[Public-list] Propeller needed

[Don Campbell]

Joe;
  If you have Excel, and would like a spreadsheet to calculate prop size contact me
off- list. I have a share-ware version that is fairly accurate. The main thing to
remember is that hull speed is about 10.5 feet per second for an Alberg 30, and
after that props or motor size make very little difference to speed. If you have a
heavy motor that increases wetted surface  area, or sets the back end into the
water, that may slow things a bit. It is a question of matching the prop to the
motor so that the torque, horse power, RPMs,  and fuel efficiency are maximized for
the motor - not the boat. You need to have the torque, horsepower curves and fuel
efficiency (if available)   for the motor you have in your boat in front of you to
properly calculate the prop that is the best compromise for your motor. With a
diesel, you really need to be on the torque curve where it starts to flatten out to
avoid lugging  the engine.
    As C.B. has said, motor sailing helps a lot and upwind, will often increase
speed by half a knot and decrease fuel consumption by a considerable amount.
     Michigan Wheel also have a design they call MP2 which gives a much greater
surface area than the standard prop. We have one on a Canadian boat and the skipper
says it affords much greater boathandling capabilities, particularly in reverse,
than he had with a regular prop.
    There should be a discussion on props in the archives about 13- 16 months ago.
    Rules of thumb,  although some sources may vary a bit:
1" of diameter = 2.5 inches of pitch.
2" of extra pitch will cut RPMs by 450.
Don.
#528

FITZONELOVE at aol.com wrote:

> One question regarding the 13' prop; my aperture is 13.5," which leads  me to
> believe I can only use a 12" prop to allow the necessary clearance.   Do
> other A 30's have more room?  I'm currently swinging a 12x12, but would  like to
> go larger, as I have a Yanmar 3GM30, and feel I am underpropped.
> Thanks,
> Joe  #286 One Love
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