Underpropped or other problemms?

[Jeffromxer]

I have a 20 ft pontoon with a 97 Force 75 ELPT. Overall my prop performs well but I cannot go WOT or the engine will just rev up after reaching about 5500 rpm. It feels like the prop is slipping. I don't think it is popping out of gear as I cannot feel it pop out or back in as I throttle down slightly. I have an Amita Solus 12.1x9 prop on it. I have read that I should have a 12 1/4x9 on it. Would that slight difference in diameter cause it to rev up at WOT? Or am I under propped? Thanks in advance for your advice.

 

[steelespike]

Sounds like your prop is ventilating.Is it a 4 blade?
What is the max rpm rating on the 75?
Have tried testing with more and less weight at the stern?
I think the 97 Force uses a merc leg so merc props should fit.
If your at part throttle at 5500 you may want to increase pitch
one step.
The merc Spitfire 4 blade may help with the ventilation.
It is said the spitfire doesn't require a pitch reduction when rpm is correct.

 

[Jeffromxer]

The prop I have on is a 3 blade. The motors max rpm are 5250-5500. Putting more or less weight at the stern makes no difference. How do you tell if the prop is ventilating? I have had it come out of the water and rev up in very rough conditions. This is different and will happen every time in calm or rough water. Could it be the prop slipping? When this happens it revs very fast and supplies no power to the prop. Feels like you are revving in neutral. Then I back off the throttle slightly to obtain my max rpm on my tach. When all this happens it is very smooth with no jerking or noise other than the motor revving. Thanks steelespike for the help.

 

[steelespike]

You can check for a spun hub by putting a water proof line across the hub and body and check the line after it "ventilates."
Usually a spun hub will gradually get worse to the point of being useless.

 

[Jeffromxer]

I marked the prop and ran it. The hub is still in the same spot so I don't think it's spun. While duplicating the problem, I noticed that it would only reach 6000 rpms If I Left It Full throttle. I must be underpropped. What size prop would I need to get my max rpms in the correct range? Also I was wondering if adding a hydrofoil would benefit a pontoon at all. Either with fuel consumption, ventilation, cavitation, or performance.

 

[jestor68]

Yes, you are way under propped.

You should be running a 13 or 15 pitch prop.

A 4 blade prop would be better at resisting ventilation.

Your WOT rpm range is 4750-5250.

 

[Jeffromxer]

I got my new prop and ran it. I opted for a 3 blade 12 pitch, fearing if I went much higher I would be lugging the motor. My wot rpms are now at 4900-5000. The prop performs well. I am having trouble with the prop coming out of the water when hitting other boats wakes. My motor is as low on the transom as it will go. Does anyone know if adding a hydrofoil would help with this? I don't want to drill on my cavitation plate for nothing. Thanks everyone for the help.

 

[buckinrut25]

I have found very little use for foils other than helping under powered boats get on plane. With your set up I don't think it is nessasary. How deep is the cavitation plate while underway? With my pontoons I find with the plate at least an inch under most of those issues go away.

 

[steelespike]

Have you tried to fine tune the trim.Usually on a pontoon trim adjustment can sometimes help venting problems
and little else.May take patience fiddling.
While I consider a hydrofoil a bandaid over a problem. You could try it,I would use doelfin If your handy an aluminum plate
attached on top of the anti vent plate may help.Don't worry about the holes if you have to remove it.
Usually a 4 blade can help in your case an 11". If there is a choice pick the largest diameter.
You might try a search for pontoon props.

 

[jestor68]

I have seen a number of testimonials/reviews where folks put the Doel Fin on their pontoon boat motor to help reduce ventilation issues; which is what you are experiencing.

They reported that it greatly reduced or eliminated the issue. It appears that the Doel Fin's position on the AV plate serves to cover the prop and helps to keep air from being sucked down into the propeller as the boat pitches, bringing the prop closer to the surface.

 

[Jeffromxer]

I am considering trying the doel fin foil that was recommended. With it being fairly inexpensive, it is worth a try. I will let you guys know how it turns out. Thanks

 

[midcarolina]

I would think adding a foil would make the ventilation worse due to the extra stern lift.......

 

[Jeffromxer]

I talked a buddy into letting me try out his hydrofoil. It was a stingray classic. midcarolina was right. The problem was noticeably worse. I will be taking it off and trying to fine tune the trim as steelespike suggested. Thanks everyone for the help.

 

[Jeffromxer]

I took the hydro off and also switched back to my 9 pitch prop. The ventilation problem is gone.(Except for the rare monster wave that you hit just right)The 12 pitch prop I got was 11.6" diameter where my 9 pitch is 12.1. Also the 12 pitch prop has smaller blades than the 9 pitch. I think this is why it wants to lose bite on the water. Are the bigger blades because it is a pontoon specific prop? I cant find a 12 pitch for my motor that is specified as a pontoon prop. Any help would be greatly appreciated.