[alberg30] Re: Gear Ratio

[Rap1208 at aol.com]

From: Rap1208 at aol.com

Everyone.    This is just general info:  I did a lot of reading about props,
thrust, and slip angles when I installed an inboard in my Pearson Arial.  I
installed a single cylinder Vire 7 hp two stroke.  It had a 2:1  reduction
gear box.  It swug a 12-6 prop at about 1400-1500 shaft rpm.  At full throttle
it wanted to bury the boat, but backed off she could do almost 7l in clam
water.  This was a 25 ft boat, full keel Alberg design, 5500 pounds.  So when
I hear of 12-6 props on a A-30  I think you are under proped.  When I read of
9" pitch I think you are over proped.  Also excess pitch is ONLY good in very
flat water.  Lower pitch, around 7" to maybe 8" has less slip and more thrust
in calm to rough condition.  Also the nore thrust the more of a load on the
engine.  Excess pitch will cause cavitation , and not thrust. Think of an
airplane, that uses less pitch for take off and climb, and increases pitch
only at cruising altitude.  Thrust is the name of the game, and a lower pitch
gives that to you when you need it.
Russ Pfeiffer #251    Ca Va  

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