Prop Shaft Wear - Does this need replacing?

[Tim Good]

Ok here we have a a 38mm (1.5inch) prop shaft connected to an aquadrive which is then powered by an M90 Perkins 80hp engine. Previous owner had a shaft locking system that wasn't serviced and since damaged the prop as shown below. There was a clutch in the middle of the black ring which we have since cut off and removed.

Next we have the very annoying situation that, when disconnected and the prop is slid backwards, it is slightly too long to come out. (Photo 3) :disgust: Guess how annoyed I am!

So the questions are:

1. The worn part of the shaft is 33mm in dia. For you non mathematicians that's a 14% reduction. Does it need replacing or are they so over engineered that it will actually be fine.

2. Any clever ideas on getting it out AND getting a new one in without raising the engine? No i haven't removed the cutlass bearing yet but I doubt it will make much difference. Once retracted up against the skeg there is about 12 inches of shaft still in the boat.

3. Any other thoughts?

 

[NickRobinson]

Belt and braces would say replace, though I have had two (different boats) welded and turned and trued. They bend big time as they weld

If the damaged area isn't touching anything, lip seal, stuffing box, gbox coupling clamping area, it should be fine, especially considering the access issues. Its obviously been coping like that for a while-

 

[TQA]

Err YES

Replace now!

 

[MoodySabre]

My propshaft was worn so the dripless stern seal was not dripless. I have changed to a PSS which bridges the worn part and does not rely on a seal against the shaft to stop water getting in.

 

[Tim Good]

My propshaft was worn so the dripless stern seal was not dripless. I have changed to a PSS which bridges the worn part and does not rely on a seal against the shaft to stop water getting in.

Fortunately this worn section is well inside the boat so concern is purely about structural integrity.

 

[oldharry]

Wear = serious, but if not on a bearing part now, probably OK - depends on the power it is transmitting. I am much more concerned about the black marks: they look suspiciously like crevice corrosion, which can penetrate a long way in and cause serious weakening. There is no way to tell how deep it is or how serious the damage is. If it is - Replace.

 

[robmcg]

The 80hp of your engine will put out a considerable amount of torque so, on balance, I would change it, especially if its a fairly long shaft. In addition, the shaft wear will cause an imbalance that will wear your cutlass bearing more than you want. As for removing it, if it won't go out backwards, then it may have to come out forwards!

 

[cryan]

Are you sure that's worn? It looks like it has been machined that way for some reason? If it was worn to that degree I would have (a) expected a lot of heat damage to be visible on the shaft, (b) an enourmous amount of noise whilst running caused by the wearing process.
If that is wear then change the shaft but make sure it was not machined that way first.

 

[Tim Good]

if it won't go out backwards, then it may have to come out forwards!

That's my worry! It's not out of the water long enough to be lifting engines.

Are you sure that's worn? It looks like it has been machined that way

Yes the shaftlock system gentle ground it down over time. The stainless filings were in a pile underneath. That's why it looks so neat.

 

[charles_reed]

The question is, will it or won't it shear. Otherwise you have the problem of having to dismantle half the boat to take it out to either rebuild or replace.
The only way is to calculate how much torque the reduced shaft will take. That need a very precise measurement and some simple calcs based on the properties of 316 stainless.
I'd guess it won't be a problem - but fracture will mean a possible loss of prop and a possible Pan-Pan call.

Why not use a mic and ask Vyv? or another reasonably competent engineer.
Anything is just guessing

 

[Yngmar]

1. The worn part of the shaft is 33mm in dia. For you non mathematicians that's a 14% reduction.

What matters more is that it's a 25% reduction in cross section (π*radius²).

 

[Scarboroughsloop]

I'm no engineer but if that were mine I would be replacing without hesitation.

Extracting the shaft is simple, just cut it as it comes out. Getting the new one in is the question. You may have enough space to get the new on in with no bearing in place, but if this isn't possible as I see it you only have the one option.....

 

[James_Calvert]

Extracting the shaft is simple, just cut it as it comes out.....

You might want to think about that a bit first...when I replaced mine (different boat), the propshaft supplier wanted the old one for a pattern for the new.

 

[Norman_E]

What matters more is that it's a 25% reduction in cross section (π*radius²).

It is worse than that. The strength of a shaft in torsion varies as the third power of the diameter. Originally 38mm in diameter, torsional resistance proportional to 38 cubed (54872) Reduced to 33mm diameter, present resistance in torsion is proportional to 33 cubed (35937) or about 65% of the original. Those black areas may as stated above be evidence of crevice corrosion, but even if they are not, any roughness of the surface is a stress raiser. Shafts are polished for good reason. It is probably down to half its original strength in torsion, and every time the engine is put in gear, or the gearbox put in reverse the stresses in that thin part are getting it closer to sudden failure. It must be replaced.

 

[Norman_E]

You might want to think about that a bit first...when I replaced mine (different boat), the propshaft supplier wanted the old one for a pattern for the new.

All anyone can possibly need is the shaft diameter and overall length, provided you also give the turner the sawn off end where the propeller fits as a pattern for taper, keyway and screw thread.

 

[James_Calvert]

All anyone can possibly need is the shaft diameter and overall length, provided you also give the turner the sawn off end where the propeller fits as a pattern for taper, keyway and screw thread.

And the gearbox end in my case, where there were some clamping bolts which needed cut outs.

Out of interest, how easy is it to cut a 38mm SS propshaft in situ anyway, it's something I've never done. And how many cuts might be needed to get it out?

 

[doug748]

Take an angle grinder to it.

If I were not planning anything spectacular this season I might leave it till next. I would take it steady though; it has not broken yet so that should buck you up.

I guess the P bracket well hidden and glassed in to the Nth degree? If you could shift that you should be in with a chance.

 

[Norman_E]

If you use an angle grinder be very careful where the sparks go. A pad of wet cloth can help, but make sure it stays wet and have a fire extinguisher handy. I have cut a stainless shaft with an ordinary hacksaw and a good bi-metal blade. You just need slow steady cutting strokes using the full length of the blade.

 

[awol]

Some good advice above - one solution not so far offered, or if it was I missed it, would be to heat and shrink a tube onto the outside. Would restore the torsional strength but would require enough space to get it on and the stern gland would not come off again without surgery.

I presume surgery on the skeg is out of the question?

 

[No Regrets]

Just imagine what would happen if that snapped.

Then replace it!! :nonchalance: