do these figures sound right?

Discussion in 'Boat Design' started by ENG Student, Oct 26, 2009.

  1. ENG Student
    Joined: Oct 2009
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    Location: London

    ENG Student Junior Member

    I dont know if this is in the right area but...

    Ive been designing a system to run a small electric boat at a speed of 3.1m/s (6knots).

    From my drag calculations for a surface area of 2m^2 and Cd of 0.06 i got a drag figure of 480.5N

    From the equation Power=Force*Velocity i deduced that the power required is 1489.55 Watts basically 1.5kW.
    Taking into account propellor efficieny of 60% and motor efficiency of 80%, i have found that i need a motor of 2.5kW to achieve 3.1 m/s...

    Now that seems an awful lot of power for such a speed when i have read on this forum of people achieving 3-5 knots on less than 1kW
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Given the 2 square meter wet surface, it would seem that your boat will be, perhaps 10 to 12 feet long. Think in terms of speed length ratio. For modest power input figure on something like S/L = 1.0 to 1.2 ... 6kn/sq rt wl =1.0 suppose the Wl is ~ 10 feet then your S/L = about 1.9 and the boat will be in transition stage and require much propulsive effort. I doubt that 2.5 Kw would do it. If the boat was a well designed splinter of 36 feet on the WL then 2.5 Kw might do it nicely. A planeing boat of that size might get more than 6kn with 2.5 kw but it would surely need help trying to get over the hump before planing began. More deterrent is provided by the weight of the batteries to supply that kind of power. Planeing boats of low power are very sensitive to weight.

    I think that you must lower expectations, make the boat much longer, and/or go back to the drawing board.

    Be assured that others will jump in here with better advice. And explore the thread on this site titled; efficient electric boat
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