Shaft alignment in stern tube

[sailingjupiter]

On my MG Spring 25 the shaft as it exits the boat under the hull is closer to one side of the stern tube than the other. Or in other words the shaft isn't central in the tube. I have had the boat 5 years and never bothered with it. But thinking about it there has always been a clatter in neutral (Yanmar 1GM10). I assume it is the shaft rattling around in the tube and hitting the side where it is closest.

If when the boat is out of the water you manually turn the shaft whilst in neutral and engine off the shaft definately gets harder to rotate at the same point for every revolution.

I assume the engine was originally professionally aligned as I have a fist full of invoices for engine work from the previous owner. Would the engine have come out of line slightly over the last 5-7 years?

I'm tempted to slacken the engine mounts and move the whole lot slightly so the shaft exits centrally. But will this be correct? Afterall, alignment remains the same although this may put strain on the cutlass in the P bracket as it may be aligned to that presently. Currently the cutlass doesn't appear to have to much play so maybe the engine to P bracket alignment is correct and the tube is very slightly off centre.

What would you suggest do....anything (as its always been OK bar the rattle in neutral) or take action? If so what?

Thanks

 

[sailorman]

1GMs are known to leap about @ idle speed. i dont think you have a real problem
My Sons Cinder 22 ( For Sale ) has the same engine, no probs when its running, it does leap about but only "rattles" when the stop cable is pulled due to the imbalanced running when stopping

 

[Tranona]

Suggest you check shaft alignment. It should be checked regularly because the mounts can deteriorate and the hull can change shape. Google propeller shaft alignment and there is an entry from www.hellersolutions.com which has an excellent set of instructions on how to do it.

Basically you have to split the coupling at the gearbox and measure the gap between the faces with a feeler guage. It should be the same at all points and stay the same if you rotate the shaft relative to the gearbox.

 

[sailorman]

http://www.randdmarine.com/flexiblescinst.asp

 

[ksutton]

Make sure the idle speed is exactly as the Yanmar manual recommends. Even 50 RPM under recommended can make a considerable difference. When you are maneuvering and you pull the lever back into neutral before the the gearbox actually fully disconnects the engine is still momentarily trying to drive the prop around while the engine is near to or at idle speed, it can make a hell of rattle. This is amplified while berthing when moving the lever quickly between ahead and astern and coming back to neutral.

Ken

 

[Davolas]

If the shaft is tighter at one point of rotation, this implies distortion rather than misalignment and shaft may be slightly bent. Difficult to prove without removal of the shaft and checking by rolling on flat surface or more technically with vee blocks and clock gauge.

As far as alignment is concerned, yes will change with age, particularly with the soft engine mounts found on Yanmar engines.

To align, disconnect shaft from engine. Assuming you have a flexible rubber tube with a convential stuffing box or deep sea seal assembly inboard. Move stuffing box side to side and up and down to find limits of movement then block in mid position. Align engine flange to shaft flange so that faces are parallel, (gaps even at top / bottom and side to side) and centred i.e engine is not high / low or to one side ( use straight edge across flange sides). then reconnect and ensure all mounting bolts locked up.

Note check that above has not made misalignment at exit point worse, this is more likely to be caused by either misalingment of original tube installation or movement of P bracket, which could result from hitting debris, which may also cause shaft distortion.

With the amount of flexibility in most boats and engine mounts alignment is not ultra critical but the closer you can get it the smoother it will run and less stress on all components will result.

Results much better than 0.5mm are probably not repeatable anyway

 

[Tranona]

Hi Sailorman
As I commented before, this reference is only relevant if the installation actually includes an R&D coupling. Does not work with plain couplings or other flexible couplings such as Bullflex or Centaflex.

 

[sailorman]

The principle is the same .
feeler gauge between the meeting faces of the flange rather than the bolts

 

[Tranona]

But confusing for somebody who has not done it before, possibly hanging upside down looking for a non-existent red bolt head!

 

[sailorman]

if he chap hasnt done it b 4 he will need a competent person 2 show him the ropes i would have thought i would guess you would agree or he could do much damage 2 his running gear

 

[yoda]

From your description it sounds more likely that the shaft is not straight causing the 'per rotation' stiffness. I would check that first before starting on a realignment.

Yoda