Propeller shaft - stern tube clearance

[SailingEcosse]

I've trawled the net and am struggling to find the info I'm looking for, so I'm hoping some of you knowledgeable folks will able to help, or at least point me in the right direction.

Background info
Colvic 26 sailer
Petter twin engine removed, stern tube assembly removed, cutlass bearing renewed, and everything refitted.
Existing shaft size 1"
New engine going in is a new Kubota 602 (16hp which is possibly a bit more power than required but it was cheaper than all the alternatives!)

Question:
On fitting the prop shaft it appears to have minimal clearance to the stern tube i.e. about 2mm total, or 1mm at any given point, is this enough and if not what should the minimum amount of clearance be?
My concern being that even small engine movement could make the shaft wear on the stern tube.

Next question (related to the above):
The prop shaft is machined down from to 3/4" od at the gearbox end to take the drive flange
Will the reduced diameter be okay for the engine HP?
Or should I be looking at a new 1" shaft, and dependant on the answer to the first question, a new stern tube as well?

Any and all help much appreciated (I will eventually stop asking questions, and may even be able to answer some in the distant future! )

 

[Tranona]

Do not machine the shaft down to fit the flange. Use the correct bore coupling. Sounds like you have a solid coupling and flexible mounts. The shaft will almost certainly foul the stern tube. With that small amount of clearance you need to introduce a bearing on the inboard end so the shaft is supported at both ends and use a flexible coupling such as a Bullflex, Centaflex or R&D so that the shaft remaims fixed and the engine can move on its mountings.

 

[NorthUp]

I have the same shaft/ tube setup. The engine fitted when I bought Conachair was a Yanmar GM20, with the soft engine mounts, and the rattle from the shaft hitting the tube at certain revs was one the factors that left me changing the engine to a Perkins 3 cylinder 103-10 (AKA Volvo 2030).
This has much more robust rubber engine mounts from R& D couplings, and was a definite improvement all round.
Despite a couple of years with the rattling yanmar, the shaft was not marked or worn, the tube slightly polished- I had changed to a cutless bearing to try and cure the rattle without success- the bearing was knackered after a year.

The reduction in diameter of the propshaft to 3/4" is perhaps a bit marginal- I would change it while the engine is out, and for good measure you can get a brass stub machined to fit the thread on the inner part of the stern tube, and fit the Volvo shaft seal!

Finally, going from 18HP with the yanmar to 25 or so with the Perkins increased max speed from 5.8 knots to 6.9k, but cruising at 5.5k is much quieter and more relaxed now.

edit- you have the tube out- I would be tempted to go up a size, but have you got enough width at the outboard end of the hull for anything bigger?

 

[Spyro]

With that clearance it will almost certainly bang against the stern tube and be in danger of damaging it possibly with dire consequences. Before I fitted my new shaft last year and set the alignment properly my shaft used to bang against the stern tube onengine start up, it was quite an alarming and worrying bang.

 

[SailingEcosse]

Okay, so if I'm reading this right

1" shaft should be retained and mated to a new half coupling which then connects to an R&D flexible coupling (I have this already) on the gearbox?

If I go with the 1" shaft though, clearance to the stern tube will be minimal and likely to hit, even with the R&D coupling in place?

I can't go any bigger on the stern tube without also machining out the hole through the hull though

How about going to a 25mm shaft for the extra .4mm, would that be a worthwhile improvement on available clearance?

Thanks

 

[Tranona]

No. My solution was to have the inner housing of the stern tube machined on the inside to take a length of cutless bearing and on the outside to take a Volvo seal. This keeps the shaft fixed and central but the engine (originally a bouncy Yanmar 1GM free to move. R&D coupling was OK but the (much more expensive) Bullflex even better.

 

[vyv_cox]

It depends whether your stern gland is on a rubber hose or a fixed one as shown for a Contessa here. I note you say you have removed the stern tube, which suggests that it is the rigid type, in which case the small clearance is OK because the shaft is always concentric in the tube.

With this assumption there are a couple of additional points.
A 3/4 inch shaft is a little on the light side for 16 HP, not so much that it won't take the power but because smaller diameters are more difficult for a coupling connection that relies on friction.

Your new engine may have softer mountings, a typical trend today. It may flex considerably more than the old one but the movement cannot be accommodated by the shaft/stern tube. You will need a flexible coupling of some sort, which I suggest you discuss with the engine supplier.

 

[SailingEcosse]

have the inner housing of the stern tube machined on the inside to take a length of cutless bearing

The inner end of the stern tube is only about 1" long, but I understand what you are getting at with this, I like this solution, but not sure how possible it would be given the current stern tube and fittings.

Your new engine may have softer mountings

Yes...very soft IMO!

discuss with the engine supplier

He said an R&D coupling would do the job, although to be fair neither he nor I knew how limited the shaft to tube clearance was at the time.

It depends whether your stern gland is on a rubber hose or a fixed one

Stern gland is not the same as the contessa one, but same as the rubber hose version shown

 

[vyv_cox]

Yes...very soft IMO!

He said an R&D coupling would do the job, although to be fair neither he nor I knew how limited the shaft to tube clearance was at the time.

Stern gland is not the same as the contessa one, but same as the rubber hose version shown

OK, your shaft/stern tube clearance is too small. My shaft is one inch, my stern tube OD is 1.5 inch and I'm guessing its wall thickness is around 3 mm (1/8 inch) so the total clearance is about 1/4 inch. Originally my engine was a Bukh which has hard mountings. I never experienced clash between shaft and tube. I re-engined to a soft mounting Yanmar 3GM and immediately began to get considerable clashing at specific engine revs. I changed the engine mountings to the harder version and have re-aligned many times for the optimum position. Only by fitting an Aquadrive did I overcome this, and even now it isn't entirely clash free.

 

[Tranona]

The inner end of the stern tube is only about 1" long, but I understand what you are getting at with this, I like this solution, but not sure how possible it would be given the current stern tube and fittings.

You may find that using a Volvo seal on its own will be sufficient to hold the shaft central in the tube. The design of the seal is such that most of its length is a fluted water lubricated rubber bearing very similar to a cutless bearing. This provides good support for the shaft and providing there is not too much distance between the seal and the coupling and the seal could limit the movement of the shaft. However, you might investigate having a new inner housing made long enough to take a 50mm or so length of cutless. Many of the older style stern tubes (such as Stuart Turner) used a fixed stuffing box with a white metal bearing. The casting is meaty enough to do what I suggest. Worth talking to stern gear manufacturers such as T Norris, Teignbridge or Lake engineering.

 

[SailingEcosse]

Thanks guys, much appreciated

I'm going strip the stern tube out again and measure it all up to see whats possible, and try and make a decision on where to go with it.

 

[pampas]

Bore the coupling to fit shaft,may need a new key way cut.