How To Align Prop Shaft After Months On Land

[Q Xopa]

Originally Posted by George DuBose

I have a 1973 Pearson 36. Between the propeller shaft and the transmission flanges there is a flexible coupling about 2" thick. My local mechanic says he can't align the flanges with the flexible coupling in place and I can't bring the two flanges close together due to the Flex-o-fold prop being 1/4" away from the P strut.

The only way to get the flanges together is if the prop is off and the boat is out of the water. Meaning any prop shaft/tranny alignment has to be done out of the water.

Unless some genius on this blog has a solution...please...

Im certainly not a genuis. In fact I would say Im dumb enough to have a go at this.

I assume you are having trouble fitting feeler gauges in around your coupling to get the gaps between the flanges.

Im a helicopter engineer by trade. We do driveshaft alignments using Dial gauges.

They are not as cheap as feeler gauges, but IMHO they are much better for a number of reasons.

The ideal setup is-
2 Dial gauges,
One attached to prop shaft, or coupling and reading the Txn output flange.
The other is attached to the Txn flange and the stylus reads on the prop shaft.

The advantages are-
You can see bent shafts, eccentric couplings, coupling angular misalignment, eccentric or bent shafts, can leave the coupling attached. You cant really check for these things with feeler gauges. You can find an anglar misalignment, with different gaps. But you cant really tell if it is in the shaft or the coupling.

Feeler gauges are really only checking for an 'angular misalignment they cant really show you an 'axial offset' ie shafts parallel, but not on same axis.

Dial gauges are also 'live'. So you can simply set to 0 at 3 o/c, check at 12, then at 9. You dont need a 6 reading. Google 'TIR' ( Total Indicator runout) to get an explanation.

So at 9 adjust half way back to 0.
Go to 12 and again adjust half way back to 0 from 12 reading.

Do this in the order-
Near near, Far far.
As in look at Txn DTi and adjust aft mounts, ie Near DTI and nearest mounts.
Do the same with Propshaft DTI and fwd mounts, ie far far.
Done!

I know there are many here that know the feeler gauge method and dont want to hear any other hoccus poccus rubbish. I have met many. Im sure they are all right.

[lordgeoff]

Originally Posted by benethridge

Hi, everyone.

My Vagabond 42 has been hauled out for many months now. I recently bought a new prop shaft and when I tried to align it to the transmission coupling, I noticed that it is about 1/2 inch too high. When I took the prop shaft off many months ago, it was perfectly aligned...or at least it came off easily. Now I can't even bend it into place (probably not a good idea anyway).

The boat rests on blocks on the keel, with jack stands as well, of course.

I'm thinking that this misalignment is due to the forward and aft ends of the fiberglass hull flexing downward over time, due to gravity. I have read that you should never try to align the prop shaft to the transmission out of the water, due to this flexing.

Do you think this is the problem? If so, how long does it need to be in the water before it will come back to its normal shape? Days? Weeks? Months?

Ben Ethridge

I had a new shaft fitted on my Beneteau 50 just after I bought it. The boat had been in the yard supported on the keel for two weeks.
The guy who supplied and fitted the shaft wanted the boat to sit in the water for 1-2 days so that if there had been any flexing then it would have returned to normal. Have not had any shaft issues since (8 years). The boat is always in the water. Only on the hard for antifoul etc

[Cheechako]

If your alignment is that far off just from sitting on the keel, you have an awful flexy boat. I'm not saying it's not possible, just that that shouldn't be the cause if the boat is well built. and.... the old one should have been off also. I would assume if this is from yard blocking etc most of it would have occurred immediately, not an "over time" thing.

If you test the deflection (bend) in a piece of material, whether fiberglass, steel or wood, it will show up 99% as soon as you place the weight on it.

-Did you check your cutlass bearing etc when installing the new shaft?
- Is it possible the rubber tube was bent over time AND the cutlass allows enough slop for that to happen?
-Can you rotate it? does rotation show the shaft is bent?