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Thread: Jet Ski Engine as an Inboard

  1. #1
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    Default Jet Ski Engine as an Inboard

    Just a quick question......has anyone out there in Forumland ever installed a jet ski engine as a true inboard, i.e. with a transmission, prop shaft at around 10 degrees, etc. etc. in a small boat?
    The mass of men lead lives of quiet desperation......Thoreau

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    Default Re: Jet Ski Engine as an Inboard

    Great article in WB some time ago, actually cover story on that issue. The boat, a down east design was at the WBS back about 10/15 years ago. Search the index. Great boat. I spent some time looking for a likely doner for another boat I was building but finally gave up. Giver er a try. It would be a great pleasure to take a chainsaw to a jetski.

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    Default Re: Jet Ski Engine as an Inboard

    It was a cool boat. Evocative of a lobster boat but small and quite light. The purpose was that the mooring was mud at low tide. They could not obtain a jet ski motor and drive unit at a rational cost. It was more practical just to buy the jet ski and throw away the hull.

    But I think the OP is more wondering about just using the motor and attaching that to a conventional prop. To my mind, it makes about as much sense as modifying an outboard motor for an inboard application. You end up with real problems of finding a transmission that will even come close to fitting.

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    Default Re: Jet Ski Engine as an Inboard

    I also think a “jest ski” engine is possibly to much of a high RPM and low torque engine to work well as a true inboard. (Maybe I’m wrong, I’ve never even touched one.) I do like the idea though, I think they are pretty interesting looking little water cooled engines.
    As Ian pointed out, the lobster boat type that was featured in WB used the entire jet drive system. It did work out nicely.

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    Very high rpm engines ( 8000 plus rpm) so consider the reduction gear needed.

    Kevin


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    Default Re: Jet Ski Engine as an Inboard

    Quote Originally Posted by nedL View Post
    I also think a “jest ski” engine is possibly to much of a high RPM and low torque engine to work well as a true inboard.

    This. Exactly this.
    Heute ist so ein schöne Tag...

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    Default Re: Jet Ski Engine as an Inboard

    Wouldn't the obnoxious sound follow the engine wherever it went?

    Jeff

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    Default Re: Jet Ski Engine as an Inboard

    Is it wrong to dream about some sort of electronic ignition frying laser gun when a gang of these abominations go past.
    I think I'll leave out the question mark.

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    Default Re: Jet Ski Engine as an Inboard

    Don't Forget the 'jet ski ' concept was based on a Kiwi designer's shallow draught vessel, enclosed prop system... Hamilton springs to mind but might be wrong

    The idea had been around for a long time before PWC s

    I heard it took off because Honda or someone found themselves with a huge unsaleable load of mcycles and got their designers to come up with a new toy that could utilize tens of thousands redundant motors... jet skis. quad bikes whatever... create a market and then fill it with what you already have laying around. Neat.
    'C'est la vie' say the old folks it goes to show you never can tell

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    Default Re: Jet Ski Engine as an Inboard

    Sorry I asked.
    The mass of men lead lives of quiet desperation......Thoreau

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    Default Re: Jet Ski Engine as an Inboard

    Quote Originally Posted by John A. Campbell View Post
    Sorry I asked.
    Dont be. Your question got answered.
    There is nothing quite as permanent as a good temporary repair.

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    Default

    What boat do you have in mind?

    If you used one of the larger 4 stroke versions, even supercharged with a V drive reduction it wouldn't be anywhere near as annoying as the 2 stroke we all love to hate.
    Depending on hull form it makes much more sense to use the entire propulsion setup and add a reversing bucket.



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    Default Re: Jet Ski Engine as an Inboard

    I've always thought the motor out of a Honda CX 500 would make a nice boat engine. It's already shaft drive.

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    Default Re: Jet Ski Engine as an Inboard

    Why are you sorry you asked?
    I’ve thought the same thing when I’ve seen pictures of the engines,..... hmmm,... neat little water cooled two cylinder engine, could it be coupled to a small Hurth reverse gear as an inboard????. Great idea until you realize they are low torque high reving engines.

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    Default Re: Jet Ski Engine as an Inboard

    The production jet drive boats that evolved from jet skis just keep getting bigger, so I have to assume the drive systems are evolving to keep up. Looks like it's just a matter of matching the propulsion system to the hull in question.

    -Dave

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    Default Re: Jet Ski Engine as an Inboard

    Quote Originally Posted by purri View Post
    I do believe Kawasaki marketed the first one.
    I think that is right. There was also the Suzuki Wetbike, those were kinda cool. You know when we were young enough to play with such things. Jet Skis didn't get annoying until they made the easy to ride sitdown retard models. Prior it took a bit of ability to ride them thus no gnat gangs.

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    Default Re: Jet Ski Engine as an Inboard

    Quote Originally Posted by Woxbox View Post
    The production jet drive boats that evolved from jet skis just keep getting bigger, so I have to assume the drive systems are evolving to keep up. Looks like it's just a matter of matching the propulsion system to the hull in question.
    Yes... but those are jet drives, not propeller drives, and still rely on high RPM, not torque. That's why they work. That's why the Lobsterboat in the WoodenBoat article mentioned above went with the entire system.

    Perhaps an engineer can explain it more accurately. Somewhere in my notes I have it that propellers start to cavitate around 2,000 to 2,200 rpm (engineers, is this correct?). A transmission then cuts engine RPM to the right range for the prop. This also relates to why those Chevy V-8s are so easy to marinize... They are reasonably inexpensive to build so the torque comes on way down low.

    The comment above re: using "extra" motorcycle engines is on point. If you have a small bike engine, but want to put it in a boat, you need to deal with how it makes power. As you go down that road, jets make more sense than props. And so it goes.

    The original poster should not be sorry that he asked... He asked a reasonable question, and it started with reasonable answers. It did get a bit off topic with discussions relating to frying laser guns, but that happens around here.

    For Mr. Campbell, my question is "What kind of boat are you thinking?" If your design could handle a small jet, then this may be an ideal solution. If you're looking for a small engine for a prop boat ... Oh, wait, now I remember: You're looking for an engine for Slippery, aren't you?

    It would be difficult to use this type of engine on that boat. I still believe you can marinize an older car engine for a lot less money than your talking for this one. Failing our earlier discussion re: a flathead Ford 60, an Iron Duke or the 4 they used in the Pinto would meet what you're trying to do, and while not perfect, could be reasonable. It would be a lot better fit than a jet ski motor. I really do think that's the way to go.

    I also think, for your solution, you don't need to worry about "most" of the marinization piece. Anti-spark in the electrics. Marine carburetor. Don't worry about the lower end so much. Build the boat. Get on the water. Have fun.
    Heute ist so ein schöne Tag...

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    Default Re: Jet Ski Engine as an Inboard

    I'm not convinced about the RPM limit theory...

    Arneson boats used a gas turbine engine spinning at about 20,000 rpm with a reduction gear that spun the prop at 6000 RPM.

    I have a little cruise and carry outboard that runs about 10,000 and spins the prop at half that. So 5000 RPM"

    Those jet ski engines would probably work just fine with an appropriate gearbox. It could be done on the cheap with a chaindrive even.

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    Default Re: Jet Ski Engine as an Inboard

    Firstly, let me say that I appreciate everyone's comments and wish to respond as follows:

    1. Don Z., yes, it's "Slippery".....I am 100% desirous of her....she is 14'3" X 4'9" of Mr. Hacker's 1920 design, intended for 40 hp Lycoming marine engine, long extinct.

    2. I am 79 years old and weigh 170 pounds.....if Slippery will carry me alone at 25 - 30 mph I will be a happy man.

    3. Over 25 years ago Mr. Nelson Zimmer, a very fine naval architect (now deceased) told me over the phone that his 16-ft "Gentleman's Runabout" (from Woodenboat Plans) would do 28 - 30 mph with the then-specified Westerbeke 26GX......that engine developed 26 hp and 47 ft pounds torque........weight was around 280 lbs.

    4. Kohler LH775 and Kawasaki FD-851-D are V-twin gasoline commercial engines...horizontal shaft, fuel injected, dry weight under 140 lbs., liquid cooled w/radiator....both rated at 31 hp and 44 ft lbs. torque......they can be bought on line for around $2800 to $3300 with no charge for freight

    5. Michigan Motorz sells the GM Vortec 140 hp 4-cylinder marine package for $6559.....am awaiting call from them as to weight (likely around 450 lbs) ......most likely this would be my best choice in the event my two sons wanted to sell "Slippery" when I croak.....most people who want to speed wouldn't go for anything less.

    6. I have a very good friend who owns a LARGE machine shop in nearby Temple, Texas who has built large scale model boat running hardware for me assured me he can adapt the Kohler or Kawasaki engine to a small marine tranny such as ZF-45C with no problem.

    7. I have absolutely no interest whatsoever in "putting pieces together" to build up an engine, no interest in a used automotive engine of unknown or questionable condition.....

    8. I'm not saying that using either V-twin would be without challenges.......dealing with exhaust would be an issue......engine compartment will be 45 cubic feet which would call for something like Jabsco bilge blower at 250 cfm to keep radiator cool.....there will be others.......

    9. At this point, I'm really getting tired of researching and worrying about this.....I think I need to run down to Austin for a pickup load of rift sawn or quarter sawn well-seasoned white oak and start making frames (or should I use some of that 18 mm Okoume marine plywood at $220/sheet?) .......the engine decision can wait a while .....just make sure the engine stringers are spaced so either V-Twin or the Vortec will fit.......Slippery will be built VERY light

    10. Thanks to all for their input.......
    The mass of men lead lives of quiet desperation......Thoreau

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    Default Re: Jet Ski Engine as an Inboard

    John,

    Are you familiar with the Smith Island Crab Skiff? As I understand the class, they all use the same Kohler V-twin engine so the drive system has been worked out to a fine degree. There wouldn't be any figuring to do as long as your boat is reasonably close in size and weight to the Crab Skiffs. As to exhaust, they are quite loud.



    -Dave

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    Default Re: Jet Ski Engine as an Inboard

    I think this is what you want. https://www.rotax.com/en/products/ro...ce-marine.html
    60HP at 126lb without transmision, 4 stroke, already marinized. You need the rpm's anyway when going fast so a standard transmission will be ok.
    The 2 stroke E-TEC versions are quite a bit lighter, but they sell them only non marine (for snowmobiles).

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    Default Re: Jet Ski Engine as an Inboard

    A jet propulsor is not efficient except on a high-speed boat. For a propeller or a jet drive, the maximum possible efficiency is

    eta max = 2/(1 + (outflow velocity / craft velocity))

    As outflow velocity approaches craft velocity, eta max approaches 100%. As the outflow velocity gets higher, eta max goes down.

    On the other hand

    Thrust = mass flow x (outflow velocity - craft velocity), so if you make the two velocities almost equal to get maximum efficiency you get little thrust. You can't win!! (As if you didn't already know that.) But, if both velocities are high their ratio can be near 1 even if their difference is substantial.

    A large, slow boat wants a large, slow propeller that moves a lot of water at low speed. A higher speed boat wants a smaller propeller that moves less water at higher speed. A very fast boat (e.g. jetski) wants a very small propeller or a jet that moves a tiny amount of water at very high speed.
    I will beg you for advice, your reply will be concise, and I will listen very nicely and then go out and do exactly what I want! (Apologies to Lerner and Lowe.)

  23. #23
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    Default Re: Jet Ski Engine as an Inboard

    A few years ago at one of the major yachting events ( can't recall which ) some production RIBs turned up with jetskis grafted in to use as mark and or referee boats, tenders.
    Build RIB with appropriate jetski shaped/size hole, drop jetski into hole , glue. Seemed like a great option for the purpose , fast ,jets and no props around lines ,and for rescue as well I imagine. What the economics were I have no idea. But considering you got the entire powerplant , fuel and steering and seating in one pod, it might be good.

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    Default Re: Jet Ski Engine as an Inboard

    Quote Originally Posted by Phil Y View Post
    I've always thought the motor out of a Honda CX 500 would make a nice boat engine. It's already shaft drive.
    Well, they were known for bad cam chain tensioners back in the day. Not sure if Honda ever sorted that out, I sold mine.

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    Quote Originally Posted by Canoeyawl View Post
    I'm not convinced about the RPM limit theory...

    Arneson boats used a gas turbine engine spinning at about 20,000 rpm with a reduction gear that spun the prop at 6000 RPM.

    I have a little cruise and carry outboard that runs about 10,000 and spins the prop at half that. So 5000 RPM"

    Those jet ski engines would probably work just fine with an appropriate gearbox. It could be done on the cheap with a chaindrive even.


    Arneson - or any surface drive-- runs super cavitating props( aka, " cleavers.") the ate designed to run half out of the water.

    They are excellent for going fast. Excellent for shallow water. At slow speed and in reverse, not so much.


    Kevin



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    Default Re: Jet Ski Engine as an Inboard

    For a small vintage reproduction speedboat how about s Universal Atomic Four? A proven little marine engine. They are still available on the “rebuilt” market. 64 cid, 30 hp.

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    Default Re: Jet Ski Engine as an Inboard

    I think that a Ken Bassett Rascal speedboat - with appropriate bottom modifications - would be a fine hull for a medium- to high-horsepower jetski motor & jet. Get that motor off the transom for better balance and not interrupting the sweet hull lines.

    Hope for the best, but plan for the worst.

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    Default Re: Jet Ski Engine as an Inboard

    Quote Originally Posted by Breakaway View Post
    Arneson - or any surface drive-- runs super cavitating props( aka, " cleavers.") the ate designed to run half out of the water.

    They are excellent for going fast. Excellent for shallow water. At slow speed and in reverse, not so much.


    Kevin
    Yes, I'm aware of that. So, I'm not sure about an RPM limit for propellers. Perhaps tip speed is the limiting factor? My "homemade" rig uses a 9"x7" two blade prop that runs up to 3000 rpm.

    (About a hundred years ago I used to make parts for Arneson (swimming pool cleaners) and had a long conversation with him about some of this. His designs were "tested" nearby. Single blade prop with a counterweight 100+ mph was no big deal yet they seemed able to maneuver and dock the boat and get it on the trailer)

  29. #29
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    Default Re: Jet Ski Engine as an Inboard

    Speed limitation for props?
    Small inboard raceboats normally turn their props at 6000-8000 RPM.

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    Default Re: Jet Ski Engine as an Inboard

    This is slightly larger than a jet ski engine but it is an outboard power head driving an impeller. Much smaller and lighter than a traditional jet.

    https://www.mercurymarine.com/en/us/...max-sport-jet/

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    Default Re: Jet Ski Engine as an Inboard

    Dave, that did it......the most encouragement I've had so far......just seeing those 19-foot skiffs running across the water with 20 hp Kohlers for power was just what I needed to see. I've made contact with Twisted Oyster for additional info and it's been really good. I checked out the possibility of using rebuilt Atomic Four and, without another Atomic Four as a trade-in, the cost would be a little over $6000. Also my contact didn't think this engine was the right way to go.....he said the 31 hp Kohler (or Kawasaki) made a lot more sense aside from cost difference. I've lofted "Slippery" at full scale and also at 1/4 scale.....the idea is to build them both with the model slightly ahead of her "Mama". I think "Slippery" has her own set of challenges with the step bottom and that very curvaceous deck and hopefully building the model will add a degree of confidence. I most definitely will be building almost all of the boat out of Joubert Okoume marine plywood.....18 mm frames, 9 mm bottom, 12 mm floor, 6 mm sides, 4 mm deck, the latter with 3/32" Honduran mahogany bonded to it. Engine stringers will be vertical grain Doug fir, all other stringers vertical grain Sitka spruce. Keel, stem, breast, and transom will be Honduran mahogany. Sheer clamps and chines will be laminated vertical grain Doug fir....3/8" layers...West System 105/206. I cleared shop space today and my very good neighbor has a 16-foot flatbed trailer and he's ready to drive us down to Clark Hardwood Lumber Co. in Houston at a moment's notice......The way I've got it figured, I'll be 81 (I'm now 79) when the model and her Mama are finished (if I'm lucky) ....then it will be time to build Slippery again....this time casket-sized....and then it will be over....plant me in a little Hacker....yessiree!.
    Quote Originally Posted by Woxbox View Post
    John,

    Are you familiar with the Smith Island Crab Skiff? As I understand the class, they all use the same Kohler V-twin engine so the drive system has been worked out to a fine degree. There wouldn't be any figuring to do as long as your boat is reasonably close in size and weight to the Crab Skiffs. As to exhaust, they are quite loud.



    The mass of men lead lives of quiet desperation......Thoreau

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    Default Re: Jet Ski Engine as an Inboard

    John, I've seen those crab skiffs in action on several occasions, and there's little doubt in my mind that the owners get tremendous enjoyment from a boat that is easy to run and maintain. Glad to be of help.
    -Dave

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    Default Re: Jet Ski Engine as an Inboard

    The crab skiffs are air-cooled, most are direct drive, some have electric clutches, I've no doubt a water-cooled Kawasaki V-twin, keel-cooled or a radiator so it could be enclosed in an engine box, with a trabsmission, would be a much nicer, quieter setup.

  34. #34
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    Default Re: Jet Ski Engine as an Inboard

    I agree......my machinist friend tells me he can adapt a marine transmission to either engine (Kohler or Kawasaki) but I sure would like to find somebody who has already done that with success. I'm doing the research now and so far haven't found it ......got a lot more phone calls to make and lot more web searching. I'm surprised something like this hasn't already been done years ago using these little "commercial/agricultural" engines.....one thing's for sure....the engine's firing would have to be muted through a damper plate or some type of "soft" coupling between its crankshaft and the transmission to eliminate "gearbox rattle". Or maybe I need to use one of my old model boat tricks where forward direction is by gasoline engine w/centrifugal clutch and reverse is by 12 volt electric motor and electric clutch. Noram Co. (North American Clutch Co.) makes a centrifugal tri-lobe that will handle up to 50 hp and elsewhere on this Forum is a fellow who recommended the Toyota starter motor for this application as it would withstand somewhat longer run time without overheating.
    Quote Originally Posted by hightop View Post
    The crab skiffs are air-cooled, most are direct drive, some have electric clutches, I've no doubt a water-cooled Kawasaki V-twin, keel-cooled or a radiator so it could be enclosed in an engine box, with a trabsmission, would be a much nicer, quieter setup.
    The mass of men lead lives of quiet desperation......Thoreau

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    Default Re: Jet Ski Engine as an Inboard

    Please do share if you come up with an off the shelf solution. I know the issue has come up here before and the answer has always been, " fabricate/ have something custom made" for a gearbox.

    Noise suppression is another challenge with the air-cooled engines.

    Kevin
    There are two kinds of boaters: those who have run aground, and those who lie about it.

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