[C320-list] Gori-Prop buyer question - Not a propmanufacturerdebate

[Warren Updike]

I recommend: "Propeller Handbook," by Dave Gerr, International Marine 
Publishing Company, 1989, as a reference for understanding propeller 
selection.  In the introduction he states, "Actually, all propeller 
selection is a process of approximation and estimation."  If you have the 
engine manfacturer's power curves, they can be useful in understanding the 
operation of your engine, and in selecting a proper propeller.

On the matter of top RPM, on page 6, he says, "If your engine is reaching 95 
percent or more of its top RPM, the propeller is probably sized quite well. 
If you are unable to reach 90 to 95 percent of the top RPM, there is reason 
to be concerned."  The reason for this being that the engine mfgr.s top 
rated RPM is based on ideal conditions.  It is more likely than not that 
your engine will not attain the stated or specified max. rated HP in actual 
use.  Therefore, it is better that your propeller power curve cross the 
engine power curve below the max.rated RPM.

The following are my own observations:
I have a two blade 15 X 11 on my 1994 C320 with the Perkins M30 engine and 
Hurth 2:1 reduction reverse gear.  To my eye, it appears that the blade area 
of a 3-blade prop (i.e. Catalina stock unit,) is somewhat greater than that 
of my two blade prop.  That being the case, to keep the same power function 
as the two blade prop relative to engine power, it would seem logical to me 
that I would need something smaller than 15 X 11 when changing to a three 
blade prop.  Infact, Gerr includes the measurement of "blade area" with 
pitch and diameter:  "Choosing the correct propeller pitch, diameter, and 
blade area will insure that power requirements of the propeller match the 
engine correctly."

Before making the selection, I would want to conduct a few trials with my 
current configuration to determine speed at various RPM levels and under 
various sea conditions.  Then, compare this to the same trials with the new 
prop to be confident that it is correctly sized.  The new prop should allow 
you to attain the same max. RPM as the old, and give you more speed and 
better acceleration in flat water as well as in rough.

Other factors you may want to consider are degree of prop "walk," and 
performance in reverse.

Warren & Pattie Updike
C320, #62, 1994, "Warr De Mar"
Frog Mortar Creek, Middle River
Chesapeake Bay