2275RL / Yamaha 115 trim issues cavitating : /

Bigtime57

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I have a 2275RL with a Yamaha 115 that has had nothing changed since day it was bought new.... with myself alone on the boat you can just start to trim the boat out and it will cavitate , and also with 6-8 people on the boat and distributing the weight out on the boat no matter how i try to use the trim it will cavitate.............. Should i try dropping the motor down a notch? try changing the prop? This is my 3rd Bennington and never had this issue before ...kinda has me stumped
 
I have the same issue with my 22 GSR and Yamaha 115. My RPM's are spot on.
 
Where does the cavitation plate sit in the water when you try to open it up or go WOT? Sure seems like a motor mount height and/or prop issue. Did you just buy it so this is a new issue to you with it that previous owners had as well, or is this something that just started out of nowhere?

Year? Which Bennington pontoon hull?
 
Where does the cavitation plate sit in the water when you try to open it up or go WOT? Sure seems like a motor mount height and/or prop issue. Did you just buy it so this is a new issue to you with it that previous owners had as well, or is this something that just started out of nowhere?

Year? Which Bennington pontoon hull?
I am the second owner but have known the boat since it was new (21 hours when i bought it ) ...so i know nothing has ever been changed since new from the dealer .... its a 2007 model with dual toons ....... this has been an issue since i purchased the boat ...just dealt with it and not used the trim ...just trying to get a little more speed and faster plane...... at WOT i can see the plate right at the top of the water
 
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I had the same issue with my boat. I couldn't trim at all without cavitation, so the trim was useless. I installed a hydrofoil on the engine and that solved the trim issues.
 
Lots of people get cavitation and ventilation confused. I believe you are having a ventilation problem where the blade is sucking air and you lose traction when you trim.

#1 As already mentioned, move the engine down a hole or 2, this way when you go and trim up, the blade still sits in the water enough for bite to lift the front up creating more speed.

#2, I've found that the larger diameter prop you use on the 115 and the closer it gets to the cavitation plate, the more its going to suck air and vent. The solution for me at least was a larger pitch and a smaller diameter. Even going from 13-1/2 to 13-5/8 diameter created tons more vent for me. I have 5 props I've tested this year alone on this HP engine..
 
I had the same issue with my boat. I couldn't trim at all without cavitation, so the trim was useless. I installed a hydrofoil on the engine and that solved the trim issues.
I have seen the hydrofoil on smaller fishing boats and wondered myself how they would work on situations like this ....
 
Lots of people get cavitation and ventilation confused. I believe you are having a ventilation problem where the blade is sucking air and you lose traction when you trim.

#1 As already mentioned, move the engine down a hole or 2, this way when you go and trim up, the blade still sits in the water enough for bite to lift the front up creating more speed.

#2, I've found that the larger diameter prop you use on the 115 and the closer it gets to the cavitation plate, the more its going to suck air and vent. The solution for me at least was a larger pitch and a smaller diameter. Even going from 13-1/2 to 13-5/8 diameter created tons more vent for me. I have 5 props I've tested this year alone on this HP engine..
I am a going to lower the motor for sure ...... see how that does for now...then after boating season do a prop swap .... I know my boat is heavy but the speed just isnt there so im going to do a prop swap...now the fun part is finding the "happy" prop which your info is a big help on where to start ..
 
OK, so maybe it's just pic perspective, but it looks like the motor mounting frame is sitting on the top of the transom now...where the bolt holes are drilled is irrelevant. In any event, doubt moving the motor down less than an inch is going to cure the degree of your problem. Need more info, like what prop is on the pontoon. Yours would not be the first pontoon to have always been running a prop that is a poor choice for the application.
 
OK, so maybe it's just pic perspective, but it looks like the motor mounting frame is sitting on the top of the transom now...where the bolt holes are drilled is irrelevant. In any event, doubt moving the motor down less than an inch is going to cure the degree of your problem. Need more info, like what prop is on the pontoon. Yours would not be the first pontoon to have always been running a prop that is a poor choice for the application.
you are correct the motor is very close to the transom ....not sure if it will even drop another bolt hole or not ..... I will have to check the prop this weekend when i go back down to the slip on the prop specs ......
 
It's very hard to impossible to read the specs on some props without taking the prop off because they are on the inside barrel....good luck.
 
If that's a white Yamaha aluminum prop which it looks like, the size/pitch should be stamped on the outside.
 
You're as low with the engine as you can go. And looks are deceiving on engine setup.

I suggest you call Bennington Customer Service and ask them the correct prop for your boat. I think they keep notes on what combos work best. The chances are you're a prop away from doing better.

The next alternative would be to put a jack plate on the transom and adjust it down--where it doesn't ventilate. They can be bought on EBay for $200.

My dealer only sells pontoon and tritoon boats, and specifically Bennington. He sets up so many engines that he knows exactly what prop's best for most uses. I use a $707 15 pitch Yamaha Reliance SS prop on my F150 engine--perfect.
 
Bamaman you are correct the motor will not go down any further without using a jack plate …… my prop is 13.5x15
 
you are correct the motor is very close to the transom ....not sure if it will even drop another bolt hole or not ..... I will have to check the prop this weekend when i go back down to the slip on the prop specs ......
My prop is 13.5x15
 
Doubt a jack plate is worth the effort for your pontoon. The modest power (115 HP) and ~22' pontoon indicate the need for a pontoon rated prop. As a singular example: a generic Yamaha 15p prop on our pontoon runs more than 30% calculated slip with the motor full down (the same was true with generic 13p and 14p too) . It's likely you will see something similar if you run your speed versus RPM in a slip calculator (you'll need the motor gear ratio too...it's likely ~2.15 to 1). Heavier loads, turning or trim are putting a prop that is already not hooking well into ventilation followed by cavitation. IF that is not the case, there may be a problem other than just the prop. The pontoon rated Yamaha Talon 12p on our pontoon will run ~30% slip with the motor up two holes and almost cuts that by half with only up one hole.

My approach is to do the math. You may wish to only use a qualified recco. In that case, try to use one that comes with a calculated slip number that is ~15% (or less).
 
Please let us know how this gets resolved. I’m m still betting a prop change (mercury diamond) will be the answer. A hydra foil is a strong option as well. ⚓⚓
 
Please let us know how this gets resolved. I’m m still betting a prop change (mercury diamond) will be the answer. A hydra foil is a strong option as well. ⚓⚓
Yeah, my thoughts too, although I'm partial to the Nemesis/Spitfire solution. IF a prop won't cure the problem, the right foil may help the prop not ventilate so easily (although even without a foil, ventilating a Nemesis is not easy). I tried running a Doel Fin a couple times (was on the motor when we bought it). It didn't help and mostly added drag. Junked it and have had the best results with a 2 degree transom wedge (our pontoon with the -80 series furniture is a bit stern heavy). No extra drag at speed because additional tuck-under trims out when not needed.
 
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