Prop shaft misalignment

[Anchorite]

Is there a 'standard' trial and error procedure to correct a prop shaft misalignment without removing the boat from the water?
It looked okay before launching, is okay on minimum revs forward and reverse, but starts knocking when the revs are increased.
Four silent blocks (one on each corner), 8° dropdown gearbox, Vetus flex coupling, Volvo seal and cutlass bearing.
All suggestions welcome: thanks!

 

[Spyro]

Is there a 'standard' trial and error procedure to correct a prop shaft misalignment without removing the boat from the water?
It looked okay before launching, is okay on minimum revs forward and reverse, but starts knocking when the revs are increased.
Four silent blocks (one on each corner), 8° dropdown gearbox, Vetus flex coupling, Volvo seal and cutlass bearing.
All suggestions welcome: thanks!

I had a similar problem with a knocking shaft. The problem was not mis-alignment it was not centred in the stern tube. I fixed mine as it was out of the water and was fitting a new shaft anyway because of wear around the stuffing box. Firstly I made a ring from an of cut of cutless bearing that fitted around the shaft. I built it up with tape so that it fitted around the shaft and just fitted inside the stern tube. I fitted the shaft to the coupling without the stuffing box in place. Lined up the engine with the coupling in the normal way with feeler gages while the shaft was held centrally by the cutless bearing in the P bracket and the made up ring holding the shaft central in the tube. Once it was all lined up I slide the shaft back out from the coupling removed the ring and fitted the stuffing box. Then fixed the shaft back into the coupling now knowing that it was lined up centrally in the stern tube.
The problem with trying to do it in the water is that it's difficult to see that the shaft is central in the stern tube.

 

[Topcat47]

Yes. This works with a deep sea seal.

Loosen the bolts on the flex coupling.

Holding the coupling flanges together, try feeler gauges around the perimeter of the flange. The most common misalignment is from top to bottom. This is corrected by adjusting the height of the engine mountings on the silentblock bushes.

Misalignment side to side has to be adjusted by twisting the silentblocks longitudinally. Not very easy to do afloat as it involves slackening off the fixings into the engine bearers as well as those holding the engine to the mountings.

You will ship a little water during the exercise.

If you have a different shaft seal, you might need to prevent the shaft sliding out of the boat. A Jubilee clip might work but I"ve no experience of this.

Hope this helps

 

[Tranona]

Yes, sounds like the shaft is not central in the tube, so even if the engine is correctly lined with the shaft it may hit the side of the tube as the engine moves around. You may find you do not have much clearance between the tube and the shaft. If you run for any time off centre you may also damage the seals in the Volvo seal. This does to an extent hold the shaft firm because of the bearing inside it, but running offset will wear the seals and bearing on one side.

Getting the shaft central is not easy in the water as you have to move the seal forward to get at the shaft. You may still find the that the shaft still touches the side at certain revs if the clearance is small. The solution, which may not be possible with your tube is to insert a bearing (either cutless or composite) in the inboard end of the tube so the shaft is supported at both ends and the engine can move independent of the shaft.

 

[Anchorite]

Never what you think!

Went down to the boat with the feeler gauges: tested Reverse gear first as that was another "small" problem. No movement in reverse...the gearbox was engaging but the shaft wasn't turning. The knocking must have been the prop crown hitting the skeg (only an inch-ish behind). Now to sort out the Vetus flex coupling (on Google I see I'm not the first to do battle with this design...).
Thanks for your answers: they would have done the trick!

 

[Tranona]

Unlikely the shaft will move that amount going into reverse. 3-4mm would be normal. Which Vetus coupling do you have - Bullflex or Uniflex?

 

[Neil_Y]

If you have a rubber bearing the shaft will not be sat central as the rubber will allow it to sit lower.

We confirm carrier/bearing alignment by having a clearance fit bearing which can slide easily into the carrier on the shaft whilst the shaft is chocked centrally.