise.
Interestingly, except for the racers, lots of "fast boaters" cruise at
about 50-70% of max power, which is of course well below max RPM. This
improves fuel economy and ride quality, as well as increasing engine
longevity. So sailboaters and trawlers aren't the only ones to hold
back on the throttle, though it's understandable that as a sort of
"lifestyle thing" they may well take a bit more pride in not feeling
the need to "give it all it's got".
Jaime Andersen
need to know to give a more accurate estimate? I assume that a 12,000# boat
built like a 12m would require less effort to glide through the water than a
12,000# boat built like a steel box.
Interesting, I would like to see some feedback on that. Does it mean that a
12hp engine will propel both at the same speed? Please give me an education!
Jaime Andersen
Bellingham, WA
Margaret and Loren Block wrote:
> I'd like to see computations (graphs) that show what power is required to
> propel displacement hulls to rated hull speed AND beyond. I've seen many
> debates about the "best size" outboard motor for a given sailboat in a
> given area. Some sailors use o/b's that are double the recommended size as
> insurance to better cope with opposing currents and/or headwinds. It would
> be nice to see some real quantitative data about this.
>
> Thanks,
>
> LB
>
> Margaret & Loren Block Georgetown, TX
> C22 #14903 "Perfect Harmony"
> Creators of BlockBase Volunteer Center Database
> http://home.att.net/~lorendi/index.htm
Paul Heath
J'h wrote:
> Here's my answer to the question.
>
> Boaters (in the U.S. market) are simply used to grossly "over-powered"
> transportation devices (especially cars) due to our low fuel prices, and
> "bigger and more is better" mentality and culture.
>
> Most of us drive motor vehicles that seldom operate at even half max rpm
> range, and sometimes only 1/3 rpm range when cruising at highway speeds.
> We tend to want the same "feel" in our boat motors.
>
> (snip)
Seems to me that you don't need all that power for cruising -- but for things
like
docking, slowing down at a downwind slip, making headway with a headwind,
fighting a 6 knot tidal flow, etc.
My boat is 26000 pounds - and has a 40 hp motor. The 40 replaced a 25 hp
motor - and boy am I glad.
Does anyone know the equivalent horsepower under sail ? I usually have
800-1000 square feet up - say its blowing 15 knots at a reach - what is the
best guess at horsepower ?
P
Matt Pedersen
Paul Heath wrote in message <3899FEE5...@us.ibm.com>...
>Does anyone know the equivalent horsepower under sail ? I usually have
>800-1000 square feet up - say its blowing 15 knots at a reach - what is the
>best guess at horsepower ?
Dave Gerr gives the following numbers:
Wind Speed (kts) HP/sqft KW/sqm
9-10 0.015 0.118
13-15 0.020 0.161
19-21 0.040 0.312
25-27 0.070 0.559
So that gives you 16 h.p. with 800 square feet flying, and 20 h.p. with
1000. The power numbers are for apparent wind. Note that it doubles
when you go from 14 to 20 knots of wind.
He also gives the following numbers for a speed estimate:
Boatspeed (knots) = 150 / sqrt (disp/hp) note:disp in pounds
= 117 / sqrt (disp/kw) note: disp in kg
You can do this in more detail, but these will get you 90% of the way
there.
Matt
C. Marin Faure
*@*.* (J'h) wrote:
> So why is so much power actually being installed by boat mfrs and
> purchased by displacement and "semi-displacement" boat buyers?
As explained to me by the local Grand Banks dealer, there are more and
more people (in this area, anyway) with a lot of money to spend, and not a
lot of time (or so they think). What they want is to be able to get
quickly to the area they want to cruise. So they order their trawlers
with big engines so they can force it through the water faster. In the
case of a Grand Banks 42, it will burn about 7 gph at 9 knots, but you
can force its semi-displacement hull to "plane" at about 14 knots, but the
fuel consumption is about 25 gph. So you increase your cruise speed by
perhaps 30 percent, but you increase your fuel consumption by over 150
percent (I think I did the math right). But many of today's dot.com
millionerds can afford this, so they don't think twice about the fuel
bill.
C. Marin Faure
author, Flying A Floatplane
Jere Lull
> Does anyone know the equivalent horsepower under sail ? I usually have
> 800-1000 square feet up - say its blowing 15 knots at a reach - what is the
> best guess at horsepower ?
For guesstimate purposes, take the hp it would take to maintain whatever speed
you're making. We cruise at about 6 knots and use about 5 hp for that (0.26
gph). So if we're doing 6 knots under sail, the forward component of the wind's
lift is about 5 hp.
Full out, our Yanmar 2GM20 can maintain at most 7 knots or so. Some days, we've
hit and maintained 7.5-8+ knots for several hours at a time. (GPS and knotlog
agreed.) Thus the wind power at those times was quite a bit higher than 20 hp.
--
Jere Lull
Xan-a-Deux -- '73 Tanzer 28 #4 -- out of Tolchester, MD
Xan's Pics & Specs: http://members.dca.net/jerelull/X-Main.html
Our BVI Vacation trip FAQ (250+ Annotated pics):
http://members.dca.net/jerelull/BVI.html
Jere Lull
The newer diesels and slower props can deliver those figures. That's about what
we're running in the Yanmar: 0.26 gph gives us 6 knots. Otherwise, we're doing 6
knots on less than 4 hp and I find that even harder to believe, not that I'm
complaining.
C. A. La Varre
>lots of "fast boaters" cruise at
>about 50-70% of max power, which is of course well below max RPM
Somebody, somewhere, sorry it's late and I can't remember who, but I think
he was local, said that a diesel engine really likes running about 90% of
capacity.
Having made my earlier contribution for HP vs displacement, maybe someone
could enlighten me and us on the most economical ratio of maximum HP for
various engines... Would that be gallons per hour per knot? or GPH/HP? If
the former, then no speed is best. Hmmm. That doesn't help. If the latter,
then I find a dip (minimum) at about 54% of maximum engine HP. That would
match Marcus' empirical 50-70% function...
Marcus G Bell
> > lots of "fast boaters" cruise at about 50-70% of max power,
> > which is of course well below max RPM
C. A. La Varre (alav...@tiac.net) wrote:
> Somebody, somewhere, sorry it's late and I can't remember who,
> but I think he was local, said that a diesel engine really likes
> running about 90% of capacity.
That was part of a whole different thread ;-) Yeah, a diesel can
run hard, but apparently it doesn't necessarily have to all the
time, and like most things the harder you run it the faster it
burns out.
Now, consider that running at full power consumes fuel at twice
the rate of 1/2 power, but the boat does not go twice as fast. So,
knots/GPH has decreased, hence knots per gallon decreases with
increasing speed; less efficient.
> Having made my earlier contribution for HP vs displacement,
> maybe someone could enlighten me and us on the most economical
> ratio of maximum HP for various engines... Would that be gallons
> per hour per knot? or GPH/HP? If the former, then no speed is
> best.
Yes, it's the former, but "no [particular] speed is best" does not
follow, because the HP per knot requirement of the boat is not a
constant. Faster is less efficient, and at zero speed with the
engine shut off we'd have an efficiency of 0/0 which is
indeterminate.
> Hmmm. That doesn't help. If the latter, then I find a dip
> (minimum) at about 54% of maximum engine HP. That would match
> Marcus' empirical 50-70% function...
When running at wide-open throttle, the piston engine is generally
at its most efficient when running at the RPM of peak torque,
because among other things the "pumping loss" of moving air
through restrictive passages is lessened with decreasing RPM. So,
as you say, there is a GPH/HP relation, and it varies with RPM but
also covariant with load and throttle setting (most diesels don't
have the latter complexity). But the RPM of peak torque is in the
right ballpark for the lowest GPH/HP.
(Note, I'm not saying it's a good idea to run an engine at maximum
throttle setting or fuel delivery rate and limit its RPM by
overpropping to try to attain peak efficiency; that's a recipe for
premature death.)
Now, put together the HP/knot of the boat with the GPH/HP of the
engine, and the best GPH/knot for the system can be calculated. We
didn't mention parasitic loss from the prop, but it decreases as
RPM decreases too.
A planing boat often has its best knots/GPH below hull speed, and
about its worst when it exceeds hull speed but is not yet planing.
Once on plane another range of good efficiency is realized, till
at high speed it drops again.
As a final note, unless you are reading speed from a GPS or other
system referenced to land, the best knots/GPH gives your best
efficiency through the water only. Throw current into it, and you
might find that you're making quite efficient headway through the
current but going nowhere with respect to land. That would be zero
efficiency in terms of getting where you need to go for as little
fuel as possible, and you'll have to speed up to get there.
A.C. Koelewijn
>
>> I'd like to see computations (graphs) that show what power is required to
>> propel displacement hulls to rated hull speed AND beyond. I've seen many
>
> any "typical" sail or trawler pleasure boat to hull speed and somewhat
> beyond. Anything more than that, is just going to create big wakes, more
> engine noise and fuel burn considerably greater than the incremental
> speeds attained.
My 5000 lbs boat has a 10 hp diesel. With that it can easely attain a
speed which is somewhat above hull speed. I have never felt a need for a
bigger engine. Which would make the "rule off the tumb" 2 HP per 1000 lbs.
My previous boat, which I gues had a net displacement of about 3000 lbs,
had a 8 HP 2-stroke outboard, and was grossly overpowered with that
engine, A 6 HP outboard would have been more then enough.
Boat displaceents are the net displacements as given by the mfg. As
sailed, they were of course heavier.
And for an outboard remember, it does not help to have extra power to run
against waves. By the time you have waves in which you would need that
power, your prop will be out in the air a lot of the time, which means you
will have to slow down anyway, or you will soon not have any engine power
left.
Aart
--
Aart Koelewijn Linux 2.2.13
email: aa...@mtack.xs4all.nl
http://www.xs4all.nl/~mtack/
eric
C. A. La Varre <alav...@tiac.net> wrote:
> David Smalley wrote:
>
> >Anyone else got more data points?
>
> Voyaging Under Power, Robert Beebe:
> http://www.bluewatercharts.com/cgi-
bin/SoftCart.exe/nauticalbooks/prodpages/
> 80190.htm?E+mystore4
>
> has some excellent calculations in Chapter Six. These give two
factors and
> the resulting shaft horsepower versus speed for a specified Speed to
length
> ratio. I've done a regression on these for my boat and the results
closely
> match the data provided by the engine manufacturer (Cummins). We have
a 120
> HP diesel that drives a 46', 45000 lb boat to hull speed (8.6 knots)
> slightly beyond; we've had 9+ knots in flat water with a clean
bottom, I
> seem to recall. That comes out to about 2 2/3 HP per 1000 lbs, but as
the
> curves will show it isn't linear.
>
Excellent thread. Just one question and it concerns HP. We are, as
you say, talking about shaft horespower but is an installed engine,
with all of its power take offs, putting out anywhere near its "HP"?
And, of course, the smaller the engine, the greater percentage effect?
--
eric
S/V Nebaras
Sent via Deja.com http://www.deja.com/
Before you buy.
Steven Shelikoff
Also, you have to consider prop efficiency. The little two bladers that
many old sailboats have can be greatly improved upon by a decent folding
three blader.
> And, of course, the smaller the engine, the greater percentage effect?
You'd hope that a smaller engine would be driving a smaller
alternater[s], wouldn't have an AC or fridge compressor to drive, etc.
But yes, for the same PTO demands, a smaller engine takes a bigger HP
hit percentage-wise.
Paul Kruse
>20 hp/gal/hr is right up near the theoretical output of a diesel engine. It
>would be considered **very** good in real life. 15 hp/gal/hr seems to be more
>in line with what diesel boat owners report.
You are right, and you are wrong at the same time. (How's that for an
answer.)
Actually, you can get up to about 24 hp per gallon per hour out of a
modern diesel, and the manufacturers are all promising that will
improve in the near future. If you drive an OTR truck, you will see
number similar to this. The trouble is that the engine installations
commonly used on boats are not nearly so efficient as an OTR truck,
since the boating community has not demanded the same sort of fuel
efficiency as the truckers have demanded.
The 20-24 number is of course the peak optimum for the best operating
conditions. Under other conditions of load and shaft speed, you will
get less. To have a better idea of exactly what you will get in a
particular set of conditions, you would have to have the complete set
of three dimensional specific fuel consumption curves. I've seen the
actual number as low as about six or eight.
But you are correct. I was figuring everything at the engine, and I
assumed operating conditions that were probably better than they
actually are. I will therefore concede that it would probably have
been better to have used 15 for a real boat with a single speed
transmission, and without a CP prop.
Boatless, but building M/V Doulos I and Doulos II
http://www.trawlerworld.com/abuilding/doulos001.html
Paul Kruse
plk...@iu.net
Port Canaveral, FL, USA
Thomas Webb
> Dave Gerr gives the following numbers:
>
> Wind Speed (kts) HP/sqft KW/sqm
> 9-10 0.015 0.118
> 13-15 0.020 0.161
> 19-21 0.040 0.312
> 25-27 0.070 0.559
> He also gives the following numbers for a speed estimate:
Note: I think Gerr also uses the equivalent of V = ((1200*hp/disp)^.33)
* sqrt(lwl) which just ain't the same as V = 150/sqrt(disp/hp)!
Gerr works an example using "Swan Song" in _The Nature of Boats_. It is
interesting that while he uses the wind speed to hp table to work the
example, he does not use the equation he gives to get a velocity
number... Instead he uses the displacement/hp to speed/sqrt(lwl) chart
to get a SL and then multiplies the SL by the sqrt(lwl) to get V. While
I'm not mathematically inclined, working a few problems using Gerr's
equation showed that the numbers for a 40 foot cat and a 12 meter (both
of which I have actual V to true wind info for) were way out of line
even for the broad reach w/o spinnaker (where Va =~ Vt). However, if
you work the problems like Gerr did, using table look-up, the numbers
come out in the right ball park. A little fiddling around on a yellow
pad suggests to me that you can get similar numbers using:
V = ((1200*hp/disp)^.33) * sqrt(lwl)
Where the (1200*hp/disp)^.33 approximates the table look-up. I don't
know how Gerr came up with the 150 / sqrt(disp/hp). It works for "Swan
Song", but it doesn't scale anything like the displacement speed chart
so it is can not be equivalent to working the table like Gerr does in
his example. So, either Gerr or I messed up, and while I generally
think Gerr is a more reliable source than I am, in this case, I'm pretty
confident that he blew it.
-- Tom.
KJenk2
the Navy. Here's what I remember:
1. Power increases approximately as the cube of the speed for a ship. The
appropriate formulas are
Water Drag =1/2*water density*(velocity squared)*Drag Coeffiecient*cross
section area.
Power =Drag*velocity
The Drag Coefficient is a function of the body geometry and the Reynolds
Number. Usually for high reynolds number it's primarily a function of geometry.
2. Hull speed=1.3sqrt(LWL) (For sailboats, may well be different for other
hull shapes)
3. If you assume that cross section area is proportional to (displacement)^2/3
and that water line length is proportional to (displacement)^1/3 then a little
algebra will give you that
Power for hull speed proportional to (Drag Coeffiecient)*(displacement)^7/6
What this means in plain english is that hull speed power is not really quite
linear with displacement and is affected by the hull shape. That's why one of
the previous posts giving details for sailboats, trawlers and other boats
showed a grouping depending on hull shape. All of the sailboats were grouped
reasonably close and so were the trawlers, but the two groups were wide apart
because the hull shape is quite different. Notice also that required power
increases a little more than linearly with displacement so you have to be
careful about the range you apply this formula to. I'd look at boats in the
same general size range if I were you.
Ken Jenkins
Jon V.
> Matt Pedersen wrote:
> > He also gives the following numbers for a speed estimate:
> > Boatspeed (knots) = 150 / sqrt (disp/hp) note:disp in pounds ...
>
> Note: I think Gerr also uses the equivalent of V = ((1200*hp/disp)^.33)
> * sqrt(lwl) which just ain't the same as V = 150/sqrt(disp/hp)!
Using the sa/hp factors provided, I get the following:
For my boat (249sq' sa, 22' LWL, 5500Lb disp):
Two-decimal HPs used just to piss off terminal number-rounders, and
intermediate results left in because I'm lazy.
HP: 9.96 (wind=20Kts)
Using 150/sqrt(dsp/hp=552)=23.5: 6.4Kts
Using ((1200*HP/disp)^.33) * sqrt(lwl): 6.1Kts
HP: 4.98
Using 150/sqrt(dsp/hp=1104)=33.23: 4.5Kts
Using ((1200*HP/disp)^.33) * sqrt(lwl): 4.8Kts
hp: 3.73 (wind = 10Kts)
Using Sqrt(dsp/hp=1472) = 38.37: 3.9Kts
Using ((1200*HP/disp)^.33) * sqrt(lwl): 4.3Kts
Overall, with the exception of the 20Kt wind where the speed is .2Kt above
theoretical hull speed for the 150/... formula, I don't think the numbers
are that far off in either case. The errors pale compared with sailor's
tales.... why, I was once in a...
-Jon
-----------------------------------------------------------------------------
"There... I've run rings 'round you logically"
-- Monty Python's Flying Circus
C. A. La Varre
Marcus G Bell wrote:
>like most things the harder you run it the faster it
>burns out.
>> maybe someone could enlighten me and us on the most economical
>> ratio of maximum HP for various engines... Would that be gallons
>> per hour per knot? or GPH/HP? If the former, then no speed is
>> best.
>
>Yes, it's the former, but "no [particular] speed is best" does not
>follow, because the HP per knot requirement of the boat is not a
>constant. Faster is less efficient, and at zero speed with the
>engine shut off we'd have an efficiency of 0/0 which is
>indeterminate.
Hmmm. How about Gallons/mile, or miles per gallon. My problem with any of
these is that the faster you go, the poorer the performance, according to
the curves from Cummins. The only one that shows a "sweet spot" is GPH/HP.
Is that counterintuitive?
>But the RPM of peak torque is in the
>right ballpark for the lowest GPH/HP.
This occurs at 2100 RPM for a machine with max RPM of 2500 (84% load)
>Now, put together the HP/knot of the boat with the GPH/HP of the
>engine, and the best GPH/knot for the system can be calculated. We
>didn't mention parasitic loss from the prop, but it decreases as
>RPM decreases too.
Based on the minimum GPH/HP (.0536 at 2100 RPM) we interpolate the GPH/KT
table to find that this corresponds to 8.56 knots for a boat with a hull
speed of 8.6 knots. In other words, flat out without wasting speed trying
to get up over the hull speed hump.
>A planing boat often has its best knots/GPH below hull speed,
which is exactly what the numbers show.... Interesting!
>As a final note, unless you are reading speed from a GPS or other
>system referenced to land, the best knots/GPH gives your best
>efficiency through the water only.
Yeh, but that is the only thing you can deal with as far as horsepower and
fuel consumption are concerned. Currents are not part of the equation.
Thomas Webb
Let's look at the 12 meter "Vim", because I have a polar for her at
hand.
lwl = 45.5, disp = 60400, sail area = 1880
I'll use Gerr's hp at 13 knots and the polar at 12 knots true since
that's pretty close.
hp = 1880*.02 = 37.6
Knots, Gerr by formula = 3.7
Knots, Gerr by my take on his table look-up = 6.1
Knots observed by polar running DDW(Va < Vt) = 6.7 (ish, probably
includes spinnaker, but less Va)
In this case Gerr's two methods are very different. The table look-up
is close while the formula is way off.
-- Tom.
Jon V.
> Sure Gerr's formula works well for some boats, but it doesn't scale.
> Let's look at the 12 meter "Vim", because I have a polar for her at
> hand.
>
> lwl = 45.5, disp = 60400, sail area = 1880
>
> I'll use Gerr's hp at 13 knots and the polar at 12 knots true since
> that's pretty close.
>
> hp = 1880*.02 = 37.6
>
> Knots, Gerr by formula = 3.7
> Knots, Gerr by my take on his table look-up = 6.1
> Knots observed by polar running DDW(Va < Vt) = 6.7 (ish, probably
> includes spinnaker, but less Va)
>
> In this case Gerr's two methods are very different. The table look-up
> is close while the formula is way off.
Not really surprising... whenever you have a formula which takes a complex
property like hydrodynamic drag and converts it into a constant, you can
expect strange behavior somewhere in the spectrum.
It isn't purely scale, though... if you use the two formulas on the
tugboat mentioned in another thread, you get 8.7Kts via Gerr's formula and
14.1Kts by your method, assuming a 400HP cruise (as somewhat indicated by
the 16GPH number stated). I would trust Gerr's method further than yours
(not that I would trust either) in that case... and 118,000 Lbs/80'LWL is
by no means small.
So it isn't scale per se, it is some other assumption which is
wrong/different in the two methods.
I once had a spreadsheet with all of the normal measurements (lwl, disp,
sa, etc) from about 35 different yachts, all of them older designs,
ranging from 16' to 45' lwl... maybe I can dig it up and program the two
guestimators into it... if I can find it. That would help to qualify when
each one was in error... but I'm betting that it is just a matter that the
150/ guestimator was intended as a simplified version for fat
(non-optimized) boats or something.
-Jon
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Familiarity breeds contempt -- and children.
-- Mark Twain
Thomas Webb
...
> It isn't purely scale, though... if you use the two formulas on the
> tugboat mentioned in another thread, you get 8.7Kts via Gerr's formula and
> 14.1Kts by your method, assuming a 400HP cruise (as somewhat indicated by
> the 16GPH number stated). I would trust Gerr's method further than yours
> (not that I would trust either) in that case... and 118,000 Lbs/80'LWL is
> by no means small.
Actually, I think they are both Gerr's formulae; he presents them on
the same page in a way that suggests to me that they should be
equivalent. That's my beef. I just converted his graph back into a
function to get the second formula. Anyway, the graphical method is
really trying to deduce the designed speed length ratio from the
designed horse power. Obviously this will not work for vessels like
tugs because their designed horsepower is not directly related to their
designed speed. As nearly as I can tell, Gerr's formula method is an
attempt to approximate his graphical method, but it is a poor
approximation for many cases.
For the tug you could put speed and displacement into the formulae and
get needed horsepower for that speed out, but you'd need more energy to
do that than I have on a Friday afternoon.
-- Tom.
Marcus G Bell
> >> economical ratio of maximum HP for various engines... Would
> >> that be gallons per hour per knot? or GPH/HP? If the former,
> >> then no speed is best.
> Marcus G Bell wrote:
> > Yes, it's the former, ...
C. A. La Varre (alav...@tiac.net) wrote:
> Hmmm. How about Gallons/mile, or miles per gallon.
Let's try it this way. We have efficiency of boat, engine, and the
combination. The boat is MPH/HP (higher is better), the engine is
HP/GPH (again higher is better), and the combination is MPH/HP *
HP/GPH = MPH/GPH = miles per gallon. So your "gallons per hour per
knot" is the reciprocal of MPG, using nautical instead of statute
miles of course.
MPG for the boat/engine combo is the bottom line that everybody
understands.
> My problem with any of these is that the faster you go, the
> poorer the performance, according to the curves from Cummins.
> The only one that shows a "sweet spot" is GPH/HP. Is that
> counterintuitive?
The counterintuitive part may be that running the engine slower
makes it less efficient, but that depends on how much intuition
you have about engines.
The Cummins data show that dropping below a certain RPM causes
GPH/HP to rise. As the boat goes slower, HP/MPH drops. The
question is, is GPH/HP of the engine rising *faster* than the fall
in HP/MPH of the boat, such that gallons per mile is rising as you
go slower? The only way to know is to have the quantitative,
empirical data for the boat/engine combo. If you can plot speed
vs. RPM and GPH vs. RPM, you can make a plot of speed vs. GPH, and
you'll thus find an RPM of highest MPG.
[snip]
> > As a final note, unless you are reading speed from a GPS or
> > other system referenced to land, the best knots/GPH gives your
> > best efficiency through the water only.
> Yeh, but that is the only thing you can deal with as far as
> horsepower and fuel consumption are concerned. Currents are not
> part of the equation.
It's kind of a sidebar, but I'll elaborate. Say the boat/engine is
most efficient at 5 MPH through the water burning 5 GPH, and if
you go to 7 MPH, you burn 10 GPH. In the former case, that's 1
MPG. But, if you're bucking a 4 MPH current, you're effectively
going only 1 MPH towards your destination and your *effective* MPG
drops to 1/5. But, if you went up to 7 MPH thorugh the water,
you're going 3 MPH towards your goal and your MPG goes to 3/10, a
50% improvement!
If you have a destination, you can get there with the least fuel
by measuring your MPH with respect to the destination and dividing
by your GPH. Run at the speed that maximizes that ratio, and you
will be maximizing your effective MPG, with currents already
accounted for. The main point is that the presence of a current
alters the effective speed of peak efficiency, and it is good to
be aware of that and know what to do about it.
Marcus G Bell
> > >> I'd like to see computations (graphs) that show what power
> > >> is required to propel displacement hulls to rated hull
> > >> speed AND beyond.
J'h (*@*.*) wrote:
> > > 5 HP per 1000 lbs displacement, should be more than adequate
> > > to push most any "typical" sail or trawler pleasure boat to
> > > hull speed and somewhat beyond.
J'h (*@*.*) wrote:
> ... note the words "*more* than adequate". That seems to have
> been overlooked in the responses to the very general "rule of
> thumb". ;-)
I think this could be because the original question was in 2
parts: (1) power required to propel displacement hulls to rated
hull speed, and (2) power required to propel displacement hulls
BEYOND rated hull speed.
If only the 2nd question is answered, then we ask an implied 3rd
question, "how *far* beyond", which requires us to answer the
first question. Lots of responses in this thread were aimed at the
first question.
> I think what I've learned in this long informative thread, is
> that *theory* may support as little as .5 HP per 1000 lbs for
> hull speed with some bigger heaviers boats in ideal calm
> conditions.
It's quite likely that *practice* supports those numbers, since
some real-world examples were provided.
> On the water *practice* finds almost nobody using less than 1 hp
> per 1000 lbs, and usually closer to 2 as a minimum.
There's a difference between "having" and "using". The engines on
displacement boats are not typically used at their full rated
power for cruising, so an observation that an engine's max rated
power is 2hp/1000# of boat displacement is not the same thing as
saying that the boat needs 2hp/1000# to cruise at hull speed.
Nevertheless, the extra power is called upon for maneuvering and
to fight the weather, so it is "used" some of the time, and if you
don't have it you can't use it. An engine selected to just barely
reach hull speed (measured or theoretical) is going to be too
small. This is where knowing what others have done and how well it
worked is quite useful.
> [snip] Calculated out across that whole table, the average of
> the boats listed comes out in excess of 8 HP per 1000 lbs.
Hmmm, can we put a few container ships and sailboats with electric
motors in the table to bring the numbers down a bit? ;-)
Which serves to underscore the point that a sweeping average
across such diverse types of boats will not be all that great a
predictor of a reasonable engine size for any single boat. And
once again we get into that "hull speed power" vs. "maximum
available power" issue, so a number representing only the max
power probably tells us only what power will exceed hull speed,
but not the hull speed itself or by how much it will be exceeded.
Such a number taken from boats similar to ours *will* tell us with
a certain liklihood what would be a good choice for our boat.
Jon V.
> "Jon V." wrote:
> >... I would trust Gerr's method further than yours (not that I would
> > trust either) in that case...
>
> Actually, I think they are both Gerr's formulae; he presents them on
> the same page in a way that suggests to me that they should be
> equivalent. That's my beef. I just converted his graph back into a
> function to get the second formula.
Yeah... I can understand why that would bother a person. :-)
> Anyway, the graphical method is
> really trying to deduce the designed speed length ratio from the
> designed horse power. Obviously this will not work for vessels like
> tugs because their designed horsepower is not directly related to their
> designed speed.
Well... I was using the cruising HP sans-tow... some 400-600HP less than
max for that vessel. I have since refined that... more below.
> As nearly as I can tell, Gerr's formula method is an
> attempt to approximate his graphical method, but it is a poor
> approximation for many cases.
Strangely so.
> For the tug you could put speed and displacement into the formulae and
> get needed horsepower for that speed out, but you'd need more energy to
> do that than I have on a Friday afternoon.
Yeah. I was sort of playing around over the weekend and used my poor-man's
pocket analysis tool (a HPC with "pocket excel") to run some numbers while
sitting in a fast food place trying not to listen to what passes for
'popular' in music these days...
I actually was sort of merging a couple of the interesting threads that
we've had lately, btw, so not everything is to the point... and I was
calculating ranges based on motoring, not sailing... obviously.
Here was the input data:
LWL: 80'
DISP: 118,000Lbs
Fuel: 2400G
HP/G: 18 (HP per gallon of fuel)
both formulas used.
I added to it by using a 18HP/gal guestimate to calculate the GPH at
various speeds. What I got was the following:
Kts(^.33) Range(^.33) Kts(150/) Range(/150) DKTS
10HP 4.21 18173 1.38 5965 2.83
100HP 8.99 3885 4.37 1886 4.63
200HP 11.31 2442 6.18 1333 5.13
300HP 12.92 1861 7.56 1089 5.36
400HP 14.21 1534 8.73 943 5.48
500HP 15.30 1534 8.73 943 5.53
600HP 16.25 1321 9.76 843 5.55
700HP 17.09 1054 11.5 712 5.54
800HP 17.86 964 12.3 666 5.51
Which isn't interesting at all when it comes to wondering which formula to
use, but is sort of interesting in the wider question about cruising
efficiency. DKTS is the difference in speeds between the two methods
(calculated from the full precision numbers).
Anyway, I understand what you are saying... and I agree that it is
annoying when an author comes up with two contradictory methods...
-Jon
-----------------------------------------------------------------------------
Nuclear powered vacuum cleaners will probably be a reality within 10 years.
-- Alex Lewyt (President of the Lewyt Corporation,
manufacturers of vacuum cleaners), quoted in The New York
Times, June 10, 1955.