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Shaft drive systems in Engineering can be quite a problem. If you look back in time workshops had long lengths of line shaft driven from a mill or steam engine. these were carried in numerous bearings. If the bearings were sat on rigid structures this was no problem a good fitter just ensured correct alignement. If they were on timber supports which flexed then bearing life was short and fires plentiful.
How does this affect a Boat? Boats flex!! Engines on soft mounts move about under differing loads!!
Reg Freeman took good account of this with the Seadog and realised that a one piece shaft through a fixed stuffing box would give trouble.
What Reg did was to treat the Stuffing box as a forward bearing using the water lubricated packing as that bearing and it works well. He then fitted a short shaft between the gearbox coupling and the final drive shaft using flexible couplings to take up any misalignement and with hull flex and soft engine mounts that cannot be avoided.
This system offers a good number of advantages such as it is easy to change a cutlass bearing without having to remove the shaft it is just slid inboard by removing the short shaft.
Softer mounting rubbers can be used to reduce vibration.
No need to remove the rudder to change the prop and thats a big job saved Etc.
Removal of the short shaft allows engine/gearbox removal with out danger of catching shaft flanges.
If a one piece shaft system is to be used satifactorily then the stuffing box must be either removed completely and mounted on a flexible rubber tube, or a carbon face type seal fitted after removing the neck ring section of the stuffing box. Otherwise alignement is impossible with two fixed points and a moving engine. It would be back to the line shaft fire problem, but on a boat it results in fatigue failure.
Also Carbon Face seals and rubber mounted stuffing boxes can fail disasterously where as the old fixed stuffing box rarely creates a major problem and can always be limped home.
Comments appreciated
Eric
SeaDog Name (if owner or crew) Ex Taliesin
Graham
Mar 23, 2010 - 4:17PM
Re: Prop Shaft Systems
I fully support all you say Eric.
I had to persuade 'Scotty' from TS Marine to retain the intermediate shaft when we re-engined Dougal but when the job was completed (including an additional removal and replacement of the propeller!) Scotty was total convinced. He said that he would recommend the retention of the BW Gearbox and the intermediate shaft for all future BetaMarine replacements on SeaDogs.
I once had to remove and replace the propeller twice in one season, without the extra shaft it would have meant removing either the engine or the rudder.
Graham
SeaDog Name (if owner or crew) Dougal
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