propeller sizing

[jimmyk]

my prop has gone west. i had the measurements and it was 15 x 18 rh on a 1 1/4 inch shaft. i ordered a new prop of that size. i am about to fit it but it looks smaller than the original. smaller diameter. not loads smaller but enough. the engine is a 12 hp petter. will a smaller diameter prop make much difference do you think

 

[theoldsalt]

my prop has gone west. i had the measurements and it was 15 x 18 rh on a 1 1/4 inch shaft. i ordered a new prop of that size. i am about to fit it but it looks smaller than the original. smaller diameter. not loads smaller but enough. the engine is a 12 hp petter. will a smaller diameter prop make much difference do you think

Have you actually measured the old & new props? If not what you ordered, query with the supplier.

 

[jimmyk]

Have you actually measured the old & new props? If not what you ordered, query with the supplier.

i think its wjhat i ordered. it has the right size stamped on it. sadly i am no expert. i wondered if modern sizing was different to old. but i suppose an inch is still an inch

 

[Tranona]

That sounds an awfully big prop (and shaft!) for a 12hp engine. Suggest you use the Propcalc programme on www.castlemarine.co.uk to size the propeller for your boat/engine/reduction gear combination, or ask your propeller supplier to do it for you.

 

[peteK]

my prop has gone west. i had the measurements and it was 15 x 18 rh on a 1 1/4 inch shaft. i ordered a new prop of that size. i am about to fit it but it looks smaller than the original. smaller diameter. not loads smaller but enough. the engine is a 12 hp petter. will a smaller diameter prop make much difference do you think

The pitch sounds a lot are you sure its 15x18 and not15x8.

 

[alahol2]

The pitch sounds a lot are you sure its 15x18 and not15x8.

Agree, I've got 13 x 8 on a 2:1 gearbox with 13HP and even that is slightly over propped. I can't see a Petter 12HP swinging 15 x 18 even on a 4:1 gearbox (which probably doesn't exist anyway).

 

[jimmyk]

That was definitely the size stamped on the original one. And i got the new one off the shelf so must be fairly common size. Dunno. Its a slow old boat at the best of times. Was hoping that a diameter smaller wouldnt make much difference