30 Mainship vibration

Hello, I recently purchased a 1999 Mainship sedan, powered with Cummings 6BT 5.9M, engine, prop is 22X26 X LH,

When I hit 2200 RPMs the stern of the boat vibrates considerably.
I understand there was issues in early models with vibration.

Would changing the prop help? And if so what size would be recommended,
I believe Manship added a five blade prop.

Thanks for all your input.

There has been much discussion about this at the Yahoo Mainship site over the years. This issue is also noted on the Pilot 34. As with many manufacturers prop tunnels address some issues very nicely. The boats are a little faster because of the lower shaft angle and the semi Kort effect of the tunnel, and the boat saves a few inches of draft, all good things.
In gaining that .5 knot the rule of thumb for prop tip clearance goes out the window, and that's often where vibration issues occur. There has also been some "tunnel burn" reported on some boats.
I do not have a P30, but if I remember correctly Mainship did switch to a 5 blade prop, and several owners of early models went to 5 blade props as a fix. Some also reduced the diameter of their existing props and increased the pitch. Some also installed damping tiles on the tunnel, which is more a bandaide as it doesn't address the core issue.

I'm sure others may chime in, maybe spend some time with the search feature on the Yahoo site, search for tunnels, lots there.

I would first have your prop scanned and trued. Then check the prop shaft for straightness. And when you put the prop back on, make sure it is seated right on the taper. Check the shaft coupler for trueness and align the engine.


After all of that, then you can think about a new prop.


David

djmarchand said:

I would first have your prop scanned and trued. Then check the prop shaft for straightness. And when you put the prop back on, make sure it is seated right on the taper. Check the shaft coupler for trueness and align the engine.


After all of that, then you can think about a new prop.


David

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Excellent advice. These are, in the order given, the usual suspects for vibration.

The 1999 Pilot 30 did not have a prop tunnel. That was added in 2003 I think.
I don't think a 5 blade would help in your situation. It's probably another issue with alignment or prop causing the vibration.

Thanks for the information.....
I had a full review of all systems, prop, shaft, engine alignment , all check out well,
Leaning to prop change

We had a customer with this very issue. In our yard we had equipment to allow us to do laser alignment. In this customer's boat the cutlass bearing housing was installed incorrectly and the bearing was aimed 1.5" to port of the engine, even when the engine was shifted all the way to port. We tried to realign the bearing housing, but the shaft would rub on the inside of the keel. We had to rebuild the engine mounts to move the engine further to port. $$$ and unhappy customer, but problem solved.

altouchet said:

The 1999 Pilot 30 did not have a prop tunnel. That was added in 2003 I think.
I don't think a 5 blade would help in your situation. It's probably another issue with alignment or prop causing the vibration.

Click to expand...

2003 is a correct statement. A 5 blade prop could help by being able to decrease prop diameter and getting better prop clearance. That or decreasing diameter on your current prop and increasing pitch...and maybe cup.

As a general rule, when designing a prop, " diameter beats pitch." if you have to reduce diameter, increasing rake and skew can help mitigate the loss of diameter.