shafted

siencyn

siencyn

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I know this topic has been discussed ad infinitum on the fora but my problem is a little different. Do you think it is possible to ''joint'' a shaft? I have to remove the shaft from my Countess 33 and the boatyard informs me that the design means either lifting the engine out or removing the skeg!! The shaft has to come out as misalignment has caused it to damage the stern tube and this urgently needs reglassing. Could you not cut the shaft and fit a good strong tight fit cylinder over the joint with bolts through the ends to secure it? Or would this affect the balance too much? Or, what about a flexible coupling in the middle? Can't be taking the engine out every time!! Thanks for you thoughts. /forums/images/graemlins/confused.gif
 
Something like these maybe?
rigids.jpg


Manufactured by These folk, distributed in the UK by Lenze or believe it or not RS Components /forums/images/graemlins/grin.gif
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"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>
 
[ QUOTE ]
Exactly! Cliff you are a gem. Thanks Butty! You wait till I show this to the boatyard!

Rich

[/ QUOTE ]If you look carefully, you will see that there is a keyway cut in each joiner. It will be a bit tricky (read impossible) to cut a similar keyway in the shaft in situ.

Its really not that hard to take the engine out. You're not going to lose the alignment - by your own admission there isn't any at the moment. Just unbolt the bits and slide it foward into the saloon. I covered everything with cardboard the last time I did it, and used the boom as a crane through the main hatch.

(Don't listen to the harbingers of doom who say you need a crane - I've done the same thing several times on boats from 33 feet to 55 feet - just make sure you support the boom with a halyard and a strop where you are lifting from.)
 
A flexible coupling would be a better solution as alignment is less critical and the coupling is likley to be better balanced and better suited to dealing with the fore/aft thrust.

Torque (Twisting force): Unless there are splines / keyway to transmit the torque you are relying on friction only between the couping and the shafts to spin your prop.

Thrust: Going forward, no problems, the prop will push into the hull. What is to stop the prop pulling iteslf out from the coupling (or even out through the back of the boat), when you stick the gearbox into reverse and give it some welly?

Be careful, be very careful

In addition, the coupling will reduce the vibration transmitted from the engine to the hull.
 
Thanks John, you make it sound so easy but the design of my boat does not allow for more than a few inches of forward movement of the engine. I would have to remove a cross bulkhead. It’s a lift right out job. I have looked at this site and I am not qualified to comment authoritatively but they have these couplings without a keyway and they seem to be saying that this is what they are for. I was thinking that once I cut the shaft out I should be able to find someone to cut a keyway in it shouldn’t I? Or could I expect it do the job without a keyway anyway?
 
OK I take that on board its all good advice. It would have to have a keyway but that would do it would it not? It will not be immersed in sea water at all [unless I'm sinking!]
 
Thanks Garden shed. I see what you mean about pulling it out with reverse engaged. I had not thought of that. But what's the difference between the shaft connection on a flexible coupling and this? The both are just keyways aren't they? What stops the shaft pulling away from a coupling?
 
[ QUOTE ]
If you look carefully, you will see that there is a key-way cut in each joiner. It will be a bit tricky (read impossible) to cut a similar key-way in the shaft in situ.

[/ QUOTE ]If you look carefully at the website you will see the couplings are supplied either keyed or plain (no key-way) however as the shaft will be taken out in two pieces each piece could easily have a key-way machined into it.
[ QUOTE ]
Torque (Twisting force): Unless there are splines / key-way to transmit the torque you are relying on friction only between the couping and the shafts to spin your prop.

Thrust: Going forward, no problems, the prop will push into the hull. What is to stop the prop pulling itself out from the coupling (or even out through the back of the boat), when you stick the gearbox into reverse and give it some welly?

[/ QUOTE ]Friction, What else do you need? These type of couplings are precision machined and can handle a lot more HP and torque than a sail boat drive train can throw at them. On some styles the clamp bolts are positioned to intersect the shafts to provide a "key" to prevent the shaft coming out in the event of excessive axial loads. You would either pull the back out of the gearbox or break the shaft before one of these clamps (properly sized and fitted) would slip.
[ QUOTE ]
Other consideration is whether these couplings are suitable for use when long term immersed in sea water.....

[/ QUOTE ]So cut the shaft inside if space permits or...use a stainless steel clamp (these type of clamps are available in stainless steel).

One thing that does surprise me though is the number of folk saying they have to remove their skeg and/or rudder to get the prop shaft out. On my boat the shaft is set at a (very) slight angle so when the prop is removed the shaft can slide past the skeg (just) - seems to me to be a good design feature which is apparently lacking on many boats.
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"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>
 
Some gearbox couplings have roll pins? Would a jubilee clip or two stop the shaft from disappearing aft if the worst happens?
 
Absolutely Cliff...
And the idea that a flexible coupling can take up misalignment is also risky. Some couplings are deisgned to do this others aren't. One of the most popular marine couplings isn't. So it depends on which type you buy.

But it is a simple process to get the shaft alignment sorted so this should never be an issue.
 
If you are intending to make the join inside the boat an alternative to these shaft sleeve type joiners would be two split-clamp half couplings and a bobbin as sold by www.asap-supplies.com (misc & sterngear/shaft couplings). But to get the shaft out you would still need a considerable distance between the stern tube and gearbox. This would also add quite considerably to the weight to the shaft. Just a thought.
 
FWIW, The standard coupling used on Bukh engines is a similar clamp arrangement to those above and I do not recall ever hearing of anyone pulling the shaft out going astern. I have heard of owners having problems getting the shaft out of the coupling even with the clamp bolts removed!
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hammer.thumb.gif
"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>
 
[ QUOTE ]
On my boat the shaft is set at a (very) slight angle so when the prop is removed the shaft can slide past the skeg (just) - seems to me to be a good design feature which is apparently lacking on many boats.


[/ QUOTE ] I am sorry to report that it is a design fault. I either have to drop the rudder or remove the engine on my boat to get the shaft out!
 
Cliff. In would endorse what you say about these couplings. They have a rated HP far in excess of the requirements in a small boat and I would further say that the ones with a keyway are arguably better but unnecessary too. I have used them on machinery drives and in one case a hovercraft main drive shaft. no probs. The ones I have used have required a small flat to be machined on the shaft and the bolts act as cotter pins like bike pedals. Easy peasy.
I further endorse what you say about propshaft alignment. Why oh why do designers not offset the shaft (and engine mounts by about 2 inches or so which, with the prop removed would allow the shaft to be removed) Do they think the boat would go around in circles??? (Doh) Lack of forethought like lots of other things. Maintenance? Whats that?
 
[ QUOTE ]
I am sorry to report that it is a design fault. I either have to drop the rudder or remove the engine on my boat to get the shaft out!

[/ QUOTE ]How do you make it out to be a design fault? What do you see as being the problem with such a design?
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hammer.thumb.gif
"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>
 
fenner can supply both rigid and flexable shaft couplings that clamp on the the shaft with taper lock bushes and depending on the HP of your engine could be used without the key but a key could also be fitted if required. The taperlock bush also will stop the shaft sliding out as the coupling is restrained laterally on the shaft so it will take thrust in both directions.
 
I agree about the pulling out of the joint scenario. I changed out the seal on my last boat and I had hells own job in getting the shaft free of the coupling. I also agree with the sentiment around bad design. It stinks that you have to either destroy your skeg or remove the engine to take a shaft out. Rubbish design I say! Anyway, it looks like you guys have come up trumps again with some excellent advice and a really thoughtful discussion. Many, many thanks. I will be putting a coupling in the shaft and see what happens.
 
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