17p or 15p for 1996 Force 40 hp

[1973Chieftain]

My '96 Force 40 was running right about 4900 rpms with a 10x17 prop, and although my hole shot was not outstanding, I was getting 31 mph by my gps. This past time out I hit a rock I think which really chewed up my prop, and my only spare was a 11.25x10 that came with the motor. Now unless the diameter effects things a huge bit, im guessing that's way under pitched for my light 15' tin boat. My goal is top speed, the only thing besides cruising I will be doing is casually pulling a tube on hot summer days. Since 4900 is right at the low end of the employees range, 5200 being the high end I beleive, would I benefit from stepping down to a 15 pitch? Would that effect my top speed? Thanks for the advice, im very much a prop noob! (And I have three engines im setting up )

 

[steelespike]

Re: 17p or 15p for 1996 Force 40 hp

If that has through hub exhaust it is a Merc leg and gives you more choice on props.
a 16 is the highest listed under Force while a 17 is listed under merc.A merc 15" is the same number as the Force.
again assuming the merc leg.
If a 15" prop gets you 5300 with about 10% slip your speed would be about 33.and the 15 should do better with a load or water sports.
As far as your 10" as pitch goes down diameter goes up. You can get around with a 10" but will have to be very careful to stay within
the rpm range 4750-5250.
Your motor height can affect performance as well.The anti vent plate(just above the prop) should be about even with the bottom of the transom.

 

[1973Chieftain]

Re: 17p or 15p for 1996 Force 40 hp

As always steelespike, you are the man! Thanks! It is a 13 tooth spline with through hub exhaust. Iboats only has up to a 16p but numbers of other places list 17p props for her. The engine is mounted rather high at 2" above the keel, I messed up when I rebuilt the transom.. a project for next winter. So you would think going to the higher end of the rpm range would be beneficial? Thanks!

 

[1973Chieftain]

Re: 17p or 15p for 1996 Force 40 hp

Update: I bought a mercury 15p, and am getting 33 mph (gps) @ about 5150 rpms.... thanks for the advice steelespike!

 

[steelespike]

Re: 17p or 15p for 1996 Force 40 hp

Wow! Sometimes you get lucky. Thanks for the feedback its bound to help others.