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Thread: Inboard Jet Boat Help

  1. #1
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    Unhappy Inboard Jet Boat Help

    After some searching around the net and this forum (great resource) I still can't find a direction to go with my new boat. So here goes. BTW I am new to jet boating;

    I traded an UTV for a used Wooldridge boat (19') vintage 1978 with an inboard engine and a Hamilton 752 Jet. Not to get into the details of what happened but I kinda got hosed on the deal, basically the engine/Jet doesn't seem to have enough power/thrust to push the boat on step.

    The nitty gritty;
    The engine is an Oldsmobile 260 bored out 30 over with an RV cam (straight from a car). Supposedly when it was new it made 105 hp, even the 307 only made 140 hp. The Hamilton 752 requires 100 - 250 hp according to their manual to operate, and have a maximum speed of 5000 RPM. The boat itself I'm not too sure about yet, still getting with Wooldridge about the model since the plate on the hull is not legible.

    So I loaded up my new prized boat and fired it up in a local lake to see what it could do before tackling a river. First thing, it is quite loud. So much so that I would have to wear ear plugs if I used it for any amount of time. The manifolds are water cooled, with risers behind them which then exit through the transom below the water level. Already I am thinking something is up with the headers.

    Out of the gate at full throttle I can only reach 3100 RPM, not enough. Only about 500 ft/lbs of thrust created at those RPM's and the boat could not get on step. This is an empty boat with 2 adults in it. When I lifted the doghouse to see if anything was wrong it created slightly more power and got on step, but not quickly. Not enough power to be comfortable going up a river with gear in hopes of getting a moose. With the doghouse back down it then lost power again. The back of the doghouse is open, so heat or exhaust fumes is causing my power loss I think.

    These are my current thoughts;
    Is the current engine even worth the trouble, given it's age, it's from a car, and apparent lack of power?
    If I get a used marine engine I might be inheriting someone else's problems, should I just save up and get a new one in$tead?
    Could all my problems be in the Jet, and a simple tune up fix it all?

    So my long winded question is this, since I am not getting any RPM's should I assume the jet unit is sound and focus on the engine or should I look at the Jet first. Money isn't very plentiful so I'm trying to do this as cheap as possible.

    Any responses would be very helpful.

    -Ryan

  2. #2

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    Ryan, Where is this boat located. If in the valley I would take a look at it with you and see what we could figure out.

  3. #3
    Member broncoformudv's Avatar
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    If you can't get rpms it is your engine. That is a very small engine for that size of boat, it should have a 302 or larger, preferably a 350 or 351 in it. Have you checked the air intake vents on your boat to make sure they are not clogged up? That would explain why your engine runs better with the dog house open. There should be one on each side of your transom, they look like chrome or black miniature hood scoops. Normally one is facing forward and one is facing backward.

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    Member Jimw's Avatar
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    ....ensure your carb is opening up all the way at full throttle.
    2005 20' Weldcraft Sabre XL 350 MP
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  5. #5

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    Unfortunately the way I see it you bought the wrong boat for your needs. Even if you do get this little bugger running right it still doesnt have the power to take you and a friend moose hunting. Inboards are noisy, we wore ear muffs when running our boat.
    Couple of choices:
    a. get the motor running right and use the boat for very light lake and river duty
    b. repower it with a bigger motor and maybe swap out the jet for a 212 - $$$$ ca ching
    c. sell it as is (if you can) and consider it a learning experience.

    It can end up a nice boat if you want to put the cash and time into it. Good luck with the project.

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    Assuming that your engine is running well and producing full power then a simple solution would be to get the correct impeller for your engine's horsepower. Right now you have more pump than the engine can pull, this is adjusted by using different impellers. It's the same as having too big of a prop on an outboard, it won't rev up all the way and performance sucks!

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    Thanks for the responses guys. Big Bend, yes the boat is in Wasilla. And any help would be much appreciated. I do like the hull design and size of the boat, so I think I am going to keep this for some time.

    With that thought it sounds like the engine is a lost cause and I should start sourcing a 350. Anyone recommend a specific inboard engine, new/used? I'm hesitant to get a used one but I think with a satisfactory compression test I might. What do you guys think?

  8. #8
    Member Jimw's Avatar
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    I'm probably stating the obvious, I'm a big fan of new, nothing wrong with used. Make sure you get a marine version.
    2005 20' Weldcraft Sabre XL 350 MP
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    a marine outfitted engine will save you the time of setting up the exhaust, heat exchanger and such, but if you feel mechanically inclined and comfortable with that kind of work, a junk yard gm 350 out of a late 70's truck is all you need. yes its out of a truck, go ahead and pull the casting #'s off any marine engine and gm will tell you what car/truck that engine was designed for. the 350 i have in my 22' weldcraft is originally designed for a 1978 chevy 3/4 to 1 ton with a 4bbl quadrajet, but its in my boat with a harding marine sticker on the valve covers. jet make sure to get one with good compression and no internal rust and go from there.

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    Member winibezold's Avatar
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    sorry its Hardin Marine, they still make top of the line marine engines, just saying that all marine engines like that start their life being made for cars, boat engine makers buy them and outfit them, then put them in your boat for twice the cost!!

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    Member broncoformudv's Avatar
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    Like everyone said getting a complete ready to go marine engine is the easiest way to go about it. Used saves you a few bucks but you never know how it actually runs till you install it and fire it up. I would start checking the boating forums and calling around to KEM and such to get an idea of prices.

  12. #12

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    If raising the dog house changes the way the engine run's that much I would check for an exhaust leak. One more thing to think about is that if your going to change the engine soon I wouldn't spend much money on impellers for your current setup as you most likely will need to change them to match your new engine.
    06 custom weld storm 6 liter 212 and other stuff
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    might want to check that the distributor is advancing and check fuel delivery. sounds under powered but someone put this combo together and i bet it worked ok, if the boat is pretty light it might work. could be as simple a as a bad spark plug too. If it were me i would try and make it run right before i just bought a motor. If you can get it working for now it will give you more time to plan and find a deal on the swap you want. The two stage hamilton should be an easy conversion to three stage with a bigger motor

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    make sure you dont have a boat full of wet foam either it adds up on weight really fast.

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