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Mercury 115 HP Four Stroke Engine Issue

S

Slainte

New member
I have a 2017 Bennington 20 SLX pontoon boat purchased new with a Mercury 115 HP four stroke. Having an ongoing issue. When accelerating, motor sounds like it's trying to speed up but boat does not. At times it surges up to 6000 rpm or so and then goes back down. When you throttle back down and try again, it runs fine. This has happened since getting the boat occasionally at first and now it's happening all the time. New prop was installed in the spring. Have had three of the marina mechanics take a look. They confirmed trim is down and no seaweed, etc. on prop. First problem was that there was no diffuser at the prop; they installed one; issue continues. All three mechanics believe the reason is that we have a three blade prop (which came with the new motor and new boat) and that a four blade prop will eliminate this issue. New four blade prop will be installed today or Monday.
Has anyone had a similar issue? Would appreciate any feedback. While this is a reputable marina and I'm certainly not a mechanic, but a four blade prop as the solution does not make sense to me.
Thanks!
 
Sounds like it is the wrong motor !-------Is it a 20 inch shaft on a boat that needs a 25" motor ??-----Should not be hard to figure out.-------A new boat should have some support from a " high end " factory.----Call their 1-800 number and voice your concerns.
 
Thanks for your reply. As it turns out, the mechanic just left. Replaced prop with a five blade prop he said is designed for the kind of performance we needed. Took a long test drive and the boat's running better than it did when we bought it. Now my concern is how will the marina where we bought this boat and who just came to fix this boat handle the cost of the whole thing! Time will tell -
I don't think we should be responsible for any cost given the boat was sold to us (new) with this motor and this type of propeller.
Anyway, I learned a lot about props, that's for sure!
 
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