Propulsion position - sailing yacht

Discussion in 'Sailboats' started by ToMeK, Feb 28, 2018.

  1. ToMeK
    Joined: Nov 2006
    Posts: 21
    Likes: 1, Points: 3, Legacy Rep: 10
    Location: Zagreb, Croatia, Europe

    ToMeK Young naval architect

    Dear everyone,

    I started working on sailing yacht concept, my first project. For the purpose of making the GA I need to decide about Propeller position, in longitudinal direction. I know that usually on smaller sailing yachts prop is saildrive or shaft type and postioned under stairs. Also detailed design CFD analysis can be done in order to capture the flow and put the prop in best position, but it is to early for this.

    Are there any good guidelines or some empirical expression to help decide on position?

    Propulsion will probably be electrical saildrive, so position is not restricted by huge engine placement.

    Thank you :)

    Kind regards,

  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    The footprint for electrical is still quite a bit bigger than for a diesel due to the present lack of systems integration in the smaller units and the desperate need to maximize efficiency. And plan on needing to access to everything more often. So you can't just pack everything in solid. And while a fuel hose is cheap and can be any length you like, the primary circuit wiring for the e-drive is big, heavy, expensive, and lossy, so the power source needs to be near the motor. Figure on about twice as much floor space and equipment volume allowance for about 1/2 the power.

    As far as prop position goes, it is more efficient to operate the the prop in the hull's wake, where the induction velocity is lower. For a convention hull shape, a prop located at about 96-98% of the WL gets best results. This location can gain 3-5% in propulsion efficiency above a prop in free-stream conditions. The hull also smooths out the inflow perturbations. On small boats, say 30' or so, you might not be able to go this far aft because the prop will pitch out of the water in waves. So you might need to scootch it forward some more to get it deeper and keep it from ventilating.
  3. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    For the same power an electric motor and batteries take more volume than a diesel engine and fuel tank. If you can afford the extra cost, the lower noise levels and lack of fumes are attractive. Also, V-drives allow positioning the motor aft of the propeller if necessary.
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