Supporting Shaft in boat

[Ryan]

I've just finished replacing a Volvo MD3B with a new Yanmar 4JH4. Everything is working great, but I'm a little concerned about the length of unsupported shaft. This is about 4' on a 1.25" shaft. The shaft is almost entirely inside the vessel (dry).
Everything seems to be fine, but there is more driveline vibration than I'd like near max RPM. A bearing or bushing would only need to contain the shaft, not support it. I can imagine a simple light duty, perhaps even composite roller bearing to support the shaft, but I don't know where to look. Any ideas?

[Alan Wheeler]

First question is, is the shaft itself vibrating at high RPM, or is there just vibration in general?
Can you actually get sideways movement by hand from the shaft? 4ft is not that long for an 1 1/4" shaft.
You do need to insure that the shaft is coupled correctly to the box. The shaft must be in verticle/horizontal line with the motor coupling. You should have not had to bend the shaft slightly to line it up. Just to be clear, The angle of shaft to engine may be different and that is OK, but you must have a flexible coupling connecting the two. It is that coupling that should have its two halves line up perfectly. If there is shaft vibration, there are two or three possibilities. The shaft is bent. The stern tube bearing is worn or the propellor is out of ballance. Or a fourth is the angle of shaft/motor alignment is too great.
If you want to place a bearing part way down the shaft, you are going to have to get something purpose made. You need to seek an engineer to fabricate something to support the bearing in the hull. The bearing itself shouldn't be too hard. There are many self aligning bearings available and an engineer should help you in what is best. I would consider the best person to talk to would be some marine engineer at your local marina.
If I have miss understood, feel free to clarify.

[GordMay]

As Wheels implied, a properly aligned prop’ shaft shouldn’t require any support inside the hull.
However, the shaft should not extend more than about 1.5 times shaft diameter (eg: 1 7/8" - 1.875") between the skeg/cutless bearing & the propeller.
Regards,
Gord

[Ryan]

To answer your questions,
Although there is no flex coupling, the faces of the coupling and engine are aligned within .005". I'd rather not have a flex coupling.
There is very little movement by hand of the shaft.
The vibration is greatest just forward of the dripless shaft seal.
The shaft is brand new.
The propeller is about .75" behind the brand new cutlass bearing in an aperature. Some of the vibration may stem from having a small aperature with an 18" MaxProp. The tip clearances are fine, but the deadwood is almost 5" so there is probably some flow disruption.
I guess I'm also concerned about "shaft whip".
This is a full keel/barn door rudder with an almost horizontal shaft (8.2 degrees).
There is almost no vibration at the engine end of the shaft.

[Alan Wheeler]

It's never a simple answer Ryan. There are many scenario's to the problem, some are possibles, some a probables and some are a mixture of inevitables.
First of all, even a .005" difference can be translated as a greater error at the opposite end. If the opposite end is tightly held, then that error has to be corrected somewhere along the shaft. It can be seen as a bending of the shaft and a thus a incorrect stress/wear load on the bearing. It can also be seen as a vibration. This part gets complicated. But it is to do with the load being transfered to the shaft via the prop blade. The less No. of blades, the more the problem. It works like this. The driving force is seen on a blade, as it is basically a rotating padel. On a two bladed prop, the blade opposing is also creating a driving force. This "push" is seen out on the face of each blade and is transfered to the shaft at opposing sides. But the two area's that have no blade, has no pushing force. Following so far? OK, so if there is inbalance along the shaft, the Pushing force can help to counteract that balance for two opposing 90 degree area's of rotation and can cuase the vibration to increase at the other two opposing area's of rotation. This force will change at differeing RPMs, thus differing rates of force. Max bend in the shaft could be seen at max RPM. You could try one method, of turning one half of the coupling 90 degrees. Thats just one set of bolt holes around. This places that vibrational stress on a different area of the shaft. But remember, that may or may not be the problem.

The flexible coupling does many jobs. Firstly, it helps to take the stress off the components from that alignment error. Secondly, helps to take the stress off components from any error arising from the prop itself (maybe cavitation, hitting a piece of debris, blades being slightly uneven in producing equall force etc) Thirdly, it reduces vibrational noise from two directions. One, the engine. The engine is a source of two distinct vibrations. One is just plain torsional movement as load variations cause the motor to flex in its bed. The second one, and one that is not commonly realised, is a phenomenen called "injector shock". This is a shock wave sent down the crank shaft and onto/into anything connected to it.
The other direction is from the propellor, back towards the engine. Remember that in most set ups, the driving force of the propellor is coupled to the hull at were the engine meets it's bed. So any vibrations produced out at the prop, which can be caused by many many things to do with the propellor and the hull, is transmitted to the engine. The flexy coupling helps reduce this noise being transmitted to the engine, thus to the mounts, thus to the hull. Oh and the mounts are another area. The wrong mount for the engine or a mounts that have lost there strength or been damaged by oil etc, will not isolate transmitted noise either.
IMO, no engine should be connected to a shaft without a flexy coupling. No matter how exacting, there are too many other influences that just can not be compensated for without a flexy. Andy yes Flexies are expensive, but once again IMO, the expensive wear created in other area's with out one, far outways the intitail cost of getting one.