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Subject: TWL: Re: Three vs. four blade props
LRZeitlin@XXX.XXX
Date: Sun Dec 01 2002 - 15:51:37 EST
In a message dated 12/1/02 5:03:21 AM,
trawler-world-list-request@XXX.XXX writes:
<< I have questions for the list. What are the advantages of three blade
propellers and the advantages of four blade propellers? Which is best suited
for 8 to 10 kt cruising? Which is the most fuel efficient? Do most of the
list use three blade or four blade and why? >>
The ability of a prop to absorb engine power and propel the boat is generally
a function of its diameter and blade area. Theoretically, the fewer blades,
given sufficient area, the more efficient. Ideally, a single blade prop would
be best if you could figure out how to do it without balance problems. With
multiple blades, each is turning in the disturbed water left by the previous
blade. Most of us run three blade props because that provides a reasonable
balance between blade area, efficiency, and vibration minimization (as the
blade passes near fixed boat structure). If you can't get enough blade area
with three blades with the maximum diameter prop permitted by hull clearance,
then four or even five blades may be necessary. The specification for a
specific boat is sort of tricky. It's best to consult the boat manufacturer
or a prop maker. For details, see Gerr's "Prop Handbook".
Larry Z
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