[C320-list] Gori-Prop buyer question - Not apropmanufacturerdebate
Bruce Heyman
BruceHeyman at cox.net
Fri Oct 13 15:30:28 PDT 2006
Warren,
Excellent research and analysis - thanks!
I took the boat out (we have the Autoprop) last weekend and ran it maxed out
for about three minutes. The engine settled in at about 3350. That says
I'm down by about 6.9% which is in your "concerned" ranged. I still have to
verify that the tach is accurate at these RPM's. When do we get the group
tool box so I can barrow the hand held tachometer.
Bruce
Somerset 671 SoCal
-----Original Message-----
From: c320-list-bounces at catalina320.com
[mailto:c320-list-bounces at catalina320.com] On Behalf Of Warren Updike
Sent: Friday, October 13, 2006 3:10 PM
To: c320-list at catalina320.com
Subject: Re: [C320-list] Gori-Prop buyer question - Not
apropmanufacturerdebate
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
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