Prop change


New member
3 Apr 2002
Stalham, Norfolk (boat)
OK, now I've got more HP to play with I'm thinking of changing the prop. Now I can use one of those prop calc thingys to get the size but is it best to go for same dia, coarser pitch or same pitch larger dia?
Whats the pro's and con's of each one, if any?


Well-known member
12 Sep 2001
Home - Southampton, Boat - Gosport
Since no-one who actually knows anything has replied, here's my 2p'th.

Prop calculation software can work. I got one from Castle Marine which gave the same answer as a posh one that someone ran for me at a boat show, but it's limited to a basic 3 bladed prop. If you can't find it to download, pm me and I'll email you a copy.

From what I've gathered, much of it from here, a bigger diameter with finer pitch will give more thrust for punching a heavy sea - or stopping in a hurry, while a smaller diameter coarser pitch will work OK at speed, but will be less good at low speed. The bigger, finer pitch will cause more drag than the smaller coarser one under sail.

There's a minimum clearance between prop and hull to avoid problems, which I think is about 10% of the prop's diameter. If you're thinking of buying new, get the supplier to check your calculations, and if you're anywhere near the 10% clearance minimum, make sure about what it should be, don't take my word for it!

Good luck!

PS I'm on the lookout for a 15X9 prop for a 25mm shaft, preferably RH, but I'm told by a tame mechanic that my gearbox isn't fussy which way it turns.