72 tri hull with 75hp force over revving

[Kevint0805]

My boat is a 1972 starcraft trihull 15' with a 1997 force 75hp it over revs with the 11p prop that came with the motor its revving at 8000 rpm but a 26p prop sounds to aggressive any suggestions and what kind of prop would be best thanks in advance

 

[SkiDad]

Something doesn't sound right. I can't see you turning that kind of rpm unless prop is slipping. What speed are you hitting at 4000. I would try a 15 pitch

 

[ondarvr]

I don't think you can get to 8,000 RPMs with that motor, so something else is going on. I'm not saying you have the correct prop right now, only that 8,000 RPMs probably isn't accurate.

 

[David Young]

Like SkiDad said, try a 15 pitch. Your Tach might be reading wrong. Your motor could blow up at 8,000 rpm.

 

[Kevint0805]

I checked the speed at wot it was 27mph but i just got the tach and it revs to 8000 i backed it down to around 5000 and im only going about 15mph if it is the prop slipping is ther a way to test the prop

 

[ondarvr]

Did you set the tach to read correctly for your motor? There should be a switch on the back, it needs to be on the correct setting or the tach will be way off.

 

[Kevint0805]

I was using a cheap tach so it could be wrong ill try a 15 and see what happens this weekend

 

[Kevint0805]

This is a picture of the tach i used

 

[SkiDad]

I have used the same tach on my boat to verify the analog tach. You have to make sure you have on the correct setting 1,2, or 3. you want it set for 1 spark every rotation. ALSO make sure it's grounded or your readings will be way off.

Try this before ordering a prop. If you give us accurate numbers the guys on here can dial you in

 

[steelespike]

Before any changes get the tach working accurately. looks like an induction tach. Usually suggested to use the bottom plug
and get the wire outside of the cowl away from stray pulses.
The 11 may be too light but you need to use the tach to help with selection.

 

[Sea Rider]

Yep, need to adjust the firing sequence of your OB to induction tach or will read way off...

Happy Boating

 

[Kevint0805]

Is ther a different tach i can get thats more accurite

 

[Sea Rider]

Is ther a different tach i can get thats more accurite

Tiny Tach, check that is firing sequence programmable...check amazon

Happy Boating

 

[David Young]

I checked the speed at wot it was 27mph but i just got the tach and it revs to 8000 i backed it down to around 5000 and im only going about 15mph if it is the prop slipping is ther a way to test the prop

My 70 horsepower 1977 15 foot tri-hull will do 35mph using a phone GPS. You should do that fast if not a couple mph more.

 

[Kevint0805]

What size prop are you running

 

[David Young]

What size prop are you running

13.25" with a 17 Pitch. Its a Solas sold here on this Forum for a little over $100.