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Thread: Engine + Jet = Hp lose?

  1. #1
    American Eagle73
    Just wondering the age old Q about how much HP do you think you loose with a jet as apossed to a ?rop boat,
    I mean you cant be exact, but is it alot more or are they fairly comparable?
    thnx Im sure there are alot of factors, but shoot em out!
    Thnx in advance
    Ty,

  2. #2
    dirtman
    I've always been told 30%.

  3. #3
    King Kuracz
    Your engine doesn't know what's connected to the back of it, whether it's a prop, jet, no matter, you don't lose horsepower, assuming whatever prop or impeller you have allows the engine to run up to the rpm where it makes the most HP.
    You don't lose HP, you lose efficiency.

  4. #4
    Wet Dream
    HP is HP, efficiency is another question.

  5. #5
    Taylorman
    Originally posted by King Kuracz
    Your engine doesn't know what's connected to the back of it, whether it's a prop, jet, no matter, you don't lose horsepower, assuming whatever prop or impeller you have allows the engine to run up to the rpm where it makes the most HP.
    You don't lose HP, you lose efficiency.
    Id have to partially disagree with that statement. I don't know about losing hp in a jet but you do in a car.
    Example, measure the hp of your car engine on a dyno then measure the hp at the rear wheels on a chassis dyno. Will the results be the same. No they won't.
    I guess you could say that is a matter of efficiency, non the less, hp at the rear wheels, prop or impeller, whatever the case may be, will be less than than at the crankshaft.
    I doubt however that you lose that much hp on a jet cause there is no gears, driveshafts or anything like in a car or I/O. I can't imagine losing 30%.

  6. #6
    Wet Dream
    If you put the engine on the dyno you have engine hp, if you put a car on a dyno and measure at the rear wheels, you have RWHP. At the prop you have shaft HP. If you want to measure efficiency rates, wouldn't thrust be the best measurement?
    And 30% is a bit far fetched.

  7. #7
    Taylorman
    I can see the future and I know where this thread is going, to a pissing match.
    Before it goes there, i think its best to clarify what the true question is, the difference in efficiency between a jet and I/O.
    So here is the true question. Same boat, engine, weather conditions and all other variables fixed. what is the difference in speed between an I/O and jet.
    My boat does 66, with a 455 and e pump. How fast would it be with a Bravo lower unit instead of a Berkeley jet?

  8. #8
    Wet Dream
    I hope this isn't going into a pissing match. I'm not trying to head it in that direction. But here is the question...
    "Just wondering the age old Q about how much HP do you think you loose with a jet as apossed to a ?rop boat."
    You should lose more HP at the prop with an I/O compared to a jet.

  9. #9
    mickeyfinn
    I agree with the above statements, you do not lose HP you lose efficiency. Or the other way of looking at it is how much HP delivered to the water do you lose. I don't know that you can come up with an exact number but I do know that with the exact same setup you will almost always get more top speed from a I/O than you will a jet. The jet will usually scorch the I/O in getting to whatever top speed it is capable of. Don't know if the losses in speed are due to mechanical differences (I wouldn't think so) but suspect that the bigger difference is the fact that gearing changes and the large number of prop surfaces that can be changed to make the I/O or V-drive more efficient. The impeller in a jet can be reasonably compared to the prop on other boats. In a jet that diameter is fixed by the bowl diameter and in a prop boat the pitch and the diameter can be adjusted to optimum sizes.

  10. #10
    American Eagle73
    Im not trying to start a shit fight here or any thing, I just thought becuse of the big pump n shit alot of power goes into that..
    Ty,

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