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Calculating prop size for sailboats

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  • Calculating prop size for sailboats

    I'm currently working on an electric saildrive of my own making. My intention is to start with a product best suited for daysailers. I hear a lot about how prop diameter for sailboats that "the bigger, the better". My understanding is that the prop size and pitch should make it possible to spin the motor at the right RPM to maximize the motors top end power. Not too slow so to avoid overload and not fast as to overrun the motor.

    ​The thing I want to know is should I use a prop that is best suited to match the motors max power or should I go with an oversized prop with maybe a reduced pitch?

    ​If this helps, the sailboat would typically be up to 27ft and 5,000 pounds. The idea is not to push for hull speed but more likely 4-5 knots so as not to have to push water.

    Thank you in advance.

    Scott

  • #2
    Take the engine's gear ratio (drive shaft RPM vs. prop shaft RPM) and relate that to the propeller pitch to get your 4-5 knots of speed. Then size up the diameter to match the power of the engine. Net result is that the bigger boats, needing a more powerful engine, will have a larger diameter prop.

    Factor in about 15% prop slip to the pitch, for reality purposes.

    Keep in mind that on smaller sailboats (20 ft range) 5 knots is going to be at or slightly above the hull speed of the boat. In reality, you may also want to increase the prop pitch slightly on a bigger boat in order to match the greater hull speed.

    Also keep in mind that an electric motor has a different torque/power curve than an internal combustion motor (engine), as far as determining the best top RPM to design towards.
    Last edited by tpenfield; May 23rd, 2017, 05:55 AM.
    Best regards, Ted . . . . Cape Cod, MA

    Formula 330 Sun Sport, O'Day Mariner Sail #3224, Sunfish
    Past Boats: Catalina 22 Sail #10531, Formula 242 Sun Sport
    Twin Mercruiser 7.4 LX MPI (0F802036, 039), Bravo 3's (0F806198, 199), Mercury 7.5 HP (1969), Johnson 4.5 HP (1980)

    My Boating Web Pages: http://www.tpenfield.com

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    • #3
      understand your market

      if the sailboat is used for any racing, it needs to have enough power on board to get to the farthest point of the race and back without wind and being too heavy

      there is a reason most sailboats have a 4hp tohatsu / Nissan / suzuki / Merc etc hanging off the back to get them from their slip out to where they can hoist the sails. it weighs next to nothing with fuel in it and can be removed to move down below for the race.

      those two factors alone are why most of the sailors that I know wont ever use electric. the weight of the electric motor and the batteries, etc is too heavy, and the amount of weight needed to push the sailboat 25-50 miles is too much of a weight hit to the boat. These are guys that have switched all their lights to LED's and gone away from group 24 batteries to two SLA-12v 10ah batteries located center near the mast so as not to affect balance and because it saved 26#

      there are people that would look at electric. they want to completely unplug - however your talking 35-50' live-aboard boats. your going to be hard pressed to match the 120hp Yanmars they have or the range of a tank of diesel when going down to the bahamas
      1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - http://forums.iboats.com/forum/owner...988-rogue-2420

      Past Boats
      1970 Wooster Hellion - Merc 9.8
      2002 SeaRay 190BR - 5.0 - A1G2 - "Cheasheads in Paradise"
      1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

      What’s behind you doesn’t matter.Enzo Ferrari

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      • #4
        Sign up today
        FYI - talking to the people at the yacht club. a 2-stroke 4hp motor is 27#. an electric trolling motor is about 27#, then they have to have a 45# group 24 battery. that weights too much for regatta racing. the 4-stroke 4hp motors weigh too much

        one guy has a 20# sears game fisher that everyone keeps wanting to buy
        1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - http://forums.iboats.com/forum/owner...988-rogue-2420

        Past Boats
        1970 Wooster Hellion - Merc 9.8
        2002 SeaRay 190BR - 5.0 - A1G2 - "Cheasheads in Paradise"
        1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

        What’s behind you doesn’t matter.Enzo Ferrari

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