Shaft & Coupling Questions

[McKee]

Long story short I had to cut the end off of my prop shaft. Now I'm done taking things apart, and I'm on to putting things back together. How does everyone feel about flexible couplings?

[BrianM]

I feel they are the best thing you can do for your drivetrain to absorb vibration.

Alignment is still critical. You've got to be patient, do it right, come back a few days later and check it. Put the boat in the water and check it again. Adjust as needed.

[Gary E]

I think it depends on the boat and the engine and it's use.
Lightweight boat, small engine not used very much...ok.. but not really needed if you do as Brian said and line things up.

Heavy boat, big engine, runs a lot and all the time... better to just have things lined up, and keep it that way. To me they are an added expense that does little good since alignment must be good anyway, but maybe I've just not seen a "good" one.

Tell us about your boat

[McKee]

If I tell you about the boat you might lynch me. Not only is it not wooden, it's a stealth glass.

I've got a Cornish Crabber... Pendragon is a 7 ton gaff cutter, about 30 LOA. Power wise there's a Vetus 3.10 about 22 hp on a good day.

[Gary E]

All of that except the 22 HP is greek to me, so, with that little HP, do whatever you want, it wont matter much.

[mmd]

Is the engine hard-mounted to the beds, or is it on flexible feet?

[McKee]

Flexible mounts.

[mmd]

There are basically two types of flexible coupling suitable for your installation.

One is a relatively hard plastic disk that fits between the transmission output flange and the shaft coupling, such as a DriveSaver. This will cut down high-frequency vibration a bit, and low-frequency vibration hardly at all. It is designed to shear if you shaft suddenly stops (grounding, rope fouls, etc.), saving your transmission from damage. It also compensates for minor coupling mis-alignment.

The other is made of two metal plates with rubber cushions between them. These allow considerable axial and linear movement between the two plates, thereby almost completely isolating the engine from the shaft. This makes vibration and sound levels drop dramatically. The down side of this type is that they are both expensive and large diameter - usually 3-4x shaft diameter.

One thing to keep in mind if you are soft-mounting the engine but hard-mounting the shaft (direct-couple or DriveSaver) is that if the engine is free to wave about, carrying the inboard end of the shaft with it, is the stuffing box capable of withstanding the axial movement of the shaft? I know of one boat that, when refitted with a new, floating, engine in place of the old, hard-mounted one, couldn't keep the stuffing box from leaking prodigiously within a few days of tightening down. The shaft moved enough that it just packed down the stuffing box packing until it leaked. Eventually, a flex shaft coupling was installed and the problem went away.

Basically, noise and vibration attenuation is a function of the amount of money thrown at the problem. I can build drivetrains that are virtually vibration and noise free, but it takes quite a chunk of change and a fair amount of space. In your situation I would go for the DriveSaver, align everything as exactly as you can, and then isolate all pipes to the engine with hose ends, and if you have a water-lift muffler or any other exhaust component that touches the hull, isolate that either with rubber hanging mounts or an anti-vibration bottom mount. Find every place that vibration from the engine or exhaust can touch the hull and cushion it.

Quiet and still is very nice on a boat. Even very fast ones.

[McKee]

Thanks for the responses everyone. Given that the engine has flexible mounts that aren't that old, I think I'll stick to the flexible theme.

[McKee]

Having said I'd put a flexible coupling I went and did the opposite. After I talked to everyone I decieded to go with the simpler answer (which is usually correct) and the cheaper one. Besides I find I need a flexible coupling later I can always shorten the shaft. Thanks again for the borrowed brain power.