prop size ? how to work out ?

thanks for that. mine is 23 x 15. previous box was 2.5 to 1. now engine overheating and wont rev much above tickover . still doing 6 or 7 kn on tickover!

are you sharing the formula for working out or is it a trade secret ?!
 
A closely guarded secret

But if you search for PROPCALC there is a bit of software to download that allows you to enter your boat and engine details and churns out the answer.
 
'Skene's Elements of Yacht Design' by Francis Kinney gives two methods of calculating prop size, and 'The Propeller Book' by Dave Gerr gives an exhausting [for the reader] treatment of it. I learned when I went to get a prop reduced in diameter, that that dimension has the greatest effect on prop performance, and so must be got right.
Peter.
 
thanks for the help.

phoned crowther's prop's to day . say they can alter pitch of prop ? apparantly the diameter { 21'' } is ok. prop is 21'' x 15.5'' prob' was that i replaced knackered 2 point five to one
box with one point five to one. boat became sluggish , overheated and was doing max hull speed at virtually tickover. also lost paddle wheel effect as couldn't rev engine.
 
I'd say that gearbag ratio is the big problem. The aim is to keep the prop spinning as slowly as possible, because marine screws are at their most efficient when rotating slowly. It might be worth while to hunt for a 3:1 box if you can. When I first bought my ketch, the gearbag was straight through - no reduction at all. It also had no reverse, as some clever-dick had welded everything together! I asked the previous owner how he had managed with it, and he said that it gave him some heart-stopping moments! The prop was an 18"x9" - very fine pitch indeed.
Peter.
 
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