Solid bush forward of cutlass bearing

[boggybrn]

I have been having problems with vibration when motoring at higher power settings, and have found lots of play in the cutlass bearing. So I decided to replace this bearing.

When I removed the cutlass bearing then I found that further up the stern tube there was a bush made of a very hard plastic. This has an inside diameter just larger than the shaft and an outside diameter just smaller than the stern tube. It is however a bit worn so the inside doesn't look that round anymore.

The basic set-up is a Yanmar 1GM10 connected to the prop shaft with a solid coupling. The shaft passes through a stuffing box which is connected with a short rubber hose to the stern tube. As I understand it the mounts on the 1GM10 are pretty soft so with this arrangement the engine is free to move around a bit. However I was expecting the only bearing to be the cutlass bearing at the far end of the stern tube.

So my questions are...
What is the plastic bush there for?
Would I be better just leaving it out?
If I refit it then where in the stern tube should it sit? (given it wasn't held in place I guess it will always end up near the cutlass bearing wherever I put it!)

 

[ghostlymoron]

The stern gland requires some water lubrication to cool it and its difficult to see how water would get through with a solid bush in the tube. So I would advise that it is removed but the question arises "why was it put there in the first place?". Is the drive shaft excessively long or thin so that it clatters on the stern tube?

 

[boggybrn]

The shaft is 25mm diameter and is somewhere between 3 & 4 feet long. There is a fair bit of clearance in the tube as this has an internal diameter of 40mm.

So I think that I may leave the bush out for now. I'm sure I'll soon find out if the shaft starts to clatter on the wall of the tube

I can always dry the boat out between tides to refit the bush if this causes a problem.

 

[vyv_cox]

I have been considering the fitting of a similar bush for some time. In certain sea conditions, mainly motoring with a following sea, my shaft resonates at about 2000 rpm (engine) and clashes with the stern tube. I assume your bush was fitted to overcome the same problem.

As you say, this is not a great arrangement to combine with a solid coupling, especially with a single cylinder engine that moves around a bit. I suspect that a flexible coupling in conjunction with the bush would be a better arrangement. Three axial grooves in the bore of the bush should be sufficient to provide water to the gland, bearings inside stern tubes are usually made this way.

 

[xeitosaphil]

A friend of mine had exactly the same set up as you on his Sadler 26. "P" bracket, Plastic bush, then a Volvo seal, solid mount onto engine. He has recently had a hip op and couldn't keep trying to burp Volvo seal when he regularly beaches boat.
I suggested he had a word with a marine engineer to install a traditional free floating stern gland/rubber tube, and remove the plastic bush and fit an R&D flexible mounting plate.

All seems well at present, can't tell you if he had any vibration problems though, but a 1GM on rigid stern tube arrangement with solid coupling does seem a little odd considering the amount of movement with that engine.

I have a similar arrangement on my engine as suggested and that's with a 2GM20, but even I get some vibration at different revs? I actually wondered if I hadn't got enough clearance on the stern tube? but you seem to have plenty.

 

[Tranona]

Think I would leave the bush out as there is plenty of clearance in the stern tube to allow the shaft to move with the engine. The solution that I arrived at with my stern tube that has far less clearance (1 3/8" id with a 1" shaft) was a short piece of cutless on the inboard end and a Bullflex coupling. However this is a bronze tube with a detachable bearing housing which I had machined to take the bearing. You could achieve the same outcome by having a composite bearing machined to fit the glass stern tube and epoxied into position. A constraint might be the clearance between the end of the tube and the gearbox output flange to fit the flexible coupling. The combination that takes up the least space is a Bullflex and a Volvo seal, although an R&D coupling and the standard Yanmar shaft coupling is a similar overall length.