Thursday, 31 March 2011

Stern Tube

Happy that the drive shaft was perfectly in line and the engine was set square I welded in the engine mounts.  This was pretty straight forward once I wriggled into position to see what I was doing.

Next job was to weld in the stern tube.  This is a bit tricky and its easy to get wrong if you don't understand the forces involved when you weld 2 pieces of metal together.  What happens is at the spot of the welding there is a small but none the less significant distortion, first expansion, then contraction.  This would be fine and would cancel its self out if no additional metal was put in, but thats exactly what happens when you weld, you add metal.  This small amount of local distortion can have a big effect on something even as short as the stern tube which is about 250mm long.

To counter this distortion its vital that the job is set up with a number of strong tack welds and that the shaft is checked after each one.  The first few are the worst, but as more are added the movement becomes less.  Its also important that the tacks are done in opposite sides, a but like when you do wheels up on a car. Once a reasonable amount of tack welds are in place longer stitch welds can be applied.

Another factor that needed consideration was that the cutlass bearing is a rubber / plastic compound and therefore would melt if too much heat was applied.  To counter this each weld was quenched immediately.

It is also very important to ensure that the drive shaft still turns easily before proceeding with the next weld.  Its a slow gentle process but its the only way to do it.  If the weld was just blatted in all sorts of nasty things could happen.

Once I was happy with the positioning of the stern tube I removed the drive shaft and the cutlass bearing, and set to with welding it all properly.

The extra plate seen here was on the hull when I bought it.  There was evidence it had been afloat at some point and that these had been welded on to close up the hole for the stern tube.  This hole would have been cut as part of the original laser cut kit.  I decided it was beneficial to leave these plates on.

Stern Tube with the cutlass bearing removed

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