Next message: Steve Littman: "TWL: 1987 Albin 37 Palm Beach research"
<< I understand the 4 blade prop generates less vibration but is less
efficient
than a 3 blade.
Can anyone tell me how much less efficient?
>>
All props are comparatively inefficient power conversion devices. Trawler
props generally convert engine horsepower into thrust with efficiencies ranging
from 30% to 60%. Typically, the more efficient trawler props are slow turning
with a large diameter. Also, the fewer the blades, the higher the efficiency
but the greater the vibration as blades pass behind sections of the boat
structure. As a rule of thumb designers estimate that a three blade prop is about 5%
less efficient than a two blade prop of comparable diameter and area. The
lower efficiency is due to blades passing through water disturbed by the previous
blades. I would estimate that a four blade prop would be about 2% to 3% less
efficient than a three blade prop. For most purposes, this is a negligible
loss. If you are really concerned about ultimate efficiency, fit the largest prop
your boat will handle and reduce the pitch so that the engine isn't
overloaded. There are several excellent prop specification programs that will put you in
the right ballpark.
Larry Z
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