What is Minimum Diameter for a Propeller Shaft

[4HMainer]

Originally Posted by foolishsailor

Regarding what has changed...there was a middle phenolic bearing that had been in there that had over time had melted and seized onto the shaft and prevented and water from coming up to the inner cutlass bearing. This is another reason I am looking to put in a larger diameter stern tube so that if I decide to put back the middle bearing then I could have the machinist cut larger water grooves...

Sorry, but phenolic bearings don't melt they harden with heat and then polish, at failure they turn to carbon! if you mean a rubber bearing in a phenolic carrier that is something completely different. Phenolic bearings are chosen in place of other materials because they don't melt.

I'm not surprised a centre bearing in the tube got warm, did you have a good water feed? was the feed direct from sea water or after the engine (which could be too warm)

I'd go for a good water feed (direct from sea) and a couple of maritex bearings a bit longer than the normal 4 x shaft. mounted in epoxy so that they can be easily removed when they need changing, but you should get at least 3 or 4 times tha life of any rubber bearing and the shaft will move less.

[alexandlorna]

I am a mere novice in the boating world , however my specialist field is the manufacture of special purpose machinery and machine tools .I do this on a daily basis and from the sharp end not from a chair .
At a glance it would seem that you must either ditch the tank or take steps to provide a properly engineered drive solution .
By this I mean a substantial shaft ,the diameter of which would be dependent on the second part of the job which would be to cut out the tank tube and replace it with a heavy duty structural tube carrying a bearing at each end and one or 2 in the centre .
This tube would of necessity involve a lot of machining work to provide the necessary bearing seats and line bored accuracy and would require the tank to be capable of providing support for the tube ,by means of welded in end fittings .
Then having achieved a true running structuraly sound bearing tube and shaft ,which in effect is now part of the tank , provision must be made to ensure the engine is coupled by some sort of cv type joint ,in order to ensure no engine movement is transferred to the tank/bearing tube.
It should be possible with careful work to align the propeller end of the assembly with the cutlass bearing .
I realise this is probably not what you want to hear , but I every week I see half done badly engineered work that costs thousands to repair and the best way to avoid heartache is to do the job properly .
There are many wonderful products available in the machine tool world for replicating shafts and bearings into place using resins etc , but these will be of little use in this case .
I wish you the best of luck with finding a lasting solution to this problem

Alex

[perchance]

If the tank is aluminium or steel and you have room to work you could weld a pair of thick pads to the tank and install a pair of flanged bearings on each end to support the shaft where it goes through the tank. Having no idea of your boats layout this sugestion may be worthless to you but I thought I'd give it a shot.