"Evan Gatehouse" wrote

I'm sort of disagreeing with Roger here with his suggestion (I
think) of flexibly mounted engines but a rigid shaft coupling.

It's probably not a disagreement but having lost track here of a point
in the thread. I'm talking about a specific drive train configuration
which is common on sailboats.

There must be a flexible stuffing box or shaft seal and there must be
no other bearings other than the stern strut. The engine can be on
very soft mounts. There is then nothing except the cutlass to pick up
any shaft vibration. The shaft is long enough to flex and accommodate
the engine motion. The full length version of this video

http://marine.unh.edu/Images/Gulf%20Challenger.wmv

shows a cup of coffee sitting on the quarter bit while the boat is
going 20 knots and there isn't a ripple on the surface. The struts
pick up very little vibration.

As soon as you introduce a second bearing into a the shaft system with
soft engine mounts, you have to have something flexible between the
gear and the shaft. I've made the shafts slightly oversize on my fast
research vessels to eliminate the need for a second bearing, way, way,
cheaper than installing and lining up that second bearing.

BTW there is a direct relationship between how much the engine can
move and the frequency of vibration isolated. Thin rubber pads take
out the high whine but getting the low throb and rumble out requires
letting the engine move 3/8" to 1/2" as you say you are doing.

--

Roger Long