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From: (no name) (no email)
Date: Wed Aug 01 2007 - 08:14:11 EDT
<<<<<<Does 'efficiency' translate into thrust? Would a certain diameter and
certain pitch 3 blade prop have more thrust than a 4 blade prop of equal
diameter and pitch? I am of the opinion that it would not.>>>>>
Edmond
The answer is yes. A three blade prop is more efficient than a four blade,
primarily because there are more blades and each disturbs the water ahead of
the following blade. I strongly suggest that you read Dave Gerr and all your
questions will be answered.
Cheers
Glenn.
For what it is worth Gerr's book says that "Slip and Efficiency are not
the same" (page 50).
If you follow your argument to its logical conclusion one would say that
a 2 blade prop would provide more thrust than a 3 blade prop of equal
diameter and pitch. I doubt that is true.
I hope you would agree that 3 blade props are not 'better' than 4 blade
props.
My understanding is that for trawlers one should swing as large a
diameter prop as is practical given the space and horse power available. Then a
calculation may be made to determine the appropriate pitch depending in RPMs
and power at maximum torque. Then one should determine if the chosen prop has
the necessary blade surface area so that it does not exceed allowable blade
loading (which causes cavitation). If you have not been able to install the
diameter you would prefer you may have to increase the blades surface area by
adding additional blades.
Edmond
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