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AuthorMessage
chrisfeast
chrisfeast

Posted: Sun Dec 20, 2015 11:50am

Post Subject: Drive shaft

Hi, To allow for some work to be done on our boat the staff at the marina have moved it each time from its moorings to their maintenance berth. Each time they have had a problem with the shaft coming out to its coupling when the engine is in reverse. Being relatively new to boating I cannot understand why this should be happening when it hasn't been a problem when out cruising. What causes this to happen and what can be done to ensure a more secure coupling? Although having only come out twice is it likely the shaft/coupling is damaged? Is it likely that going from forward to reverse at high engine speeds would cause this problem as I always pause between forward & reverse and try and avoid maximum revs., in either mode.

Drive shaft
Tony-B
Tony-B

Posted: Sun Dec 20, 2015 2:37pm

Post Subject: Drive shaft

I am sorry but it photos put on the forum end up so small to be useless. Please email the photo to tony@tb-training.co.uk. It is perfectly possible that the experience yard hands are using far more engine revs that you do so they can use propeller walk to help position the boat. That is probably why you have not suffered the problem. Most shaft couplings nowadays clamp onto the shaft in one way or another. I hope to be more specific when I receive your photo. I think yours may have some alan-key type grub screws or hexagon set screws in one of faces that tighten a cone onto the shaft. I would hope the yard would have slackened then, replaced the shaft, and then fully tightened them. If they did not then the shaft and coupling might be OK, but if they did then I fear one or or both may well be worn so far the coupling will no longer exert sufficient grip upon the shaft. I will reply again when I have the photo.

Tony-B
Tony-B

Posted: Mon Dec 21, 2015 9:03am

Post Subject: Drive shaft

Thanks for the email. I fear things are probably worse than I feared. Now I can see the coupling it is clear that its a simple clamp on type secured with the four hexagon bolts shown in your photo. If the yard loosened the bolts, replaced the shaft, and then tightened the to the correct torque then I fear you are correct that the shaft, coupling or both are worn, The shaft now needs drawing back and the diameter and coupling bore size measuring with something like vernier callipers. I see that you have no flexible coupling so if the engine is flexibly mounted it will "work" the coupling on the shaft unless you are using a packless sterngland and Cutless rubber shaft bearings. You do have a packless gland but I am not sure about the shaft bearings. I expect they are rubber though. With this set-up it is VITAL that the coupling and engine are properly aligned to within 0.05 mm in both axial and angular alignment (see gearbox manual to confirm). If it has flexible couplings that are ageing the mounts may have collapsed. Sorry for the bad news.

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