"Small" vs. "Real" jetboats
r...@recon.org
jetboats in particular, from this newsgroup - thanks!
I have known about two classes of jet boats, the PWC-engined
ones and the Mercury Sport Jet ones. Now I've learned of a
third class, what some people call "real" jet boats. These
seem to have big-block engines in the 400+ cubic inch range,
and jet drives by companies such as Berkeley. They appear to
be built on both fiberglass and aluminum hulls. Most
interesting, I've seen these "real" jet boats for sale,
used, in the classifieds for $2500-4000. That's a _heck_ of
a lot less than new or used Sport Jet powered boats, despite
the 2-to-1 or greater difference in horsepower and supposed
10-25 MPH speed difference.
Factoring out the PWC-engined boats, which approach is
better and why? It's tempting to think that one could
purchase twice the horsepower for less than half the money.
Maybe these new Sport Jet powered craft are still a
"novelty" and thus commanding a premium. Perhaps the "real"
jetboats are costly to maintain or have other problems.
My interest in jetboats is primarily as a turn-on-a-dime
playboat, only occasionally used to pull skiers. Perhaps the
(typically) shorter Sport Jet boats are better for that,
while the "real" jet boats trade agility for top-end speed?
What's the story?
gblb
<r...@recon.org> a écrit dans le message :
37caed90....@207.126.101.100...
zdial
Lifespeed
made in the 70's and 80's.
They are indeed superior to those lame oversized jetskis.
Yes, they turn on a dime, are cheap to maintain, and are
usually faster than the "big jetskis" also.
If you like jetboats, then you should check into a "real"
one. A company called Supermarine in Florida makes a 17' 350
Chevy powered model with a Dominator jetpump for $18K, new.
How much do they sell those silly jetski-boats for?
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r...@recon.org
wrote:
>If you want a play boat, get a Speedster SK.
No PWC engines for me, thanks.
Larry KN4IM
Smart man. He wants to spend his weekends ON THE WATER, not hanging
out around a motorcycle shop screaming in agony. Been both, prefer
the former, myself.
AS to the differences, the only thing the V-8 powered jet pumped river
racers has in common with the little jetboats is they both squirt
water out the back to propel themselves. It all stops there.....
The big jetboats are MUCH faster than my Sea Rayder with the V-6 Sport
Jet. Mine only does about 56-58, 53 with the bimini top up. The big
jetboats go a hundred! They are very heavy and VERY expensive to
maintain the big V-8, the drivetrain and its alignment, and the big
pump. Those ones you've seen for $2500 are very old and there are no
pump parts left to fix them. The early pump manufacturers went bust
trying to service them. They are very inefficient, compared to, say,
an I/O drive on the same engine. They gave jetboats a bad name and
you'll see old timers on here telling you how inefficient ALL jetboats
are because of their experiences with these inefficient pumps, many of
which started out life as farm pond pumps designed to pump a LOT of
water at relatively low head pressures, using impellers. The
repair/upkeep costs are why it's only $2500, now....and may be
unrepairable.
The new breed of jetboats, in two categories, both owe their roots,
not to the farm pond multi-stage jet pump, but to the PWC's very
simple pump. Early Sport Jets, powered by Force outboard heads, had
an impeller that closely resembled one stage of a farm pond pump. The
impeller was very oddly shaped and quite inefficient. With the
upgrade to the 175hp Mercury V-6, Merc simply adopted the "swirl-type"
impeller, which looks like a normal outboard stainless steel prop with
its outer edge cut round to fit inside the stainless wear ring. Being
a prop, not an impeller, efficiency went way up. My Sport Jet's pump
uses the same technology as the high speed PWC pumps....an enclosed
prop in a fore/aft tunnel with a VERY short water path, shorter than
even the PWC's use. Lots of modern yacht designers are using HALF
this technology by designing propulsion systems in shallow draft
"tunnels" where the truncated props are recessed into 1/2 circle
indentations in the hull, itself. There are also tunneled props
showing up on the bottom of outboard motors turning the pump, itself,
not the nozzle as we do. Tunneled prop technology isn't new. I
believe Penn Yan had tunnel hulls back in the 60's with props recessed
in short waterpath tunnels.
The differences in the pumps adds to the efficiency of the modern,
small jetboat (and PWC). If you look at a Berkeley jetpump, you'll
notice it has a rather smallish intake quite a ways ahead of where the
impeller is separated by a "pipe" for the water to run through. The
pump protrudes out the back. The pump, itself is a widened section of
this pipe which then reduces its diameter at the outlet to the
impeller. Now look at the PWC or Sport Jet pump. The "intake
grate", designed to keep the big stuff out of the pump, like logs and
sticks (not ski roped, dammit all) is located VERY CLOSE, almost under
the front edge of the prop. The intake channel is LARGER than the
prop so it adds to the RAM effect, the boat's forward motion SHOVING
water into the prop...just like your regular prop does.
The outlet of the pumps is also "different". The Berkeley pumps
merely squeezed the diameter back towards the nozzle. Designing PWC
pumps on SHORT water paths, it was found the rotation of the water
column detracted from the thrust. So, to compensate, a hydrodynamic
set of "stator vanes", from 5 to 7 vanes, is mounted a fraction of an
inch behind the spinning prop. They are designed to straighten out
the water flow into a straight back, no swirl flow, and are the same
diameter as the prop/wear ring themselves. Now, after the water flow
has been "straightened", water enters a chamber whos output diameter
is approximately 1/2 it's inlet diameter. Uncompressible, water has
no choice but to rapidly increase its speed so the water flowing out
the nozzle, which is directly at the exit to this chamber, is far
faster than the water being pumped by the prop into the chamber. To
smooth the flow, eliminating eddy currents, a reducing "cone" is
bolted to the back of the stator vane assembly where the driveshaft's
rear bearing is housed. The flow must be kept smooth with a minimum
of drag and eddy currents that would decrease efficiency.
The outlet nozzle, unlike the Berkeley, isn't "steerable", it points
straight back. To turn and control the boat, a separate steering
nozzle is aft of the nozzle's exit and not connected to it except for
a bearing surface. When the steering is pointed straight ahead, water
flows through the steering nozzle unimpeded because the steering
nozzle diameter is LARGER than the exit nozzle's diameter. No drag
going straight ahead is more efficient, of course. As the steering
nozzle is turned into the rapid flow, water bounces off one side of it
causing the whole flow to be deflected, one way or the other and, in
the case of some performance PWC, up or down to trim the thrust on the
boat, though this makes the pump more inefficient if the thrust is
touching the nozzle. The total length of the Sport Jet's water path
is only about 16", not counting the steering nozzle. Drag is
minimized so we can use a smaller engine. To decrease drag caused by
water pulling over the old Berkeley pumps outer surface, a metal plate
we call a "ride plate" covers the entire pump in the plane of the hull
making a very smooth transition surface for water passing under the
pump. The rear end of the Sport Jet's plate is also hinged and
adjustable, a trim tab, to help adjust the boat for maximum thrust in
the correct direction. The raised thrust centerline of the PWC/Sport
Jets is neatly in line with the CG of the boat being up above the
level of the bilge. Changing thrust on a PWC/sport jet doesn't shove
the bow UP like the thrust from an outboard, I/O or conventional
prop/rudder. Thrust so centered makes the boat VERY responsive to
changes in thrust without driving the hull up into a "plow" attitude.
Of course, as it comes on plane, the boat must respond to climbing out
of its displacement trough and over the bow wave. This takes
approximately 1 second on the Sport Jet because as it climbs the bow
wave the rear of the pump nozzle is DOWN driving hard up over the bow
wave. It's very violent coming on plane with CG thrust.
At WOT, the 175 Sport Jet is pumping approximately 55-58 gallons per
second rearward at twice the hull speed forward. 55 x 6#/gallon =
around 330 # of water displaced per second. I have no figures on how
much thrust this is. It's enough thrust to spin the boat around on
its central axis at 50 mph forward speed almost instantly. That's
what's so fun about the little jetboats. The old Berkeley couldn't do
any of these fun tricks, it just went fast on sheer power.....
Larry....sorry I got longwinded. I sure love playing with it. Most
fun boat I ever owned. Everyone from my ragbagger friends to my stink
potter friends LOVES to ride in it, too!
Douglas King
> (interesting and enthusiastic post snipped for brevity)
> ... Lots of modern yacht designers are using HALF
> this technology by designing propulsion systems in shallow draft
> "tunnels" where the truncated props are recessed into 1/2 circle
> indentations in the hull, itself. There are also tunneled props
> showing up on the bottom of outboard motors turning the pump, itself,
> not the nozzle as we do. Tunneled prop technology isn't new. I
> believe Penn Yan had tunnel hulls back in the 60's with props recessed
> in short waterpath tunnels.
It goes back 'way further than that. William Atkin was designing
tunnel-drive flats boats in the 1920s. William Burgess built a
water-pump propelled sub chaser for the Navy in the late 1930s. I've
seen photos of Harkers Island fishing boats with outboards in a well
connected to a tunnel which I believe date to the 1930s.
You might even consider Albert Hickman's "Sea Sled" a version of a
combined tunnel-drive and air-cushion vehicle. Hickman's patent dates
back to 1907 IIRC.
Fair Skies- Doug King
--
This is what we look like when we're at our best:
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George Jefferson
:Jet. Mine only does about 56-58, 53 with the bimini top up. The big
:jetboats go a hundred!
do you think? I knew a few gear heads back in those days who got very
frustrated as they could only get those big blocks into the high 70's.
The conclusion was if you want to go 100 you need a prop.
Twice the power twice the gas and 10x the noise for maybe 10mph about
sums it up I think. Some of them sure were pretty though.
--
george jefferson : geo...@sol1.lrsm.upenn.edu
to reply simply press "r"
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Lifespeed
nospam@micro$haft.com (Larry KN4IM) wrote:
>The big jetboats are MUCH faster than my Sea Rayder with
the V-6 Sport
>Jet. Mine only does about 56-58, 53 with the bimini top
up. The big
>jetboats go a hundred!
They don't go that fast. 60 mph is pretty typical, and maybe
70 mph if you have one heck of a motor. Racing jetboats have
been known to do 100 mph, but that is far from the norm, and
probably requires a minimum of 1200 HP.
> They are very heavy and VERY expensive to
>maintain the big V-8, the drivetrain and its alignment, and
the big
>pump. Those ones you've seen for $2500 are very old and
there are no
>pump parts left to fix them. The early pump manufacturers
went bust
>trying to service them. They are very inefficient,
compared to, say,
>an I/O drive on the same engine.
Jets are the simplest boat drivetrain available. They
require minimal maintenance, and are inexpensive. It is
quite easy to maintain a V8, and they don't usually require
much work anyway.
>They gave jetboats a bad name and
>you'll see old timers on here telling you how inefficient
ALL jetboats
>are because of their experiences with these inefficient
pumps, many of
>which started out life as farm pond pumps designed to pump
a LOT of
>water at relatively low head pressures, using impellers.
The
>repair/upkeep costs are why it's only $2500, now....and may
be
>unrepairable.
ALL jets are inefficient relative to props. Most newer model
jets are considerably improved in efficiency relative to the
older models. Dominator, Aggressor, and yes, Berkely,
incorporate the features you describe in your sport jet pump
for improved efficiency. Parts ARE still available, if you
buy one of the brands I mentioned.
Your Merc sportjet isn't the only pump that has been
improved over the years.
Oh yeah, the "big" V8 jetboats will spin on a dime, and
perform all the usual jetboat antics, just like your little
boat, but with more horsepower.
Jeff Thieme
>The big jetboats are MUCH faster than my Sea Rayder with the V-6 Sport
>Jet. Mine only does about 56-58, 53 with the bimini top up. The big
>jetboats go a hundred! They are very heavy and VERY expensive to
>maintain the big V-8, the drivetrain and its alignment, and the big
>pump. Those ones you've seen for $2500 are very old and there are no
>pump parts left to fix them. The early pump manufacturers went bust
>trying to service them. They are very inefficient, compared to, say,
>an I/O drive on the same engine. They gave jetboats a bad name and
>you'll see old timers on here telling you how inefficient ALL jetboats
>are because of their experiences with these inefficient pumps, many of
>which started out life as farm pond pumps designed to pump a LOT of
>water at relatively low head pressures, using impellers. The
>repair/upkeep costs are why it's only $2500, now....and may be
>unrepairable.
>
Larry, where do you get your facts, do you make them up as you go along?
First of all most V8 jets will not do 100 especially the ones you see for $2500,
60 - 70 is more common. There are jets that will do far in excess of 100, it
all depends on the type of the hull, the weight of the hull, how well setup your
boat is and of course how much horsepower you have pushing it. Not all jets are
heavy either, the drag boats usually have a very light layup and some weigh less
than your minijet. A stock V-8 motor is one of the cheapest to buy parts for, I
can go to any Auto parts store and buy parts for my Olds455 even 25 years after
it was made, try doing that with a mercruiser or a PWC motor. The reason that
they are more expensive to maintain has to do with the age of the motor not
because it is a V8. The same goes for the pumps, parts are plentiful and
relatively inexpesive for Berkeley, Dominator and Jacuzzi pumps. The biggest
reason that jetboats stopped being made in the mid-70s was the gas crisis, the
automobile manufacturers stopped making as many big block V8's and the jets
didn't fare as well as the stern drives with small block engines. Another
problem was that a lot of boat builders (Sea Ray being one of them) who knew
nothing about proper jet setup converted some of their existing models with
usually disastrous results, i.e. no top end and horrible handling. From most
accounts I've read, jet drives originated in New Zealand designed for shallow
river boating and not as farm pumps
>
Jeff Thieme
If you want to do doughnuts and 180's a big jet boat probably isn't the way to
go, they weren't designed for that. I have the best of both worlds, a '75 18ft
jet boat that I bought 15 years ago and still runs strong and a regular PWC for
when I want to get wet. You can get some pretty good deals on older jets but be
prepared to do some work since after 20 years or so things need to be replaced.
If possible try to get one with a Berkely or Dominator pump as they are easier
to modify and get parts for than the Jacuzzi or Panther jets. First choice for
a motor would be a 454chevy, but a 455olds or 460ford is fine too. Stay away
from the small block 350chevy or 400olds. If you have a particular boat in mind
stop by the message forum at www.realjetboats.com and post the particulars, the
guys there will let you know if it sounds like a good deal or not.
There are still a lot of boat manufacturers, especially on the west coast that
build the big jet boats. For example the most recent Hot Boat magazine has a
killer looking 19' Ultra Shadow which lists for $24,900 with a 454Chevy, the
model they tested had a few upgrades including a 502Chevy and went 76mph and
stickered for $33,370. One of the best things about the real jetboats is that
say after a couple of years you find that 76 is too tame, simply for over
another few grand add a procharger or whipplecharger and suddenly you have an
85mph boat. If that's to tame, pony up some more money and get a real blower
setup and you have a 100mph boat. You just aren't going to do that with a
sportjet.
Larry KN4IM
Jefferson ) wrote:
>:The big jetboats are MUCH faster than my Sea Rayder with the V-6 Sport
>:Jet. Mine only does about 56-58, 53 with the bimini top up. The big
>:jetboats go a hundred!
>
>do you think? I knew a few gear heads back in those days who got very
>frustrated as they could only get those big blocks into the high 70's.
>The conclusion was if you want to go 100 you need a prop.
>
>Twice the power twice the gas and 10x the noise for maybe 10mph about
>sums it up I think. Some of them sure were pretty though.
>
One of my main interests in jets is the safety factor. It's awfully
hard to run the prop into underwater objects (rocks, bottom, swimmers,
manatees), no matter how much the greenies want to blame PWC for the
sewage plant ruining the coral reef, if you're driving a jet with an
INTERNAL prop. I've skinned, inadvertently, over a HUGE log that
would have knocked any outboard, no matter how it was mounted, or an
expensive I/O RIGHT OFF THE BOAT! We were going 40 at the time and
the log was barely under the surface of the water, waterlogged. I
even turned around and used the jetboat to tow it to shore so other
boaters, unfortunate enough to have "things hanging down" the log
would have just torn off, wouldn't end up busted! The log banged into
something under there, but it WASN'T my prop, foot, rudder, skeg,
outdrive.....
It's also MUCH safer at the beach. I can motor right up until the bow
kisses the sand to let off passengers and unload the fun, without
having to worry about my drive "contacting the bottom".
I also have NO RUBBER IMPELLER to tear up. There is NO water pump on
the Sport Jets. engine cooling is simply a pipe that protrudes into
the high pressure water flow (about 45 psi at WOT on my guage!) of the
cone section of the jetpump. If I see 60 gallons per second spraying
out the back...my engine IS getting cooling water! I have no "intake
strainer", that little screen over those TINY holes to plug up with
marine crap, either. I've never seen anyone plug up the cooling water
on a Sport Jet. My impeller is HUGE and made of STAINLESS STEEL.
Those reasons, alone, are why I'd buy another jet.
Larry
Jack McClure
>
> They don't go that fast. 60 mph is pretty typical, and maybe
> 70 mph if you have one heck of a motor. Racing jetboats have
> been known to do 100 mph, but that is far from the norm, and
> probably requires a minimum of 1200 HP.
>
Here's a little info that might help get you up to speed on "real" jet
boats. My 21' Hallett cruiser seats 7 comfortably and tops out at 56 mph
with 2 aboard. Power is a 330hp/454 BBC turning a Berkeley drive. Parts
for both are readily available. My 19' Liberty has a 360hp/396 BBC
turning the Berkeley impeller and does high 60's with 2 jetters onboard.
Lastly, a well setup 19' jet boat can hit 100 mph with about 700 hp.
1200hp will get you 130+ mph, but the acceleration will make you think
it's 230.
Happy jetting,
Jack
Dave Hall
>
> It's also MUCH safer at the beach. I can motor right up until the bow
> kisses the sand to let off passengers and unload the fun, without
> having to worry about my drive "contacting the bottom".
>
> I also have NO RUBBER IMPELLER to tear up. There is NO water pump on
> the Sport Jets. engine cooling is simply a pipe that protrudes into
> the high pressure water flow (about 45 psi at WOT on my guage!) of the
> cone section of the jetpump. If I see 60 gallons per second spraying
> out the back...my engine IS getting cooling water! I have no "intake
> strainer", that little screen over those TINY holes to plug up with
> marine crap, either. I've never seen anyone plug up the cooling water
> on a Sport Jet. My impeller is HUGE and made of STAINLESS STEEL.
>
>
You might regret the fact that you have no strainer. I used to own a
"big" jetboat about 15 years ago, and we used it in shallow water alot,
with frequent beachings. As you probably know, the thrust from a jet
tends to stir up the sand and muck from the bottom, when you beach.
Well, alot of that stirred up stuff ended up being forced into the
engine with the cooling water. One year I had to replace a freeze plug,
and when I looked into the cooling passage, I couldn't believe how much
sand and fine gravel had accumulated in there. I'm surprised that I
didn't have hot spots (I probably did, but I just didn't notice).
A buddy of mine ran hard aground with his jet one night in Double Creek
channel in Barnegat Bay NJ. When we finally pulled him off the next day,
one of his exhaust manifolds clogged up from the sand and we had to pull
it and flush it out.
I'd rather have a strainer plug up with bottom goo, than my engine. It's
tough to clean that stuff out of an engine block.....
Dave
Lifespeed
<jmcc...@pacbell.net> wrote:
>Here's a little info that might help get you up to speed on
"real" jet
>boats. My 21' Hallett cruiser seats 7 comfortably and tops
out at 56 mph
>with 2 aboard. Power is a 330hp/454 BBC turning a Berkeley
drive.
Is that a Vector or 210? Hallet makes a great boat. I've
skied quite a few races behind them.
>Parts
>for both are readily available. My 19' Liberty has a
360hp/396 BBC
>turning the Berkeley impeller and does high 60's with 2
jetters onboard.
>Lastly, a well setup 19' jet boat can hit 100 mph with
about 700 hp.
>1200hp will get you 130+ mph, but the acceleration will
make you think
>it's 230.
I didn't figure on getting the HP figures exact. I run a 21'
Schiada twin-turbo V-drive, so it takes a bit more to get
the big boat up to speed, about 800 HP for 100 mph.
19 footers take less power, especially the air-trappers, but
I'm not sure how comfortable I'd be driving one at 100 mph
for more than a few seconds.
Don't forget the whirl-away for safety in a fast jetboat :)
David Smalley
> INTERNAL prop. I've skinned, inadvertently, over a HUGE
> expensive I/O RIGHT OFF THE BOAT! We were going 40 at the time and
> something under there, but it WASN'T my prop, foot, rudder, skeg,
> It's also MUCH safer at the beach. I can motor right up until the bow
> I also have NO RUBBER IMPELLER to tear up. There is NO water pump on
> strainer", that little screen over those TINY holes to plug up with
> on a Sport Jet. My impeller is HUGE and made of STAINLESS STEEL.
Uh, Larry,
You don't have to scream and shout. Netiquette asks that you _emphasize_
-by-using- *either* the _ * or - keys as demonstrated on both sides of
the word or words you want to stand out.
Capital letters are generally considered shouting and therefore rude.
Thanks.
WHardy1902
>and when I looked into the cooling passage, I couldn't believe how much
>sand and fine gravel had accumulated in there.
That sand and gravel might be the reason you popped a freeze plug. Lucky it
didn't crack the block. Let me explain. A few seasons back while winterizing my
GM 350 I pulled the drain plugs from the block. It dripped a few times and
stopped.
That's strange, usually a gallon or so gushes out each side and I ain't puttin
this thing to bed until I'm positive it's empty. Got a piece of wire and probed
for a while,,,,,Put my eye up to the drain hole for a peek and WHOOSH!
%$^%$#^%$#^%$#^%$#^%$^%$#^,,,,,,,&^%&^%$^%$#^%$#^%$Damn
Bayliner*&^%*&^%*&^*&^%*&^%*&It's empty now%$&^%&^%&^%$&^%&^%*
Now I get a garden hose and flush it out. Manifolds too.
"Guns are no more responsible for killing people than spoons are
responsible for making Rosie O'Donnell fat."