Direct drive inboard.. Affects of added speed?

Discussion in 'Powerboats' started by elwesso, Sep 4, 2007.

  1. elwesso
    Joined: Nov 2006
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    Location: Indiana

    elwesso Junior Member

    Im thinking about picking up another project. At my marina, they have an old correct craft. Its the same style as the ski tique (and for all intents and purposes the same boat), however its older than that (but fiberglass). Probably about 17ft or so, sorry about the sketchiness of the details.

    Anyway right now its equipped with (I think) a 327 with 210HP. I have no idea how this boat performs, but judging by its size im sure it moves along nicely with that power.

    I talked to a person and they had a boat with a similar setup. They had a motor that had about 150 more HP that they swapped right in. They said the biggest problem they had was that the back end of the boat wouldnt stay in the water. I guess thats because of the shaft angle, trying ot get the boat on plane at a certain speed and if you increase speed it will keep trying to force it down. He said that he eventually caught a wave wrong, the boat did a 180 at high speed and threw him from the boat, and that was the end of that boating experience.

    Im thinking about doing something similar to this. Im wanting to bump up the power to about 300-320HP. So lets just generalize here. Assuming I did that, what sort of issues am I looking at. I know that really im only gaining about 10MPH MAX (maybe more like 5-8, assuming 12HP/MPH)...
  2. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    Hi elwesso,
    Before you even consider the possibility of engine swapping, you really need to get a feel for how the boat performs with what it has now.
    A ski boat is not designed for killer top speed. Its hull shape is optimized for several things, among them:
    - Quick rise to plane with minimal bow rise and no ploughing
    - Small, smooth, non-turbulent wake from about 25 to 50 mph
    - Very good directional stability from 25 to 50 mph
    Add a few more criteria to provide a good experience for the guy on the business end of the towline, and most ski boat builders end up with a very shallow V (5-10 degrees), mid-mounted engine with straight shaft drive, spade rudder, and sometimes a few tracking fins on the keel. The result meets the criteria well in the skiing speed range.
    Now take this thing up over 50 mph (damned fast by most standards). Lift increases and the boat rides higher out of the water. Suddenly the balance of forces on the planing surface, which is bloody complicated to start with, is disturbed in a way the designer never expected. The upward force at the entry increases, the entry point moves aft, the downward force at the aft edge increases, and the hull is no longer supported as it was intended to be. The lateral force balance is thrown out of whack as a result, and now a beam wave that would have been manageable at lower speeds is too much for the faster boat's reduced (and imbalanced) lateral resistance.
    In short, it spins out.
    The bottom line is, skiboats have big engines to provide huge torque at low speed for getting the skier up and keeping his/her towline at constant speed through the slalom buoys or ramp approaches. Trying to get them to go much faster than skiing speed puts them beyond their design envelope, resulting in unstable behaviour.
    You'll have to test the boat to be sure, but my bet is it will go like stink with the 327 ci motor and would be unstable and dangerous at speed with the extra hundred horses.
  3. elwesso
    Joined: Nov 2006
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    Location: Indiana

    elwesso Junior Member


    Thank you for your reply. I know this boat does not have any tracking fins on the keel so I would assume that could only make things worse. Would adding things like that help?

    I dont want ultra high speeds.. 50-60 would be adequte I think. Mainly what IM interested in is the engine swap itself...

    From what Im getting from you, and I dont know why I didnt think about it before, is as the boat rises out of the water, the water that was giving it lateral stability is no longer present, therefore it doesnt have very good stability.

    Therefore, would it be fair to say if you could keep things the same by adding ballast that would help? I guess the question would be is if the hull could handle the beating...

    do you have any suggestions on what might be a good chassis to start with to do say 70MPH? I dont really care if its a stern drive, or inboard, i just want something thats relatively cheap to pick up sans motor. I have access to plenty of stern drive boats, like old century's, cobalts, and so on (V or tri-hulls).

    Again, thank you very much for your reply.
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