[Public-list] Prop Size

[sousa, stephen (ENG)]

Richard,

I find the same with the Yanmar 13 HP, I reach a point which is approx. 2800
RPM after that cavitation begins and I see a drop in hull speed. The bottom
line is that owners with 20-29 HP have the extra punch to push through the
winds and chop at a greater hull speed. These boats are heavy by today's
standards and were underpowered by various owners choosing a 13-15 HP
diesel. 

I would love to have the 3GM or 22 HP version which I feel the boat requires
in certain conditions.

Stephen 
#114 

-----Original Message-----
From: public-list-bounces at alberg30.org
[mailto:public-list-bounces at alberg30.org] On Behalf Of
richard.hazlegrove at wachovia.com
Sent: Wednesday, September 21, 2005 4:29 PM
To: public-list at alberg30.org
Subject: [Public-list] Prop Size

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. 
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