Clonking prop?

[fastjedi]

1GM10 to 25mm shaft (200 hours from new)

At the end of last season the shaft was clonking for the first few revolutions when engauging gear. When we went ashore I checked the play (about 1 - 2mm) and decided to fit a new cutlass. I have to say the new one was a very similar fit. (Checked the shaft with a micrometer. There is no wear) It seemed marginally better for a while ... but not anymore (8 engine hours since relaunch) When cold it doesn't knock at all. After a 30 minute trip up river to our swinging mooring it will continually knock when backed off to tickover in gear (forward or reverse) It doesn't knock in neutral or under normal drive load. There is no play in the shaft (tested from inside the boat), no reason to beleive the shaft is bent and there is no noise from the gearbox. I haven't checked the engine mounts yet as I didn't have a bar handy to lift the engine

I'm two weeks away from my 2 week sailing holiday ... which comes but once a year. Not sure if I should go sailing or spend my holiday trying to find the problem? Anyone had similar? /forums/images/graemlins/frown.gif

 

[bumblefish]

A friend of mine, no really, had a loud banging noise when going astern. We feared the worst, but it was a loose drive shaft anode!

 

[MacW]

Had this myself a good few years ago - in my case the washer between the prop nut and the prop had disintegrated, allowing the prop to move back down the taper a fraction when not under forward thrust, and clonking on the shaft key. lot of noise for such a little movement, and not easy to see looking at the prop.
Before drying her out, in this case by leaning her against a handy jetty for a tide, I `proved` the case by grasping the inboard bit of the shaft just aft of the coupling, engine off and out of gear and turning it quickly to and fro,if it is the prop, you should be able to feel the free play. Run the drive astern for a few seconds prior to this test to `pull` the prop off the taper, if that`s where the problem lies.
Only other things I can think of is wayward engine /gearbox mountings, loose coupling bolts ( even just one ), or loose engine/gearbox flange bolts.
Hope this helps.

 

[wragges]

Have a good look over the engine mounts first. I had exactly the same problem and found that one mount was donking at certian revs on the base plate (the rubber had colapsed). It sounded like I had something around the prop/p-bracket from the cockpit area, but when I got someone else to take the wheel I soon tracked it down by pushing /pulling the engine from one side to the other whilst running.

 

[awyatybw]

Well I suppose it depends upon the exact nature of the clonking. There have been a couple of useful suggestions already.

I was concerned about a similar noise - also only while in tickover ahead or astern. I asked a marine engineer, one whose opinions I particularly respect, and he thought that it was most likely transferred noise from the Stripper rope-cutter. Probably exacerbated by ours not being installed as per the instructions (due to constraints with the cutless-bearing housing) - if I recall correctly, the striker plate should be at the bottom while ours is offset a little from the top.

Good luck,
Alan.

 

[fastjedi]

Having asked around I am now thinking it might be as simple as a lumpy single cylinder diesel on soft engine mounts with a solid prop. This would certainly explain the difference between cold and hot. Changing the slow run speed might change everthing?