Prop required for a 16 foot displacement boat.

Discussion in 'Props' started by Ramona, Jun 26, 2018.

  1. Ramona
    Joined: Feb 2007
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    Location: Australia

    Ramona Senior Member

    My mate has this 16 foot fibreglass clinker displacement boat. The engine is a Sabb 10 hp big single with a 2:1 ratio gearbox. This engine has ship loads of torque and the idle speed is about 200 and max rpm is about 1000. Looking to do a little better than the hull speed and would be happy to do 6 knots at 3/4 throttle. The problem is the maximum size prop that will fit with the correct clearance is 13 inches! Using the online prop calculations the ideal prop is a 4 blade 22.6 in diameter with a pitch of 30 inches.
    I think he should make a Korts nozzle and make a 4 blade propeller with the same size area blades as the ideal prop has. He is quite capable of making SS propellers. My suggestion to him was to machine up a hub and tack weld 4 blades on and keep experimenting till he gets it right. It's only a couple of hundred yards to the boat ramp.
    The prop on the boat in the photo has no load on it and is obviously meant for a high speed power boat. 4hp outboard on the bracket drives the boat nicely! Mate is a diesel enthusiast and will be keeping the engine.
    Any suggestions will be gratefully appreciated. IMG_20180626_142128.jpg
     
  2. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    You need to go back to your propeller calculator and enter the correct data!
     
  3. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    This engine has a continuous power output of 10 hp / 1800 rpm, ie at shaft speed 900 rpm. BTW, the power is actually taken from the camshaft, which is the reason for the 2:1 ratio. It was delivered either with a controllable pitch propeller (2 blades dia 450 mm) or a fixed one (3 blades dia 406 mm / 16").

    Edit: The square keel ending aft is ruining the flow into the propeller as is, and more so with a nozzle. It would be far better spending the effort on rebuilding (sharpening) the aft end of the keel, rebuilding (lowering) the shaft bearing and the "heel" to accommodate the correct sized propeller, than fiddling with a nozzle.
     
    Last edited: Jun 28, 2018
  4. Ramona
    Joined: Feb 2007
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    Ramona Senior Member

    Thanks for the reply. I should have mentioned the gearbox is not the stock item. The engine is out of a yacht that did have a controllable pitch propeller but this did not come with the engine. It is looking like lowering the shaft bearing and swinging a larger propeller is the answer.
     
  5. baeckmo
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    baeckmo Hydrodynamics

    /........the gearbox is not the stock item./

    Ok, then the question is: what is the gearbox ratio? If it is the standard SABB reverse gear, "Model H" (not the CPP unit), then it is a "straight through" 1:1 ratio, but if it is a non-standard box then what? The engine power output is still via the 2:1 ratio camshaft, so if the reverse unit has something other than 1:1, then you are in trouble.
     
  6. Ramona
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    Ramona Senior Member

    The gearbox ratio is 2:1.
     

  7. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Ok, since there is first the integrated ratio of 2:1 within the engine, and in series with that a second 2:1 transmission, the total ratio is 4:1. With the crankshaft rotating 1800 rpm at max speed, you have a propeller shaft rotating at 450 rpm! This is never going to work (you'd need something like 26" diameter to make anything happen)!

    The reverse transmission must be 1:1 on this engine due to its integrated 2:1 reduction.
     
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