Motors for 40' Cruising Cat

[fastcat435]

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

Greetings

[Nordic cat]

Originally Posted by fastcat435

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

Greetings

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

[Mustang Sally]

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.

[fastcat435]

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.
Greetings

Gideon

[KGP]

Originally Posted by fastcat435

the quote comes from
http://sailmagazine.com/boatworks/en..._propeller/and ...

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.