Stern tube and shaft

[Malcolm3]

Ive fitted a new sterntube in my boat but the gap between the tube and the shaft isnt big. The inboard end has an offset engine (80hp) and the connection is with an aquadrive so though on a bulkhead the mounts are rubber. The outboard end has a sternlog with cutlass bearing.
Is there a rule for what the gap should be - its about a matchstick thickness all round
thanks

 

[abbott013]

no expert on this, but if the engine is aligned properly then there will be no vibration and no chance of shaft and tube coming into contact, what was the original arrangementof shaft and tube ?

 

[yoda]

From your description I think your shaft is supported by a cutlass bearing at the out board end and the Halyard Thrust bearing at the inboard end. There could also be a bearing at the inboard end of the stern tube. The gap you refer to between shaft and tube is quite big enough to give a clearance which is all that is required. If the gap is between shaft and bearing then the bearing requires replacing. Engine alignment is not an issue because of the CV joint part of the Aquadrive.

Yoda

 

[Spyro]

With that small a clearance all round I would be very surprised if you don't hear the shaft hitting the stern tube at times. When you first start up I suspect it will hit it. Have you seen how much an engine moves when it first fires and also when shutting down. When I got my boat the engine alignment was out and the shaft was not located centrally in the tube. It was quite an alarming couple of thuds on start up. I suspect it might also touch it when motor sailing if you are at all healed. If it was a new stern tube I would have fitted a bigger one.

 

[jfkal]

Second that

Mine has about 4mm clearing and it clanks when the soft mounted engine starts.

With that small a clearance all round I would be very surprised if you don't hear the shaft hitting the stern tube at times. When you first start up I suspect it will hit it. Have you seen how much an engine moves when it first fires and also when shutting down. When I got my boat the engine alignment was out and the shaft was not located centrally in the tube. It was quite an alarming couple of thuds on start up. I suspect it might also touch it when motor sailing if you are at all healed. If it was a new stern tube I would have fitted a bigger one.

 

[Neil_Y]

It depends on the shaft diameter, shaft length, bearing positions, bearing lengths and thrust.

To disagree with the first answer it is not engine alignment that gives rise to shaft flex/vibration, it is thrust and lack of bearing support. With any engine on rubber mounts, engine to shaft coupling alignment changes as soon as you change direction or go over the first small wave, this is what a thrust bearing and CV coupling as you have takes care of, but on any shaft drive the bearing support in the shaft should be trying to keep the shaft from flexing too much.

The rubber bearing I guess you have may allow for the amount of deflection that will see a 1-2mm flex at the shaft mid point. As you apply thrust the shaft will want to bend simply because the forces are never going to be directly in line from prop to coupling.

If you are stuck with this size of tube and clearance and you hear contact you may want to consider hard shaft bearings rather than rubber as these will not allow as much flex as rubber.

Is the boat steel? is the shaft tube steel? could you heat it? does it have a good water feed at sea water temperature?

 

[vyv_cox]

I have almost the same arrangement as you, cutless bearing in a P-bracket and an Aquadrive. Shaft is 1 inch, tube about 1.25 inch bore. Despite several attempts at realignment my shaft occasionally clashes with the tube, especially in chop.

 

[Malcolm3]

Thanks . Cox site I must say doesnt show a much bigger gap . Think at end of day will just have to try it and see. The stern tube is phosphor bronze but its in a fibreglass pipe so increasing it would be problem as its 1.5 metres long and fuel tank above so difficult to drill out accurately. Dont mind noise but last stern tube fractured (not sure why but think becos the stuffing box gland bound to the shaft and something had to give) and half atlantic poured in - luckily there was a hoist close by that time!
malc

 

[Neil_Y]

You might have room to put a bush in the stern tube 2-3mm thick, you would also benefit from using a hard shaft bearing rather than a rubber one.