T&T: propeller question

[Capefearboat at aol.com]

<<<<<<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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