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  1. #1

    Horsepower at the pump.

    I don't know if any of y'all remember me, but I'm the fella wanting to put a Seadoo engine in a flat bottom aluminum boat to run shallow, rocky rivers. Here's the thread I posted some of the details in.

    Anyway, I found my engine, 650 Rotax out of a 1993 XP. Wasn't quite the engine I wanted, but I couldn't beat the price (bought the ski and trailer for 300, sold the trailer for 250, and the hull for 45, previous owner had an issue with the thru-hull shaft seal, and thought the problem was worse than it was. Engine and heck, even the hull were in good condition). Anyway, if I am not mistaken, this engine is 70 hp at the crank. Is that correct?

    Here's my question. What kind of horsepower am I looking at down at the pump? I've been working with boats for years, but all with outboards, and outboard jets. On a prop drive outboard, about 10% of the hp is lost between the crank and the prop (in the mid 80s, the hp ratings were changed - previously, the listed hp was the crank, afterwards, the listed number was what is actually available at the prop). On a modern outboard jet, around 25-30% is lost over that powerheads prop rating (therefore, a 65 jet uses the powerhead a 90 prop motor uses, and a 25 jet uses a 40 powerhead, etc).

    I know this is an impossible question, but what sort of hp is at the pump, or more specifically, what size outboard jet would be comparable?

    I'm trying to build the boat around the engine, and at this point, I have no clue what kind of power I am really working with.


  2. #2
    idoodou's Avatar
    Join Date
    Mar 2009
    the ski you mentioned will go about 45 to 50 with avg. size rider hope that helps

  3. #3

  4. #4
    I've seen that link. Actually where I got the idea from. Much of my design will be very similar to his. My intention is to build a hull with very similar dimensions (14 - 15ft long, 48 inch bottom). However, my load is going to be heavier, due to my fishing needs.

    First of all, the boat will likely be a little heavier, as I need a bit more durability for my river. My interior layout is going to be similar to his, thus the weight will likely be similar. However, I'm needing to have a 20 gallon bait tank, and I am really hoping to be able to cover the bottom in UHMW plastic, which for 3/8 thickness, will be right around 100 pounds on a 14 or 15 footer. I'm not sure what he had in the way of batteries, but I will be needing two group 27 or 29s.

    I'm not looking for much speed (although it won't hurt, as we do have some long runs), my main goal is to run shallow, thus planing the hull well is my most important need. Obviously, weight distribution will play an important role there.

    I'm trying to compare to boats with similar powerplants, to see just how much boat I will be able to work with (I will very likely be building the hull - can't find one that fits my needs otherwise). The problem is that very few Seadoo engines have been placed in boats like this. However, as far as smaller river jets for striper fishing, where the load is going to be near identical to mine, outboard jets are prevalent.

    If there was any way to see how this Seadoo engine will compare to an outboard jet, I could look at all the local jet rigs of similar horsepower, and see what kind of results certain boat achieve with loads identical to my needs.

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