Calculating Propeller / Reduction: HP or Torque?

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

  1. Emmett Smith
    Joined: Apr 2014
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    Location: Clayton NY

    Emmett Smith Junior Member

    Forgot to mention that calculation was with a 1.9 reduction, the original had none. So its not far off, if I am understanding you right?

    Really who knows what the hp rating of the original engine was based on. Also, the original prop was variable pitch. I don't have that prop, unfortunately. What I do have is a 20 X 16.

    Interesting idea... what do you think would be the proper hp to choose?

    The sidewheeler sounds like a pretty interesting project... wow. So you are putting stabilizing outriggers outside the sidewheels, and trying to make them look right? Have you considered wheel covers? Lots of river boats had the sidewheels covered. If you did that you could hide the floats under the covers. Wheelcovers were often very docorative, too.
     
  2. Rurudyne
    Joined: Mar 2014
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    Location: North Texas

    Rurudyne Senior Member

    You might try using what Eric Sponberg calls the "B" version of Gerr's power equation, which he gives in his essay "The Design Ratios" here: http://www.sponbergyachtdesign.com/THE DESIGN RATIOS.pdf

    This is ...

    SLR = V/LWL^(1/2) = 2.3 - (((displacement / SHP)^(1/3))/8.11)

    V = speed in knots
    LWL is in feet
    displacement is in pounds

    I've been using the WebCalc 2.0 a lot: http://web2.0calc.com/ (it will solve for "x").

    According to Sponberg, Gerr stated this formula was more accurate than the one he gave in his propeller book for speed length ratios under 2.

    That should get you into the ballpark. To take that and then find our what rpm works with your prop for the desired speed and, combined with actual engine data, you can come up with your gear reduction for that prop. You may want to hire a NA, though, in the final effort just so there are no costly and/or annoying issues. I only humorously suggest bribing one to do the job for free by promising to let him study rare hulls under your care.



    As for my own mad ideas, credit inspiration from a collision between what I'd read about stabilized monohulls, fast ocean going (and beautiful) sidewheel steamers, and the mission statements of Phil Bolger's State cruisers. Eh ... also the Great Loop.

    I believe that the former, while still producing a comfortable boat, could solve some of the issues with waves and rolling and the relatively light hulls and shallow drafts the combination would permit works well with the latter mission statements. I also believe that, for a smaller easier to build boat, some boxiness isn't something begrudged current American paddleboats, and may even be expected by some.

    In the extreme that last idea had me come up with a 99' two story houseboat for sheltered waters only ... a bit too tall to do the Great Loop. I also have a far prettier and also utterly-unaffordable-to-me model of a 118', 36 ton cruiser (a very lean 9:1 LBR for the main hull) with a raised pilot house up front and gobs of deck space over the cabin (to put under a bimini, of course) that I never quite got supporting wing or wheelhouses on (I may go back now, though). Power requirements for the latter, per Gerr, to cruise at 12kt would be modest indeed. The latter would have as little climbing up and down while inside as possible because mine is a family of weak knees (and backs) plus one handicapped little sister.

    Generally, though, I've tried to make MUCH smaller versions look okay.

    While various features have fallen into place one frustration had been the wing structure: far too modern whatever I had contemplated.

    Then I recently recalled the bridge-like hog trusses used on actual paddlesteamers and wondered if some designer back in the day, were he to try the outrigger idea for either extra stability or to try for better behavior in rough water, wouldn't have used the same to support the add on hulls. It seems likely that if one had done so that the kings post used to support the heavy wheels would have also been absorbed into some cross-form superstructure. IF that had happened and IF it worked they could have become quite common, at least until the civil war. I figured that, in this alternate history, since many blockade runners were either either tired, or overloaded, or lightly built there could have easily been some infamous wrecks to ruin the reputation of such craft. At any rate, it is just a believable enough a "what if" story ... I love telling stories ... to provide a reason for it to appear on a boat like what I'm dreaming up now.

    As far as the cover is concerned, I'm actually thinking less is more. The amas already obscure the wheels so maybe adapt something a bit like a 1920's car fender with some small length of "running board" both fore and aft? In a relatively big boat built on these lines (probably bigger than the 118' foot version) these "running boards" might be a nice place to stow launches or lifeboats.

    Next I'm going to try to shrink it further from 40' to 26' ... getting closer to what I might be able to afford. I'm actually leaning a bit towards a scow-like hull in an effort to get more accommodation with the cabin sole lower down and with lower building cost than options like a true trimaran or catamaran.
     
  3. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    I would check the log book for the past speeds and engine RPM.

    If the speed for cruising was as you desire the fuel burn will give a great clue to the HP the engine was producing at that RPM that you need to match..

    Older diesels were not as efficient as todays electronic monsters , so figure each Gal per hour burned was about 12 HP.

    Older gas engines seldom made 8 hp per gal per hour.

    Many do much better, but it could be a good guide to the hp required to go at the PROPELLOR rpm of the old setup.

    Engines sadly seldom match the HP/rpm required as most use off the shelf truck or lawn implement engines.

    Have any speed /rpm/ fuel flow data as she ran?

    First you guesstimate the hp required at each speed.

    Then you figure the biggest diameter 3 blade prop you can swing quietly .

    Then you match the engine power required with the prop requirements , selecting the gears to ease the match.

    Gas or diesel most engines are most efficient (fewest GPH pet HP) near the peak in the torque graph.

    Skeens elements of Yacht Design, for the math.
     

  4. jonr
    Joined: Sep 2008
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    Location: Great Lakes

    jonr Senior Member

    I agree with others - if the old 28HP engine worked fine and calculations show a need for 53HP, you probably want to use 28 HP. Or at least investigate more.

    A 100HP engine producing 28HP might get 6% worse mileage than a 60HP engine producing 28HP.
     
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