Wiggo
Well-Known Member
So I got an update from the engineer earlier. They pulled the port shaft and ran it between centres and there was a 0.020" bend in the middle. Having straightened it and refitted it, the alignment was wrong on the coupling so ignoring the instructions on the coupling, they removed the coupling and did the alignment flange to flange.
This showed the gearbox was still low, so the mounts were adjusted to line everything up and the coupling refitted. This time everything came up true. The real proof comes tomorrow when they run the thing up and see if the vibration has gone, but for now here's the thoughts:
We probably struck some floating debris (the bend was midway between the P bracket and stern gland) and bent the shaft by 20 thou. That was enough to cause vibration, but not enough for it to be noticeable. The vibration shook one of the rear engine mount nuts loose and that allowed the back of the engine to sag enough for the heads of the coupling bolts to start striking the flange. At that stage, the vibration was clearly noticeable, so we had her lifted.
The 'obvious' cause was a loose engine mount, so that was rectified and everything realigned. Unfortunately, it only aligned correctly because there was too much play in the coupling itself as it had been damaged by the vibration. Two hours running later and the coupling failed:
A new coupling was fitted, but to a bent shaft, and the new coupling did not have the give that the damaged one had, so once everything was tightened up the bent shaft now started to shake the back end of the engine round, at which point we again had her lifted.
Fingers crossed, but that's the last of it. The engineers agree that the only logical cause was underwater damage, but slight enough not to have been felt immediately. Interestingly, the day that it happened, as we were limping into Lymington, Solent CG was broadcasting a warning about floating debris off Portsmouth, so we could have struck something up towards Southampton and it took another hour's running to work the engine mount loose. We will probably never know, of course...
This showed the gearbox was still low, so the mounts were adjusted to line everything up and the coupling refitted. This time everything came up true. The real proof comes tomorrow when they run the thing up and see if the vibration has gone, but for now here's the thoughts:
We probably struck some floating debris (the bend was midway between the P bracket and stern gland) and bent the shaft by 20 thou. That was enough to cause vibration, but not enough for it to be noticeable. The vibration shook one of the rear engine mount nuts loose and that allowed the back of the engine to sag enough for the heads of the coupling bolts to start striking the flange. At that stage, the vibration was clearly noticeable, so we had her lifted.
The 'obvious' cause was a loose engine mount, so that was rectified and everything realigned. Unfortunately, it only aligned correctly because there was too much play in the coupling itself as it had been damaged by the vibration. Two hours running later and the coupling failed:
A new coupling was fitted, but to a bent shaft, and the new coupling did not have the give that the damaged one had, so once everything was tightened up the bent shaft now started to shake the back end of the engine round, at which point we again had her lifted.
Fingers crossed, but that's the last of it. The engineers agree that the only logical cause was underwater damage, but slight enough not to have been felt immediately. Interestingly, the day that it happened, as we were limping into Lymington, Solent CG was broadcasting a warning about floating debris off Portsmouth, so we could have struck something up towards Southampton and it took another hour's running to work the engine mount loose. We will probably never know, of course...
