A quiz on your own boats

[superheat6k]

This week I asked 2 separate mechanics to quote for changing the cutless / shaft bearings. One quoted for 2 and one quoted for 4. The one that quoted for 4 also wants to replace the stuffing boxes and the other one just wants to re-pack them. Which would you choose???

I would insist upon PSS seals, but it won't be a cheap job. DIY ~ £600 per shaft for bearings and new PSS seals assuming 40' boat.

 

[jimmy_the_builder]

None!

 

[john_morris_uk]

On my boat (which admittedly has white flappy things) I only discovered how many cutlass bearings there were when the shaft was removed. The inboard end is a Halyard Marine 'Aquadrive' and there's a cutlass bearing in the stern tube where the shaft exits the boat and another cutlass bearing in the 'P' bracket.

Hence I answered 'two'.

 

[oldgit]

This week I asked 2 separate mechanics to quote for changing the cutless / shaft bearings. One quoted for 2 and one quoted for 4. The one that quoted for 4 also wants to replace the stuffing boxes and the other one just wants to re-pack them. Which would you choose???

Why on earth would anyone want to replace a stuffing box if it is in seviceable condition.
The device is a time proven method using simple basic engineering to be able to easily change the inexpensive packing material on a regular basis over the lifetime of boat.
All they need is the occasional check every so often and as all Fairline Turbo 36 owners know all to well, an ability to actually see and get at the device.
Even neglected stuffing boxes can last for years, a recently purchased T36 had its "hole in the shower floor" cut,to reveal a stuffing box ,apparently having not seen daylight (or adjusted) for years.

If you like stern glands you will love this site.
http://coxengineering.sharepoint.com/pages/sternglands.aspx

 

[MapisM]

Why on earth would anyone want to replace a stuffing box if it is in seviceable condition.

+1.
In fact, I'm almost beginning to regret the stuffing box in my old tub, which is still doing its job nicely, after 20+ years. :nonchalance:

Back to the point, time to spill the beans, I reckon.
The thread was inspired by what I discovered after deciding, in the contest of other refitting jobs, to have also the mechanical seals checked/refurbished.
Btw, the seals are made by a company I never came across before (they deal mostly with industrial stuff in oil and gas industry, and commercial/bigger boats).
But I must say that they are indeed beautiful bits of engineering, and I'm glad the boat was built with them.
This is the thing, if anyone is curious to have a look.

And this is how the shaft exit looks like:

Now, it was easy to see that there is a cutlass bearing at the end of the shaft exit. And inside, only the shaft seal was in sight.
So, that's 2 for each shaft, including the P bracket, right?
Nope. As it turned out, the shaft exit is built around a bronze tube, diagonally "cutting" through the hull bottom and completely glassed, both inside and outside.
And there's a cutlass bearing at both ends of it, so that's 3 for each shaft.

The reason why I believe Jez is right in saying that he has two on his boat is that afaik Ferretti use a different construction for the shaft exit, i.e. the one below (the pic was taken on a F165, but I think most other Ferrettis are similar):

Now, I'm not sure of which solution is better from an hydrodynamic viewpoint. No prize for guessing what DP and Ferretti respectively would answer to that question...
But one thing that the first shaft exit above does assure is a better/stronger constraint of the shaft, with 3 points at just about the same distance along its length, without introducing an intermediate P bracket (as I've seen in some boats, like Traders and Hatteras, IIRC), which is bound to create more turbulence on the water flow.
So, all in all, a solution which I like, and fwiw even if I never had the opportunity to check the internal side, it seems to be adopted also by SL, at least in their 62 and 72.

One drawback of this solution is that the removal of the internal cutlass bearing is a real PITA, due to its position/accessibility.
This is in fact still in progress - fingers crossed for next week, when they will try to pull it out with a special extractor...
Time being, below is how the tube looks like from outside, after the shaft removal
The cutlass bearing is barely visible along the external border (out of focus and enlarged from an overall awful pic, sorry but that's all I managed to make), and the flange at the end is the BPM V-drive where the shaft is attached.

For myself, it was one of those "every day is a school day" occasions - I hope you also enjoyed the construction insight, even if for a rather minor thing...

 

[kashurst]

This week I asked 2 separate mechanics to quote for changing the cutless / shaft bearings. One quoted for 2 and one quoted for 4. The one that quoted for 4 also wants to replace the stuffing boxes and the other one just wants to re-pack them. Which would you choose???

Assuming the bearing in the P bracket by the propeller is worn that's 2 (1 per shaft)
The bearing immediately behind the stuffing box should wear very slowly unless engines are out of alignment, (probably not as you would have noticed other issues) - so worth checking the wear first as they can be a pig to replace. Had mine checked at 1000 hours and they were not worn at all.
if you have traditional stuffing boxes I would stick with them and have them repacked. Nothing to go wrong, no fancy seals to fret about and any boat yard can repack them easily.

 

[Uricanejack]

I picked 2

A Small Yanmar 3GM30. Engine, Gearbox, Shaft seal, Cutlas then max prop,
so 2 between gear box and prop

Though technically, There must be a number in the gear box including a thrust bearing. so I suppose the real answer is I haven't a clue.

 

[MapisM]

The bearing immediately behind the stuffing box should wear very slowly unless engines are out of alignment

Interestingly, this is not necessarily the case.
In fact, as I recently learned, not only with propellers, but also with shaft sizing there's a bit of black art involved.
In principle, shafts must be the smallest possible diameter adequate to withstand the rated power, for very obvious reasons.
But quite often, particularly on rather long shafts with a long distance between the bearing(s) and the g/box (or v-drive), builders prefer to oversize them, in order to avoid/reduce the so called whiplash effect while spinning (mind, I'm translating that literally from IT - not sure if there's a more specific EN wording, but I think it's easy to see what I mean).
In fact, the propeller thrust vector is by definition at a steeper angle compared to the shaft, which implies that the shaft tries to flex outward of its axis while rotating.
In turn, this can create some wear also on the cutlass bearings which are closer to the more constrained point (g/box or v-drive).
But as I said, this very much depends on some builders' choices - I'm saying all this just to explain why the bearing on the hull can wear more or less slowly, even without any alignment problems.

 

[Trundlebug]

I have 2 cutlass bearings per shaft.
I know that because I replaced them a couple of years ago.
In order to do that I removed the shafts and checked their straightness.
When I looked down the tubes there weren't any bearings I'd missed.

I have one Volvo shaft seal and one HMI. So no gland packing. But not sure that would affect it.
I think the design just varies boat to boat, and will also depend on shaft length I guess.

 

[MapisM]

In order to do that I removed the shafts and checked their straightness.

Doing that too!
Ain't it nice, when you start thinking to change just the P-bracket cutlass bearing, and then the "while I am at that, it's worth also..." reasoning starts, and you end with refurbished props and shaft seals, 3 cutlass bearings per side, shafts straightness check...?
Not to mention swmbo applying the same logic on cushions, sheets, covers, and so forth! :ambivalence:

 

[markc]

So now the boat is actually mine I'm going to be booking it into the yard in the next week or so to have these done. I was wondering why one yard had opted to quote for new stuffing boxes because AFAIK the current ones are fine and I'm quite happy with old school ones. It just seemed a bit of a liberty to put that on the quote. OTOH the other yard just assumed that only 2 bearings needed to be replaced without considering the need to check and maybe replace any in the shaft tubes.

 

[MapisM]

First of all, congratulations for the completion of the deal! :encouragement:

Ref. the quotations you got, hard to give you any advice, other than I would rather change all bearings together, in principle.
Otoh, I also agree that there's nothing wrong with stuffing boxes in general, and unless they are de-zynced/corroded/cracked/whatever, I see no reason to replace them.
The only thing that pops to my mind is that if the hull bearings require the shafts removal to be replaced, which is rather likely, the yard who quoted also the stuffing boxes thought that it was worth replacing them regardless, considering also that the parts as such should be rather inexpensive (though you never know, nautical stuff and all that...!)

Then again, at that point you might as well consider fitting some more modern seals...
Mind, as I said I see nothing wrong with stuffing boxes, but I was VERY impressed by the performance of the mechanical seals on the new to me DP: 1400 hours with zero maintenance whatsoever, and just one of them was beginning to drip a little bit, and only while running. Essentially, I had them checked and refurbished just along the lines of the "while I am at that..." principle that I previously mentioned!