[C320-list] Prop question

[Scott Thompson]

Sorry to bring up religion, but I've not been very satisfied with the 
Gori 2 blade folder I installed on Surprise this Spring.  I have not 
been on the water much, but when I've been out, there has been a lot of 
vibration and it seems like the engine is working too hard.  I have the 
16.5in diameter 11in pitch model, RH drive.  This is what was 
recommended to me by AB Marine, who arranged delivery after I ordered 
the prop from Defender.

I replaced the Catalina factory 3 blade fixed prop.  I have a 1999 boat 
with the Yanmar 3GM30F.  Yesterday doing sea trials on the Magothy River 
I found I could get the engine to 3300 RPM under no load, suggesting my 
tach is probably reading a little low.  However, when under way I could 
not get the tach past 2700 RPM, and there was severe noise and vibration 
at that RPM.  With no obvious current the GPS was reading 6.9-7.0 at 
this time, with the knotmeter (not recently calibrated) reading a bit 
less, as it usually does.  The vibration is significant at anything 
above 2500 RPM, and the engine temp starts climbing over 180 too.  With 
the fixed prop I could get the tach up over 3000 RPM without any 
problems.  I had thoroughly cleaned the prop just before this trial. 
The bottom is not pristine, but was painted just before launch in May, 
so isn't terrible either, with a bit of light slime only.  I was towing 
a rigid dinghy in light chop, with wind abeam at the time, and so did 
not expect to hit full hull speed.  On the other hand, I never had 
problems hitting max RPM while towing the dinghy with the old prop, 
except when it was severely fouled.

I think I'm over-propped.  Any suggestions as to what my next step 
should be?

Scott Thompson
Surprise, 653