Rope round prop damage.

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funkster

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Started the Bukh DV20 this morning in the LM27 that I'm Lucky enough to enjoy with the father in law and it died instantly with a "Clonk". After further inspection we noticed that a stray nearby floating mooring rope had jammed round the prop (not good). After about 10 mins struggling we managed to free the rope with the aid of a winch. Once motoring across oulton broad we both realised a considerable vibration, we lifted the engine cover to investigate and found the engine to be moving from side to side considerably. I took the boat out of drive and watched as the prop slowed and so did the wobble unitil it stopped completely. As I engaged drive again and the prop spun slowly it started to move the engine again in rhythm, so the instant thought was a bent propshaft. We got back to the mooring and split the propshaft to gearbox couplings and spun the propshaft by hand and all seemed ok so we started the engine with the prop disconnected and noticed that there is movement in the flange on the rear of the hurth box. Has anybody had this problem before and would you think the gearbox shaft could of become bent or is it more likley the flange itself. Any help would be appreciated. Matty.
 
Hi Matty.
Did you acyually find movement in the flange, ie, was there up and down,(radial) play in the box output shaft?
The hurth have a taper roller bearing on the output shaft, just inside the square alloy plate with 4 bolts in it which is just in front of the flange, check that plate for secureness.
There are shims between that plate and the bearing outer shell which determin the preload on the bearing and end float on the shaft.
Have you checked the engine mountings havent split with the torsional forces caused by the rope locking up the shaft, as all that torque has to be obsorbed by something.
Let us know the verdict when you find out.
Good luck.
 
I checked the engine mounts for play with a pry bar and also rocked the engine up/down and sideways which seems very secure. I could not feel any movement/play in the flange/shaft but the flange definitley does not run true. I just hope that the gearbox shaft has not bent and we have just overstressed and bent the flange but i wouldn't of thought that was possible as it would take a massive amount of force the bend a cast item like that. fingers crossed. I just wondered if this was a common thing from getting roped up. The flange does not run out by a large amount visual by eye but it is noticable and i suppose once its coupled up to the prop the movement is exaggerated.
 
I dont think its anything to do with the prop blade as the movement is there with the shaft spinning at a very slow speed eg 60 rpm. Surley the prop would need to be spinning at a decent rate to cause shaft wobble.
 
I lost one blade of a folding prop once, except in tick over the vibration was severe enough to make me worried about the mast. I managed to get the boat into a nearby marina berth using just tickover but you could still feel the vibration throughout the boat. So I wouldn't necessarily rule out prop damage.
 
You need to decide whether it is the flange (hopefully) or the shaft that is bent. I would use a dial indicator guage, that is a guage that you might be able to borrow from a friendly engineering machinist, which you mount (clamping its magnetic base on a solid place somewhere handy on the gearbox) so that its probe runs on the outer circumference of the shaft and the dial indicator moves to show the radial change in position (in 100ths of a millimetre (or in "thous" of an inch)).

It may just be possible to use this in the "centre" in the end of the gearbox shaft whilst the flange is still fitted. If not you will have to remove the big nut inside the flange, draw off the flange and locate the probe on a smooth part near the end of the shaft.

Turning the shaft slowly (by hand) the shaft should run true within a few thou or a few 1/100ths of a mm. (1 thou = 2 1/2 100ths of a mm). A total indicator reading (TIR)(max to min reading on the guage) should be less than 5 thou.(ideally 2 thou). If the reading is a great deal higher than this, you have a bent shaft.

With the flange in place on the shaft, place the probe on a smooth region near the outer edge of the face. It should run true to no more than 5 thou TIR (ideally 2 thou). If not the flange is bent (assuming that you have already tested the shaft and found it OK)
 
Sorry to hear this, a real pain to say the least!

We got a rope around our prop when we owned a Sadler 26, that was also a Bukh. Like you it was a stray warp, in a marina.

The rope wound itself between the prop and the P bracket, in our case fairly dramatically! As the Bukh has so much torque it just kept on winding! The damage was also extensive. We had a ZF gearbox which provide rear mounting for the engine/gearbox installation. The side of the cast ally gearbox were torn out!

Now to the engine damage and my theory over your problem. The front of the Bukh had a plate which is bolted to the block behind the timing gears, ours got bent, visibly. If yours has been bent a little, maybe you can't see it, it may be enough to knock the engine/gearbox to prop out of alignment. A bitch of a job to put right on a good day.

You may do well to check there is no water leaking from where the P bracket is bonded into the hull, that may be lose.

Best of luck, let us know what the outcome is.
 
Managed to get the gearbox flange off today and took it to a friend to check on there lathe and all seemed ok. So equipped with a dial gauge and magnetic vee block we decided to check it on the gearbox. I would like to of checked the shaft but the end only protrudes the box by about 20mm and and is heavily splined. The reading is 30 thou movement on the outer flange. My question ? Does anybody know what 30thou run out would be in degrees over the 3 1/2" flange size because I have read that some of the flexible couplings can correct up 2 degrees? Hopefully Matty
 
30 thou movement, or+/- .015" over 3.5" represents a wobble of about +/- 0.3 degrees if my maths is right. However, I don't think that you should be worrying about whether your flexible coupling can accommodate this. If I understand your description correctly, ( but I'm not certain that I have ), you have a bent gearbox output shaft. This is a hefty piece of metal, and must have experienced a considerable load. The roller or ball race on this shaft will have felt this load, and might be damaged, the box internals carried by this shaft will be running slightly out of true, and the oil seal outside the bearing will be suffering. The box may carry on running, seemingly OK, for some time, but if it was mine, and the shaft really has been bent, then I think that I would strip it down for a full check-over. Not an easy job, I realise, it is up to you. Perhaps you should confirm, with further, careful, measurements before committing to the work.
 
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Are you sure?

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No, that's why my reply was rather tentative, but dreamoflife did previously say that the prop-shaft seemed OK. If the flange is undistorted, but wobbles when mounted on the output shaft, and not connected to the prop shaft, then there aren't a lot of explanations other than a bent output shaft or the splines are slack. If it is bent then it's most likely for the bend to be on the smaller diameter aft of the rear bearing, the diameter forward of the bearing being considerably greater. If this is so, the internal components, (friction cones etc.) could still be running reasonably truely. Of course, it's just possible that the shaft may have been bent a while ago, and he's only now been prompted to look at it closely. This can happen, and the recent damage may be elsewhere. I'm only going on the evidence presented so far.
 
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