Force 15 hp prop pitch efficiency ?

[pecheux]

Good day, I have a 14 footer fiberglass boat that only weights 250 pounds, alternatly using 2 OB's on it, a Force 15 hp and a Jonhson 9.9 The prop pitch on the Force is a 8 pitch, and on the Johnson a 10 pitch.

Using the boat with 2 people onboard the Force 15 hp as an edge in getting the boat on plane and has a faster top speed. On the other hand when I am alone in the boat I get a noticeble faster top speed, better plane, and higher RPM when the Johnson 9.9 is mounted.

Would that mean the Force 15 hp should use a higher pitch prop like a 9 pitch (which is the next higher up) ?

If not what can explain this ?? Thank you.

 

[steelespike]

Re: Force 15 hp prop pitch efficiency ?

Not knowing the year or the gear ratios Its difficult to be accurate.
It appears the 9.9 came with a 10 standard and I think your 15 would have come with something like a 10.The 8 probably is intended for a load.
and you might want to throttle back a little or you may overev the 15 when alone.
By the way the Force 15 is rated at 5,250 the 9.9 6,000 or 6,500 The Johnson 15 at 7,000.

 

[pecheux]

Re: Force 15 hp prop pitch efficiency ?

Not knowing the year or the gear ratios Its difficult to be accurate.
It appears the 9.9 came with a 10 standard and I think your 15 would have come with something like a 10.The 8 probably is intended for a load.
and you might want to throttle back a little or you may overev the 15 when alone.
By the way the Force 15 is rated at 5,250 the 9.9 6,000 or 6,500 The Johnson 15 at 7,000.

The Johnson is a 1981 (little usage) and the Force is a 1997 (still like new) I know the Johnson is equiped with the standard prop since I bought it new. I dont know about the Force since bought as pre-owned, and I dont know what the orinal standard prop pitch should be.
As for overeving I would be more concerned with the Johnson that seems to run in the 6000-6500 range when I am alone in the boat. As for the Force 15 hp I dont notice much difference in RPM from either 2 passenger or alone in the boat and it does sound like it reving in the 5000-5500 range. I have not found an internet site that would give me the standard prop pitch for the Force 15hp.

I once had a 1979 Johnson 15 hp mounted on an exactly same boat and if I remember well ... I obtained a noticeble higher top speed with it than with the Force 15 hp I am using at the moment.

So is the prop pitch a clue on the Force OB ? ... or could it be the difference in the motor brand itself ?

Tx

 

[steelespike]

Re: Force 15 hp prop pitch efficiency ?

On the OMC 81 prop chart it shows 8,9 10" pitch for general use.So your
10" isn't likely to overrev.Also list 5" for heavy boats and a 10" weedless.
Iboats lists 15hp Force props from 5 1/2" to 10 1/2" roughly the same as Johnson. The 77 Johnson catalog lists the 10" as standard for 9.9 and 15.
I would think The Force would be close with a 9 or 10" standard and with the 8" for average boats with above average load.The force probably has a different gear ratio as it operates at a lower rpm.F=5,250 ,Johnson 15 7,000.
9.9 6,000.
Its pretty obvious the Force with a 8" isn't likely to to be as fast as the Johnson with a 10"

 

[pecheux]

Re: Force 15 hp prop pitch efficiency ?

On the OMC 81 prop chart it shows 8,9 10" pitch for general use.So your
10" isn't likely to overrev.Also list 5" for heavy boats and a 10" weedless.
Iboats lists 15hp Force props from 5 1/2" to 10 1/2" roughly the same as Johnson. The 77 Johnson catalog lists the 10" as standard for 9.9 and 15.
I would think The Force would be close with a 9 or 10" standard and with the 8" for average boats with above average load.The force probably has a different gear ratio as it operates at a lower rpm.F=5,250 ,Johnson 15 7,000.
9.9 6,000.
Its pretty obvious the Force with a 8" isn't likely to to be as fast as the Johnson with a 10"

Thank you for all that precious information. It does lead to the fact that my Force 15hp OB would be under-pitched with an 8 pitch prop. There are only 2 numbers up in the pitch ladder ... 9 or 10 pitch ... which brind the next question to my mind ... which one will make the speed performance of that Force 15 hp at par with the Johnson 15 hp without a decrease in power by being overpitched ? Have I read somewhere that I could order a prop from Iboat on a trial basis ?

Regards.

 

[steelespike]

Re: Force 15 hp prop pitch efficiency ?

This is only a best guess based on the somewhat incomplete info we have.
If the 9.9 does well with the 10" it would seem the 15 would do at least as well with the 10 1/2.If you do run with 2 people often you might go with the
9". I believe to prop exchange is ony on larger more expensive props.
You might find a local shop or dealer that will let you try props if you intend to buy.