Engine/propshaft alignment

[mattonthesea]

I have just unattached the prop shaft, replaced the cutless bearing and repacked the stern gland. I find that the prop shaft 'doughnut' socket centre for the shaft is about 2-3 mm below the shaft centre. IE they don't match by that distance. It is possible to lift the gearbox to insert it by hand.

I can fairly easily raise the doughnut by raising the rear engine mounting nuts. As a complete novice to this sort of engineering is it really this simple? Or will I have to do something like get a laser set to point at the shaft seal in the hull and line up all sorts of things like a theodolite (never used one)?

By doughnut I mean one of those shaft connectors that clamps to each end and each side is attached to the other by alternate facing bolts

 

[vyv_cox]

Reconnect the flexible coupling bolts but do not tighten them at this stage. First of all check that the clearance between the shaft and stern tube is roughly equal all around. If not, adjust rear engine mounts until it is. Now tighten the coupling bolts until there is a small clearance between the flanges. You now aim to get this clearance equal all around, top and bottom, side to side. Do this by changing front and rear engine mount heights, always bearing in mind the shaft to stern tube clearance. Rotate the shaft and flanges from time to time to ensure all is parallel. You need feeler gauges for this, ideally there should only be a difference of 1-2 thou, although a flexible coupling will be more tolerant.

 

[macd]

As Vyv wrote.
Note though that some "flexible" shaft couplings are flexible to little more than the extent of not transmitting so much noise and vibration as a rigid coupling to the hull. These are tolerant of very little misalignment. (The amount should be specified in the installation instructions, if you can identify the type.) One of the most common types, the R&D, has a shallow domed head on one of its fixing bolts. This is to specifically accept a feeler gauge for measuring alignment.

Images of R&D couplings here: https://www.asap-supplies.com/propeller-drivetrain/boat-flexible-couplings/flexible-shaft-couplings

 

[Graham376]

When aligning, I remove the flexible coupling, slide the shaft forward and make sure it's packed centrally in the stern tube and then adjust engine/gearbox until the two flanges align within a few thou all around, metal to metal. Then replace the coupling and check though 360 degrees on the dome head bolt to make sure within around 5 thou or less.

 

[mattonthesea]

Thanks all. I now have a shaft that slides into its housing so smoothly...

What a wonderful forum with good people on it!

Matt

 

[keithb]

All good points. However don't forget to centralised the prop shaft within the stern tube first if it has a flexible hose seal (as most do). You may need to undo and slide the flexible hose forward and then i chocked propshaft against hull in the centralised position, and then can do engine alignment

 

[keithb]

Graham 376 has said this more eloquently!