[Public-list] Prop Size

[richard.hazlegrove at wachovia.com]

Al and Roger, 

You need to determine your transmission reduction ratio in order to 
compare apples to apples. 

I have a BMW 12 hp diesel with a 2.7x reduction gear.  This gearing 
produces less than 1,100 prop shaft RPM at optimum engine cruising speed 
(80% of max RPM).  This produces a relatively slow shaft speed with a fair 
amount of shaft torque and allows for a big prop with lots of pitch, 
13"x14 pitch.  I seem to get away with the 13" prop which is close for our 
appature probably because it isn't spinning very fast.  In fact,  I can 
note some minor cavitation noise if I push the engine RPMs much beyond my 
2,900 engine RPM cruising speed.  I suspect those whose transmissions 
produce faster shaft speeds are the ones that suggest that 12" is as big 
as you want to go for our boats. 

I can get about 5.5kts in flat water with a clean bottom,  about 5 kts. 
with a little wind and chop and only about 3.5kts-4kts.  when the seas get 
above a few feet and the wind over 15kts or so.   That's when an extra 10 
hp would come in handy.