Unsupported length of prop shaft too long?

[kacecar]

Hi

I've posted on here before about vibration in the drive train, and P-brackets, and dripless seals etc. and have valued everyone's input. Having taken the forum's advice, the vibration hasn't been so bad for a while but, while choosing a replacement shaft seal recently I had an interesting conversation with an engineering company - and it's set me thinking. (Dangerous!) So, at the risk of wasting everyone's time, here’s a long preamble and a few related questions.

My boat, a Sigma 362, has a 1" diameter prop shaft. The gearbox coupling is (as far as this query is concerned) rigid. The stainless-steel shaft passes through a short stern tube then on to a cutless bearing in a P-bracket. The distance from the gearbox flange to the forward edge of the cutless bearing in the P-Bracket is approximately 45”.

The engineering company told me Lloyds recommendation is that a 1" propshaft should have a maximum unsupported length of 30" (the guideline being 30 times shaft diameter).

When the 362 was designed/built and gained Lloyds approval, it was fitted with a traditional packed stern gland. I now understand that this would have counted as a fixed point, so the longest unsupported distance, between the rear of the stern-gland to the front edge of the cutless bearing, would have come in at about 35”, presumably close enough to the guideline for Lloyds not to be too concerned. However, with the fitting of a modern flexible seal, like my new one and the one that it replaces, there is no longer any support where the stern-gland used to be, so the unsupported length becomes the full 45” – 50% more than the guideline indicates appropriate.

I’m not overly bothered by this at all as it’s been like it for more than 10 years and, slight vibration aside, runs OK. However, now that I understand a bit more about it, should I be even slightly concerned about the non-compliance with "unsupported length" guideline? And, if so, what would be the most practical, cost-effective "fix"? For example should I, if I could, fit a short cutless bearing in the inboard end of the stern-tube to re-instate the intermediate fixed-point? Or, would that tend put too much pressure on the shaft as the motor moves about on its mounts, or on the mounts themselves?

 

[TimBennet]

Ideal solution is to fit an aquadrive CV joint and thrust bearing between the stuffing box and gearbox (as close to the stuffing box as possible).

This should stiffen the shaft, take the thrust of the prop and allow constant alignment of the shaft through the stuffing box at all angles of heel.

However it's not cheap and would require a small bulkhead to be glassed in.

 

[dunedin]

One question. Is there any gap at all between the P bracket and the prop.

When we got our boat the shaft vibrated badly. A flexi joint was positioned between the gearbox and the shaft.
But I was advised that the mere 30cm gap between P drive and prob could be the issue. I was unconvinced, but they removed the coupling, hence brought the prop close up against the P drive - and magically the problem was sorted completely.
Not sure yours will be the same, but simple things can make a big difference

 

[John the kiwi]

Whirling speed

Way back in the dark ages when i was training as a mechanical engineer we learned about "whirling speed". This is about the speeds at which spinning masses become a bit unstable. For example, as your washing machine ramps up in speed you may notice that the vibration is worse at some speeds and smooths out at others.

A good mech engineer could calculate the parameters of your situation (this is what Lloyds will have done in reaching their rules.) and advise a solution.

Really though you have only 2 options: increase the shaft size or shorten the unsupported length.

 

[rjcoles]

One way to solve this problem is to install an inboard half cutlass bearing directly behind the drip-less seal. You may have to have the new cutlass bearing OD machined to match the ID of the shaft tube and then you just push it in and then assemble the drip-less seal in front. For further info talk to Paul Bell at First Marine Propulsion in Southampton, he should be able to help you, he did my shaft,seals and half cutlass for me!

 

[kacecar]

Thanks for the replies.

TimBennet: I am aware of the Aquadrive solution and in some ways it's quite attractive but it would be a tight fit as well as expensive - and there are other things I should spend money on first.

Dunedin: On my boat here is, as standard, a 30mm gap between prop and p-bracket. With a flexible coupling in place that gap extends to 58mm (i.e. about 60mm). Your experience is interesting - and I take your point about simple things sometimes making a big difference. Perhaps a user of a flexible coupling should be advised to shorten the propshaft accordingly. It might prevent vibration but it would also remove the coupling user's ability to take it out again. Having said that, shortening the shaft was certainly something I considered at the time.

John the kiwi: I appreciate the maths that goes into these things (although I don't understand it - I found a formula for calculating shaft size on the web but didn't even understand the parameters!) but, being as Lloyds is an insurance company and insurance companies always err hugely on the side of caution when giving advice, I'm still unsure of how concerned I should be about compliance with this guideline. Of the two options you suggest I think only shortening the unsupported length might be practical.

 

[kacecar]

rjcoles: "One way to solve this problem is to install an inboard half cutlass bearing directly behind the drip-less seal."

That's precisely the solution I was thinking of - its encouraging to know that it's been done before, presumably with some success. Do you know how he got the half-cutless to stay in place?

 

[penfold]

rjcoles: "One way to solve this problem is to install an inboard half cutlass bearing directly behind the drip-less seal."

That's precisely the solution I was thinking of - its encouraging to know that it's been done before, presumably with some success. Do you know how he got the half-cutless to stay in place?

Not sure what the proper way to do it is, but assuming a cutlass bearing could be found that was a sliding fit in the stern tube I'd be tempted to just bed it in silicone or lifecauk and secure with a self-tapper through the stern tube and tightened onto the outside of the cutlass bearing.

If you can't find one with a sliding fit you may need to have a sleeve made up and glued into the sterntube.

 

[essexboy]

I had a similar issue and added a half length cutlass in the stern tube just behind the PSS seal
and it fixed my problem.

 

[Neil_Y]

Shaft bearings can be supplied to any ID, OD and length, but ideally you would want a water feed.

Prop to P bracket ideal is 12.00 mm for a soft mounted engine not 30mm and definately not over 50.

I guess you have a rubber bearing in the P bracket? as you said it was a cutless type. You could try a hard bearing which will not allow the shaft to flex as much.

It's a bit late for maths...

 

[Tranona]

The original shaft seal would not have been rigid, but mounted on a rubber tube, specifically to allow the shaft to move if you have a rigid coupling. If you do put a bearing (either cutless or composite) in the stern tube inboard end you will have to fit a flexible coupling or the vibrations will increase and the bearing will be wrecked.

The setup you have is very common and generally works well, but is susceptible to wear or out of alignment of the cutless in the P bracket. When you have it apart, change the cutless, check the shaft is straight and most importantly line the engine up correctly. If it is set up with the shaft out of true with the P bracket you will get vibration as the whole engine/box shaft moves as one. Also worth (if you have not done it already) replacing the engine mounts.

 

[lw395]

Is this actually causing you a problem, or are you just on the wrong side of a 'rule of thumb'?

A lot of these rules are fine things, but in proper engineering they are used with a bit of understanding.
It may be the case that the permissable length is a function of RPM as well as material and shaft size.
Sometimes, in reality things are fine but if you fail the 'rule of thumb' you have to do all the hard sums, which probably are not all that hard, to see if there is a problem.

 

[lw395]

There is a suitable 'hard sum' in para 6.6.2 of this

http://www.abycinc.org/committees/P-06.pdf

Unfortunately it is in redneck units, but excel is your friend!

 

[vyv_cox]

The shaft on a Sadler 34 has very similar dimensions to yours and I have always suspected that it goes through a critical at around 2000 engine rpm. Despite my considerable efforts in eliminating the resonance at these revs, including an Aquadrive, it persists, although not too badly. I try to avoid motoring in the rev range 1900 - 2100, which of course is precisely the boat speed I want!

Be careful where you site a shaft anode, for least effect on shaft vibration put it close to the P-bracket. Your packed gland would have had almost no effect on stiffness if it was the normal type mounted on rubber hose. I replaced mine with a PSS, with no noticeable effect on the vibration.

 

[Ruffles]

I replaced a traditional stern gland with a deep sea seal. This left the shaft completely unsupported and therefore lots of vibration.

When this wore out I fitted a volvo seal which dramatically reduced the vibration. They contain cutless style splines internally to help align the seal.

Might work for you. You could try it for a season. Won't cost you much.

 

[dunedin]

Thanks for the replies.

Dunedin: On my boat here is, as standard, a 30mm gap between prop and p-bracket. With a flexible coupling in place that gap extends to 58mm (i.e. about 60mm). Your experience is interesting - and I take your point about simple things sometimes making a big difference. Perhaps a user of a flexible coupling should be advised to shorten the propshaft accordingly. It might prevent vibration but it would also remove the coupling user's ability to take it out again. Having said that, shortening the shaft was certainly something I considered at the time.

Just looked back at the photos. We had a gap of about 70mm between prop and P bracket, wiith a shaft anode part filling the gap. In this mode vibration started about 4.5 knots, and couldn't motor above 5 knots (36 footer).
As noted, the engineer condemned this on one glance - saying gap should be less than the shaft diameter. And was proven correct in our case.
Now gap is down to I think about 10mm - with shaft anode ahead of P bracket. Now cruises comfortably at 6 knots, and happy at 7 (vibration wise) if we need it for short periods.
Good luck with your investigations

 

[Neil_Y]

10.00mm I hope, not 10cm?

 

[wotayottie]

I’m not overly bothered by this at all as it’s been like it for more than 10 years and, slight vibration aside, runs OK.

First rule of boat maintenance - if it isnt broke, dont fix it.

 

[vyv_cox]

First rule of boat maintenance - if it isnt broke, dont fix it.

Often quoted, but nobody actually obeys it because it is nonsense.
Don't change filters until the old ones are blocked up?
Don't replace chafed sheets with no outer braid left on them?
Don't top the oil up until the engine seizes?
Don't tighten loose keel bolts until the keel has fallen off?
Leave drive belts alone until they break?
And in the current case, the small vibration being experienced will be causing wear to the cutless bearing and maybe seal faces and the gearbox. A small adjustment or change may be sufficient to eliminate it altogether.

Preventive maintenance is the key to reliability. Ever hear 'A stitch in time prevents nine'? A far better one.

 

[boatmike]

Shaft bearings can be supplied to any ID, OD and length, but ideally you would want a water feed.

Prop to P bracket ideal is 12.00 mm for a soft mounted engine not 30mm and definately not over 50.

I guess you have a rubber bearing in the P bracket? as you said it was a cutless type. You could try a hard bearing which will not allow the shaft to flex as much.

It's a bit late for maths...

I agree the maths are interesting and while as a chartered mechanical engineer I am quite capable of doing a whole lot of calculations to support what Niel has said (yawn) I think from a practical point of view I support what Niel has said entirely. A previous owner has turned a less than ideal (30mm) situation into a disaster by fitting a flexible coupling. The 50mm between the prop and the bearing is your main problem. The vibrations set up here are being magnified down the unsupported shaft. In fact with this length of shaft from bearing to gearbox I would question the need for a flexible coupling at all. If you retain it you should definitely shorten the shaft to a maximum of 1.2 x the diameter of the shaft which in your case is circa 30mm, but ideally if you are not fitting a rope cutter then Niel's 12mm is the ideal and should be quite sufficient to accommodate any movement when on maximum thrust.

As a matter of interest I recently fitted Niel's rope cutters on both shafts of my Aquastar and we discussed the fact that someone had fitted flexible couplings that cause my prop to be far too far aft of the bearing. After much thought I just removed them and while I do indeed have a forward water fed cutless as well (which makes the shaft more rigid) The vibration has decreased rather than increasing. These are 200HP Volvo TAMD41a's though and probably much harder mounted than yours.

It does seem that many R&D couplings are sold to people to retrofit without advising them that the shafts should be shortened accordingly.