Unsupported length of prop shaft too long?

[lw395]

The concept of a flexible coupling and only one bearing on the shaft seems quite absurd to me.
Compounding that by having the mass of the prop significantly beyond the bearing seems shoddy in the extreme.

 

[kacecar]

Thanks everyone for all the info and opinions - and for a useful debate. Let's wrap this one up . . .

I think what I've learnt with regard to the original question is that a stern-tube cutless bearing might be useful but that, as the boat wasn't built with one, it probably isn't critical. However, what I've also learnt is that the distance between the cutless bearing in the P-bracket and the propellor (what some call the "over-hang") is a more critical measurement than I thought.

But, before outlining the impact that all this learning has had on my plans here, in fairness to suppliers of flexible couplings, is a short but related aside: When I purchased my flexible coupling a few years back the supplier did warn me to check the over-hang, although I don’t remember getting specific measurements as part of the guidance. At the time, as vibration hadn't originally been a problem with the 30mm over-hang, I hoped an extra 28mm wouldn't be too significant. Furthermore, during a recent discussion about whether or not keeping the flexible coupling in line remained worthwhile, the supplier’s reasoning was that, in a well-aligned engine / shaft / p-bracket set-up, their design of flexible coupling, which features registers that ensure concentricity at all times, doesn’t allow the sort of flexibility that creates the problem of “too many flexible components” in the shaft.

Anyway, back to the plot (no worries, we’re near the end now).

So, with everything that's been said in mind, I will now remove the flexible coupling as planned, fit a new cutless bearing (which I would normally do anyway when the shaft is out) and then monitor results. If that set-up, standard except for the replacement of a traditional stern-gland with a modern one, still gives vibration I'll put the flexible drive back in, check it all again and see if that makes any difference at all. The theory is that it should be worse because of the excessive over-hang and, if indeed it is, I'll then shorten the shaft so that the distance from P-bracket to prop is closer to the 12mm suggested as ideal. Then, if the vibration problem still remains I’ll have nothing to lose, so will re-instate a solid coupling (yet again) and fit a small stern-tube cutless bearing.

And if, after all that (and it might take me years) I’m still unhappy with the vibration then I'll be considering judicious placement of an explosive charge. Er . . . no I won’t. I’ll probably just stop using the engine!

Thanks again for all the ideas, info, suggestions, time, effort and interest.

 

[martinriches]

I had the same problem on my ALBIN25 when I replaced the stuffing box with a PSS seal. I ended up adding a cutless bearing just astern of the seal.I had the OD machined to fit and glued it in with Araldite presicion.That was 3 years ago, no problems so far.

Martin

 

[mikemonty]

Thanks everyone for all the info and opinions - and for a useful debate. Let's wrap this one up . . .


Thanks again for all the ideas, info, suggestions, time, effort and interest.

Just before you go...

Assuming the vibration is being caused by some kind of "shaft whip" due to the relatively flexible shaft - and it's bad enough to need fixing - why not sleeve the original shaft? Can't imagine dynamic balance would be a big issue on a slow turning, thin rotor like that and a sleeve might make all the difference to stiffness.

You could always have them balanced as an assembly anyway if it was an issue.

Or perhaps the shaft has a slight bend as it is? Have you had a dial gauge on it?

I agree with "if it ain't broke don't fix it" - what a pile of ballox from whoever deliberately (I hope, for their sake) mis-construed the intent of that advice.

 

[lw395]

Just before you go...

Assuming the vibration is being caused by some kind of "shaft whip" due to the relatively flexible shaft - and it's bad enough to need fixing - why not sleeve the original shaft? Can't imagine dynamic balance would be a big issue on a slow turning, thin rotor like that and a sleeve might make all the difference to stiffness.

You could always have them balanced as an assembly anyway if it was an issue.

Or perhaps the shaft has a slight bend as it is? Have you had a dial gauge on it?

I agree with "if it ain't broke don't fix it" - what a pile of ballox from whoever deliberately (I hope, for their sake) mis-construed the intent of that advice.

I thought of that, but the problem is not that the shaft is whipping, but it is located by the fixed P bracket at one end, and a flexibly mounted engine at the other, so making the connection to the engine flexible needs a lot more thought.
The front of the shaft is probably moving well beyond the range of a dial gauge.
What would happen if you caught a pot line? It could well be very destructive.

 

[Plevier]

Just for a comparison -
I have a VP MD2020, rigid coupling, 25mm dia shaft with 34" (865mm) from the coupling to the cutless in the P bracket, VP shaft seal in between. There is then 40mm overhang to the KiwiProp.
The original shaft had less overhang, I had to replace it because of corrosion and increased the overhang to allow for a ropecutter (not fitted yet). If this dimension is supposed to be only 10-12mm how does anyone fit a ropecutter?
Max shaft speed is 1450rpm at 3400 engine rpm.
I have no detectable shaft vibration/whip at any speed. According to the info in this thread I should have!

 

[Tranona]

If this dimension is supposed to be only 10-12mm how does anyone fit a ropecutter?

It is not a case of "supposed to be" it is what is needed to allow water to flow through the cutless and perhaps more importantly allow for the fore and aft movement of the whole powertrain as you go in and out of gear. With soft mounts as much as +/- 6mm. Many standard installations have much larger gaps - Beneteau are around 30mm as standard for example. Typical 40-50mm when a cutter is fitted does not seem to be a problem.

 

[lw395]

It is not a case of "supposed to be" it is what is needed to allow water to flow through the cutless and perhaps more importantly allow for the fore and aft movement of the whole powertrain as you go in and out of gear. With soft mounts as much as +/- 6mm. Many standard installations have much larger gaps - Beneteau are around 30mm as standard for example. Typical 40-50mm when a cutter is fitted does not seem to be a problem.

But do they have flexible couplings and only one shaft bearing as standard?

 

[Tranona]

But do they have flexible couplings and only one shaft bearing as standard?

They may (or may not) have flexible couplings but they do only have one bearing, plus a Volvo seal which does provide some support.

Anyway at the sort of overhang we are talking about here it really does not make any difference. There are many boats that run happily with more than 2* diameter overhang and hang props far heavier than a standard fixed prop off the end without any problems.

 

[Plevier]

Many standard installations have much larger gaps - Beneteau are around 30mm as standard for example. Typical 40-50mm when a cutter is fitted does not seem to be a problem.

That's exactly my point. Experts posting in this thread have been saying a gap of more than about 15mm is a recipe for vibration.

 

[Plevier]

But do they have flexible couplings and only one shaft bearing as standard?

In the case of Ben and Jan in my experience, no. Rigid coupling and one shaft bearing.
Volvo Penta specifically advise against having a flexible coupling if you only have one shaft bearing and a flexible seal such as their own.

 

[Neil_Y]

Ideal is getting gap to a minimum on your set up, with rope cutters the cutter/spool may take up 20-25mm so you could see a gap from P bracket to prop hub of 32-37mm as Tranoma sais this is within the normal build gaps. So you have room to fit a cutter without any increase in shaft length on most vessels.

If you don't want/have a cutter then you can reduce the gap down to 12.00mm on a soft mounted drive.

On a new cutter install and soft mounted drive we use shaft movement max/worst case of 18.00mm (as this is what has been seen normally it is less, 9 fore and 9 aft) This means you need a gap after fitting of spool/cutter of 12.00mm, 9 forwards leaves 3.00mm for water to exit bearing. The cutter can add extra distance from bearing to the prop and you should just try to keep this as small as possible. Min gap we need for a soft drive and cutter depends on prop hub shape/size if we can't sleeve the prop hub with spool then we need 32.00mm from prop to bearing, in which gap we would put a 20.00mm spool leaving the 12.00mm.

Long over hangs definitely accelerate bearing wear, bearing wear is caused by a shaft trying to flex and move and causing the water film to collapse thus running the bearing effectively dry.