Calculating Propeller / Reduction: HP or Torque?

Discussion in 'Boat Design' started by Emmett Smith, Apr 10, 2014.

  1. NavalSArtichoke
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    NavalSArtichoke Senior Member

    And save a lot on fuel costs to boot.
     
  2. Emmett Smith
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    Emmett Smith Junior Member

    Diesel, yeah. I have thought about that, too, but decided not to go that route. I know about and have access to gas engines, and could not really afford to buy a decent diesel. I also can't stand the smell, which inevitably and permanently permeates a diesel powered boat. Especially if you buy a used engine. This is not a big boat, and the engine is basically in the cabin.

    It would be cool to have a powerplant that was much lower power, like 50hp or less, but that is not really available in gas.
     
  3. Rurudyne
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    Rurudyne Senior Member

    People have suggested variable pitch props, right?

    But beyond that I'm curious what some of the more informed folks around here would think of working on the assumption that the original prop and engine combination were close to what might work best for the boat, assuming of course it was well designed. By this I mean starting with a modern prop of similar functionality to the original, taking into account the desired performance, and then selecting the new engine based on it?
     
  4. Emmett Smith
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    Emmett Smith Junior Member

    Yeah, interesting question. The boat was designed by Morris Whitaker, one of the foremost designers of small powerboats at the time. But, the gasoline engine was also very new technology. The original motor was a 4-cylinder 2-cycle, 5" bore 5" stroke, 28 hp at probably 800-1000 rpm. The original prop was 24 X 30 reversible, and the boat made 11 knots.
     
  5. Rurudyne
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    Rurudyne Senior Member

    Yeah, sounds like one of those early torque happy, low revving, real-horse-not-pony engines to me.

    One thing I would question is was the old engine's manufacturer doing what seems common practice these days: citing maximum power rather than what you normally get. If the latter something like John Deere's 25 HP may suffice (with some gear reduction) driving a similar, or even the same if you still have it, wheel. They claim ("claim"), if I recall correctly from recent reading, 18 HP per gallon per hour for these.

    EDIT: does the original motor, if you have it, run at all? If you do it does something you might try to help you know what gear reduction to use would be to have it run on a dynamo once you have it out. At least it may give you an idea of what's needed even if it isn't performing to par. Also, unless it's totally seized up I hope you're not discounting finding a buyer who likes period correct everything and loves doing restoration jobs. Or maybe a boat museum. Whatever your preference.
     
  6. jonr
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    jonr Senior Member

    > Diesel, yeah. I have thought about that, too, but decided not to go that route... the engine is basically in the cabin.

    Last month I was visiting friends in the hospital. Their gasoline engine boat blew up and they were recovering from the burns.
     
  7. Emmett Smith
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    Emmett Smith Junior Member

    I wish I did have the original but I don't. Even if I did I would not use it, that engine was from sort of the first phase of gasoline engine development. Every large town had a foundry that were making basically the same two-stroke engines in blocks of one, and the more power you wanted the more they bolted on. This was a 4-cylinder. It was made in the same town as the boat, Watertown, NY. There are only a few Watertown engines left, none that big. They were really pretty poorly-performing engines, in all likelihood. The boat was repowered in 1912 with a four-stroke T-head from Sterling Engine Co in Buffalo. That would be a decent motor, but I won't find one cheap!

    I actually am curator of a boat museum, and generally someone who likes things correct. But this is a long-term project, and step one is get the boat back in the water.

    I don't know of the John Deere, I would be curious about other gas engines for marine use you would recommend in the 25-50 hp range.
     
  8. Rurudyne
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    Rurudyne Senior Member

    About the John Deere, I just mentioned it because I'd recently read its brochure while seeing what engines might be usable for a boat design I'm tinkering with (presently a 40' houseboat, 5-to-1 LBR, shallow draft sidewheeler with stabilizing amas flanking the wheels and a traditional launch-style house whose roofline follows the sheer, to support the amas I think a bridge-like hog truss of painted riveted beams (aluminum) rotated 90 degrees might look believably Industrial Age had someone had actually tried to use outriggers to help reduce rolling issues for side wheelers all those years ago ... I too want to at least stab at "believable" if not correct).

    The engine I mentioned falls at the low end of claimed fuel economy for their marine diesels, one larger size, can't remember which, has about 21 HP per gallon per hour claimed (can't stress "claimed" enough) ... which isn't bad at all by any measure.

    I'm not sure how much heat a small diesel throws off, which I'm guessing is the concern for a boat that has its engine in the cabin.

    Humorously: considering whatever belts or exposed moving or hot parts there must have been we may lump these old boats into the category of yet another thing kids managed to survive before mandated warning tags on everything. ;)
     
  9. Emmett Smith
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    Emmett Smith Junior Member

    Ok, so here is my very basic question: If I want to size a prop to take best advantage of my engine at partial throttle, not WOT full load, do I choose a prop that is bigger or smaller than what is recommended for WOT/ full load?
     
  10. jonr
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    jonr Senior Member

    Pick a prop with more pitch to achieve higher loads at lower rpm. A calculator should work if you enter the HP and RPM that you want to use (vs what the engine is capable of at max output). Stick to ~80% load or less and the engine will never exceed the torque seen with a more standard prop selection.
     
  11. Emmett Smith
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    Emmett Smith Junior Member

    Right, but is higher load what I want? If I size a bigger prop for lower rpm will that make the engine top out at that point at WOT? I shouldn't need full power to achieve my speed.
     
  12. Rastapop
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    Rastapop Naval Architect

    You should be running your engine most of the time at the rpm where it's most efficient (check with the manufacturer) - more power output, less fuel burnt.
    The power output it gives at the back of the boat should be the power needed to drive your vessel at the desired speed.
    If it isn't you should consider a different engine.

    After you have the correct power you can worry about prop size and gearing for prop rpm.
     
  13. jonr
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    jonr Senior Member

    Yes, for better engine economy
    Maximum achievable RPM will be a little more than this target operating RPM.
    Correct, but don't worry, such a prop will not allow you to achieve full rated power output no matter what you do with the throttle.

    A *very* rough guess is that an engine will deliver best engine economy when delivering about 1/2 of its rated power output.
     
  14. Emmett Smith
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    Emmett Smith Junior Member

    Ok, so plugging in 92 hp @ 2600 rpm, which is something between peak torque and peak hp, I get a 20 X 16 prop. The calculator says I can achieve my target speed of 11 kts with 53 hp, about half of the possible engine output.

    Does 20 X 16 seem like a good size? If I plug in max rated hp/rpm, I get a recommenced prop of 18 X 13.
     

  15. Rurudyne
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    Rurudyne Senior Member

    If the old set up got upwards of 11kt with a 24x30 on a bit more than half the power of the new engine I'm guessing that something is amiss with these assumptions so far or else that slip with the original set up was very favorable.

    Or could it be that the old engine's manufacturers were just quoting the actual expected power for hour after hour use rather than a maximum power that can't be sustained but which can be had for a while?

    If you want to keep the old prop you may be able to find the approximate gear reduction needed by plugging in different RPMs for constant HP till it recommends something like what came with the boat.
     
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