Rough ride at low/mid RPM - possible sign of drive problems?

[ChrisW23]

Hi all - we upgraded our boat last fall from an 18' bowrider with 4.3L, V6 MERC (Alpha 1?) to a 20' bowrider with a 350 MPI, dual SS prop, Bravo III MERC. The dealer had done some maintenance on the 20', including a new water pump, and advised us to lay low for the 1st 10 hours or so. We are now clear of 10 hours and getting back into tubing and boarding and generally faster cruising.

My question - the new boat rides very smooth at very low RPM and at higher speeds/RPM (2500+), however it is a noticeably rough ride between ~1000-~2500 RPM. Dealer has stated all is well; they went through the entire boat before selling and everything is fine. They have also stated what I am feeling is the difference in a single prop, V6 and dual SS prop, 350 with way more power.

What do y'all think? Would you agree with the dealer or should I have another mechanic give things a look to be safe?

Thanks for your input.

ChrisW

 

[mpsyamaha]

Re: Rough ride at low/mid RPM - possible sign of drive problems?

So what you are saying is that the engine runs rough when you are putting a large load on it? Plowing at less than planing speed is when the engine works the hardest. Or are you referring to the way the actual boat rides?

 

[Another retired guy]

Re: Rough ride at low/mid RPM - possible sign of drive problems?

In that rpm range the boat is struggling to get up onto ( or coming off ) plane. Is that what you're experiencing? Bow- up & rough ride?

 

[ChrisW23]

Re: Rough ride at low/mid RPM - possible sign of drive problems?

Thanks for your reply and my apologies for not being more clear. The engine runs rough between ~1,000-2,500 RPM; boat shakes in general and I can really feel it and even see it in the steering wheel.

 

[mpsyamaha]

Re: Rough ride at low/mid RPM - possible sign of drive problems?

Are you sure the props arent bent or maybe even the prop shaft (causing it to look like the prop is wobbling slightly when you spin it by hand)? Any growling or other noises when turning sharply while underway(ujoints or gimbal bearing problem)?

If not, then its likely a motor problem.