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Old 14-07-2009, 07:14   #46
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Not really Alan , just quoted Nigel Calder in one of his posts
I have read more about this the last 30 years and efficiency,s of props vary a lot as you can see in the tests. I am not trying to sell anything , just trying to make you aware of huge differences between props.
This is a part of the write up by Nigel Calder about a 3 blade Gori prop

"On most sailboats even a well-matched three-bladed propeller is, at best, only 60 percent efficient. The other 40 percent of shaft energy is dissipated into the water to no useful effect. Some two-bladed folding propellers are as little as 20 percent to 25 percent efficient."
Gori three-bladed folding propeller test

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Old 14-07-2009, 09:47   #47
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Not really Alan , just quoted Nigel Calder in one of his posts
I have read more about this the last 30 years and efficiency,s of props vary a lot as you can see in the tests. I am not trying to sell anything , just trying to make you aware of huge differences between props.
This is a part of the write up by Nigel Calder about a 3 blade Gori prop

"On most sailboats even a well-matched three-bladed propeller is, at best, only 60 percent efficient. The other 40 percent of shaft energy is dissipated into the water to no useful effect. Some two-bladed folding propellers are as little as 20 percent to 25 percent efficient."
Gori three-bladed folding propeller test

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So now we go from 60% loss (your claim) to 60% efficient like I said for a decent setup.

Never admit an error Gideon........

Can you please point me to the post from which you took this quote by Calder, very interesting...

Alan
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Old 14-07-2009, 18:54   #48
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We have 3gM30's and since weight is critical on a cat i would not go any larger. Our Yanmars are great running engines. Our boat is 44' x 25' which has a lot of windage. If your boat is going to be 40' they should be lots.
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Old 15-07-2009, 02:00   #49
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Hallo Alan there is no error , we are discussing folding and feathering props here , the 60 % efficient figure is only right for slow rotating fixed high efficient propellers like you can find for large sailing yachts with max.prop rotations of around 750.
As Nigel states 60 % at best and I must assume that is for fixed high efficient props.
the quote comes from
http://sailmagazine.com/boatworks/en..._propeller/and yes it is interesting to see that a 20 HP engine with a efficient saildrive and efficient prop could in theory match a 40 hp inefficient diesel saildrive with a inefficient prop in performance.
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Old 28-07-2009, 18:30   #50
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Can't always believe what we read. I believe Charles Kanter has a section in his book Catamaran Communique that has erroneous information about African Cats.
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