Repower Vibration Concern

[zstine]

Recently had a Nanni 37hp replace an old 24hp on a Tartan 41. Yard has checked alignment and says it is fine, but there's still a bit more vibration than before the repower. The drive is smooth at idle up to ~1800RPM, but then begins to vibrate as RPM exceeds 2000. The shaft, stuffing box, cutlass and prop were all replaced with the new engine, However, the same diameter shaft was used.

Does anyone have rule-of-thumb info for appropriate shaft diameter & unsupported lengths for a given power out put? Maybe I worry too much. But has anyone else run into shaft size issues when repowering with higher power?

[Dulcesuenos]

Does it vibrate in neutral at that rpm?

[delmarrey]

Run it full bore in reverse. If no vibration, then it's the shaft diameter. In reverse it's pulling the shaft, not pushing it.

I re-powered mine a couple years ago and even went to a Pythondrive, and still ended up going to a larger shaft. A 1'' shaft, 5' long isn't really enough to push a 40' boat at hull speed. It starts to whip under the stress.
My old system had less torque but with higher RPM's. I went to a 1-1/8" shaft and it runs dead smooth now.