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Thread: Jetski Engine As An Inboard

  1. #1
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    Default Jetski Engine As An Inboard

    Small new inboard engines of 50 to 100 hp are fast becoming extinct except for diesels which are extremely expensive. If anyone disagrees with this, I would like to know where you've found them. In any case, I'm wondering if anyone has ever adapted a PWC engine (without the pump) as a "true" inboard operating at a 10 degree shaft angle and with a transmission, either an 8 degree down angle or no angle.
    The mass of men lead lives of quiet desperation......Thoreau

  2. #2
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    Default Re: Jetski Engine As An Inboard

    Someone tell me why the following won't work:

    Picture a 50 hp 2-stroke PWC engine driving a 10 degree angle prop shaft through a heavy duty centrifugal clutch (made by North American Clutch). Add a heavy duty roller clutch with pulley to the drive shaft output side of centrifugal clutch. Add a 12 VDC automotive type starter motor with pulley and belt drive to the roller clutch pulley. Add an electric 12 VDC water pump to cool the engine. We need a 12 volt battery anyway to start the engine.....it can also serve the 12 VDC automotive type starter motor when REVERSE propulsion is needed so........FORWARD propulsion is merely a function of engine RPM/centrifugal clutch (direct drive).....REVERSE is merely a switch on the dashboard turned on when engine is at idle.....engine turns CCW rotation, starter motor turns CW rotation. The two clutches would be in an assembly that would include a thrust bearing to handle prop thrust load and would include the shaft coupling. Gasoline = forward, Electric = reverse.......this is the same as my 76" X 18" "Rosita" radio controlled model will use except in the case of the model, I used an electric clutch instead of roller clutch 'cause I didn't think of the latter early on. Comments would be appreciated.....if you haven't guessed by now, this whole idea revolves around "Slippery" .......see elsewhere here.
    The mass of men lead lives of quiet desperation......Thoreau

  3. #3
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    Default Re: Jetski Engine As An Inboard

    Not sure what you mean "without the pump", isn't that what drives it? but there was an article in WB magazine a while back about a guy in Ct. I think who built a down east style boat using a PWC motor. If I remember correctly he stated that you couldn't buy a bare PWC motor so he ended up buying a complete PWC and removed the motor from it, basically he built the boat around the motor, the only complaint was that in shallow water it sucked up a lot of bottom debris, personally I think it's a neat idea.

  4. #4
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    Default Re: Jetski Engine As An Inboard

    Maybe five years ago there was an excellent WoodenBoat Magazine article on this. A fellow wanted a sort of "spirit of lobsterboat" but small in a light planing hull and the permanent berth had very little water. They toyed with all sorts of ideas before hitting on power by jet ski. For that project, it worked out cheaper to simply buy a jet ski, extract the engine and drive, and toss the hull. Anyway, it worked.

  5. #5
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    Default Re: Jetski Engine As An Inboard

    It was Tommy Townsend of Thomas Townsend Custom Marine Woodworking that had the lobsterboat with the jetski engine in it. It was on the cover of WoodenBoat # 198.

  6. #6
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    Default Re: Jetski Engine As An Inboard

    Good catch Steven.

  7. #7
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    Default Re: Jetski Engine As An Inboard

    What sort of boat are you talking about using this on John? A keel-boat? Fishing boat? Pontoon boat? Punt? House boat?

    Why throw the jetski away at all?

    I can’t find a picture of them but I’ve seen pontoon boats and small work barges, built here in Oz, driven by jet ski’s that can be docked/un-docked in the same way as this RIB:

    Larks

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    “It’s risky”, said experience.
    “It’s pointless”, said reason.
    “Give it a try”, whispered the heart.

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  8. #8
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    Default Re: Jetski Engine As An Inboard

    I think what the OP means by 'without the pump' is using the engine to drive a shaft/running gear/prop, i.e., instead of using the waterjet drive and impeller. A transmission may be necessary depending.
    "It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."

    -Mark Twain

  9. #9
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    Default Re: Jetski Engine As An Inboard

    This 50 hp PWC engine will be a 2-stroke thus no concern about lubrication of the engine installed at a 10 degree shaft angle......and yes, I want it to drive a prop shaft (NO JETDRIVE PUMP). I'm wanting to build the little 14'3" Hacker stepped bottom hydroplane "Slippery" as I have talked about on this Section of the Forum and I think the PWC engine will do the trick as Hacker specified a 40 hp Lycoming engine. I will need some guidance on prop selection, that's for sure, so obviously there's more research to do. I don't want to use a transmission......and if I did, I would want 1:1 ratio in forward so why not eliminate the transmission altogether? and, as described above, reverse is accomplished with ROLLER clutch and a 12 volt electric motor spinning clockwise direction.
    The mass of men lead lives of quiet desperation......Thoreau

  10. #10
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    Default Re: Jetski Engine As An Inboard

    I don't think props in water go well at 8000 rpm. And your 12v reverse power will be totally different characteristics.

  11. #11
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    Default Re: Jetski Engine As An Inboard

    Yes you can. It would be easier to just put an Evinrude Etec on the stern of course.
    If you want you can emulate the old speedboats from the GDR. Wartburg 900-1000cc 2stroke 3 cylinder 50-100hp motor, thrust bearing, prop. Usually no clutch, raw water cooling via propellor back pressure, no reverse. Not that I recommend this setup, but the engines are still available in Germany. Nowadays many are converted to the conventional setup of dual circuit cooling and transmission.

    New 2 stroke engines can be had from the UL flying people, motorcycles, outboards, snowmobiles and of course PWC. It does not pay to muck around with multiple clutches, a normal transmission with reverse and integrated thrust bearing is best. Also a engine driven cooling pump solves the cooling rate because it is coupled to rpm's, no controller needed. But you can eliminate the transmission, use direct drive, and take a paddle for reverse.
    If weight is paramount then the UL engines are best. Examples: Rotax 582, 65Hp, weight 65lb. Rotax 600 Etec, 118Hp, 75lb (naked engines weights. Real world weight is higher).

  12. #12
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    Default Re: Jetski Engine As An Inboard

    Quote Originally Posted by John A. Campbell View Post
    This 50 hp PWC engine will be a 2-stroke thus no concern about lubrication of the engine installed at a 10 degree shaft angle......and yes, I want it to drive a prop shaft (NO JETDRIVE PUMP). I'm wanting to build the little 14'3" Hacker stepped bottom hydroplane "Slippery" as I have talked about on this Section of the Forum and I think the PWC engine will do the trick as Hacker specified a 40 hp Lycoming engine. I will need some guidance on prop selection, that's for sure, so obviously there's more research to do. I don't want to use a transmission......and if I did, I would want 1:1 ratio in forward so why not eliminate the transmission altogether? and, as described above, reverse is accomplished with ROLLER clutch and a 12 volt electric motor spinning clockwise direction.
    John anything can be done with the right engineering.
    There is a world of RPM difference between the 40hp Lycoming and the 50hp Jetski engine - the Lycoming is maxed at about 2500 (I guess) whereas the jetski 2stroke will probably do 8000 plus.
    But for a hot rod boat such as the Hacker, it may suit.
    In its favour it has a cooling system with the engine manifolds in place, and it is built for marine use - so very good choice there.
    The Lycoming 40hp was originally an automotive engine I believe, but I can be corrected.
    Choose your Jetski engine and get the power curve - how many rpm on one axis and the available shaft horsepower on the other.
    Then work out the prop gap - available diameter for the propellor, twice the distance between the keel and the shaft at the propellor.
    Then you can work on the prop choice, but I think because of the high rpm you will need some extra advice.
    Then go an speak to an out and out inboard Hydroplane racing enthusiast, preferably one who owns a boat, as they are the experts in high speed engines, direct drive, minimum rudders, water pickups and high rpm props.
    The good news is that there are guys out there who have developed the underwater gear (the work has already been done) you need for your new boat.
    Go for it.
    Last edited by Don MacLeod; 07-17-2017 at 12:35 AM.

  13. #13
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    Default Re: Jetski Engine As An Inboard

    Quote Originally Posted by John A. Campbell View Post
    Someone tell me why the following won't work:

    ...the 12 VDC automotive type starter motor when REVERSE propulsion is needed so........
    I normally prefer to be suggesting ways around problems that are supposedly insoluble, but this time I'm dumping a bucket of ice cold "that won't work" on part of the idea.

    Starter motors generate lots of torque by drawing lots of current. That means they heat up very quickly. To avoid it's early demise, you need to adhere to a 1% duty cycle, with about ten seconds maximum run time. So you're looking at waiting at least ten minutes for the starter to cool down before your next 10 second burst of 0.5 HP in reverse.

    The end result would be similar to a large vessel trying to use it's jacking engine for reverse.

    regards,
    Joe
    These days, everything I do is just "puttering around"

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