Prop vibration and knocking but only when under way,

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kacecar

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Hi Forumites

Today, my first motoring of the season (launched last week), was marred by a knocking from (I think) the prop shaft. At low revs it is non-existent, at medium revs it doesn't start until the boat picks up speed, at high revs with the boat already moving it gets scarily loud. The problem only evidences itself when under way - I can drive at any number of revs while moored in the marina and don't get the same effect. Can anyone tell me what might be going on and how it should be further investigated or fixed?

Of possible relevance: the yard fitted and aligned new P-bracket last year (only 250 miles and 25 hours motoring ago - it was a bad year for my boating but good for my business); there was significantly more vibration after the work than the year before; I was worried about vibration so this winter have checked the propellor (OK), fitted a flexible coupling, re-aligned engine (it appeared to be out - I thought I had found something wrong so was glad I was fixing it); these measures appeared to have been successful - on the mooring at least, have greatly reduced the vibration. Another clue(?) - there appears to be a fresh water leak from around the P-bracket (thought I'd finished with leaks there) although it looked perfectly sealed when inspected a few weeks ago. Also, in case it helps, the installation is a VP 2003 driving a 5-6 foot 1 inch diameter shaft that sports a two-bladed folding propellor just aft of the P-bracket and its single cutless bearing. The shaft seal is a Halyard type (oil bath type - previously perfect but this year weeping oil).

Please note I will now be off-line until sometime Monday - all responses will be much appreciated but, clearly, I will not be in a position to reply or provide further clarification until then.

Thanks in advance. Regards to all.
 
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Hi Forumites

I can drive at any number of revs while moored in the marina and don't get the same effect.

[/ QUOTE ]

Are you in gear when you drive it in the marina? I can rev my boat, (Moody 44 Volvo Penta MD22L), to at least 2000 revs on my berth, as long as I increase gradually and let the lines take the strain, so you should be able to do the same on your berth.

If you are concerned, perhaps put some extra lines out.

Does it make the same noises on the berth as when moving?

Cheers

Richard
 
As Sarabande suggests, there needs to be sufficient clearance between the P bracket and the prop to absorb the longitudinal movement of the shaft when in gear. The increased knocking at high revs points to this as a possible problem.

Is the 'fresh water leak' where the P bracket seals to the hull?
 
Is the 'fresh water leak' where the P bracket seals to the hull?
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If it is haul the boat out and check pronto! It would not be the first time I have seen a "yard installed" P bracket work loose. Knocking is bad! Find the source ASAP and if the yard fitted the P bracket get them to pay for hauling out and putting it right! The fact that your engine alignment changed may be a clue to the fact that something is radically wrong at the prop end.....
Apologies for making it all sound dramatic but I really don't like what I am hearing here.....
 
For kace and oters we have seen in worst cases 9.00mm of forewards movement, so a gap of 12.00mm is a safe minimum.
However most smaller drives see less than this 4-5.00mm is typical.

How many bearings are on the shaft? is there one at the through hull? the flexible coupling could (slim chance) allow the shaft to move more now with thrust applied, so look at the shaft at the through hull when the knocking starts.
 
What sort of flexible coupling do you have? Some seem to allow more fore and aft movement than others.

Is the shaft donking the stern tube somewhere? Just thinking about the halyard shaft seal leaking - is the shaft centered on the stern tube ok?

By the way, I have a slightly similar donking - not bad and only appears at medium revs whilst under way - never berthed up whilst in gear! Have never tracked it down. My guess is that the prop cavitates on the berth at high revs so doesn't do the same things to the stern gear as it would whilst under way - eg torque affecting shaft centering; effect on flexible coupling.

Checking P bracket does sound a priority, but will be interested to hear what you eventually track it down to!
 
It could be any of the above suggestions or maybe the prop is too big and not enough hull clearance, from memory should be 10-15% of prop diameter.
Or maybe rope around the prop. I'd suggest you get someone independent to take a look.
 
I think the clue may be the shaft seal, if its now leaking in conjuction with bad vibration i would suspect the shaft itself, mine was bent fractionaly by the prop hitting somthing hard in the water. to measure it seemed very minimal but under load it caused the shaft to bend more and almost whip.


steve
 
Hi all.

Many thanks for the thoughts and responses. Given the timing of it all (just after winter "refit") I suspect the knocking problem is most likely to be due to something I have done. (The P-bracket leak could well be a separate issue - disappointing and disturbing but part of a long running saga and not necessarily directly related.)

Having considered all of your input I suspect that it is the forward movement of the shaft on the coupling that is the most likely culprit, perhaps complicated by the shaft not being properly centred in the stern tube. Consequently, I plan to:
a) Get someone to drive the boat at a speed at which the problem starts to show and, using the old engine-tuner's trick of listening through a stick/rod to identify the location of a knock, will try to identify if it is coming from the shaft seal / stern-tube or from the p-bracket / prop end. I will also monitor the water leak and measure the forward movement of the shaft under load (can be seen at the seal).
b) If I can confirm it's at the seal/tube I will re-align the engine and shaft (crew: "oh no - not again!") and see if that has any effect. If it doesn't then I suspect I'll have to haul-out again to check the seal and rest of the stern gear - not something that I really want to do.
c) If it's at the P-bracket and the water ingres isn't too bad then I will remove the flexible coupling and re-test.
d) If it is still knocking and/or the leak is excessive then it's the haul-out again (yet more expense and further disruption to my sailing season - sometimes one really does wonder why we do this boating malarkey).

I'll let you / the forum know how things go. Thanks again for all the ideas / input - hugely useful.
 
I wondered if you found the source of your problem. I have a Peterson/Formosa 46 center cockpit sailboat.

This year I had the stuffing box replaced. Put the boat in the water, picked it up in a hurry, motored only about two miles, smoke from the shaft revealed they did not allow enough water (any) to drip from the shaft.

Thereafter at anything over 1000 rpm the shaft appears to knock, louder as it gets up to 1800 rpm and then as you bring it down to below 1000 rpm even louder as it slows down.

Additionally the yard has now had difficulty getting the shaft out. They have disconnected the cutless bearing, stuffing box, and perhaps even the Pillow bearing and bracket and the flexible coupling before that to the transmission. No luck.

The told me they did not do a realignment because they were replacing what was there and thus no alignment was needed. Odd since they had to machine the new stuffing box to replace the old one as I was told they did not make one like I had on the boat and thus a new one needed to be ordered that would not fit unless some machining was done.

I was just wondering if you found your problem.

I should also mention that when I was out with the boat and had just picked it up when the shaft overheated, I went below with the boat on autopilot (I was solo that day) and it appears that at some point I picked up a crab pot and lodged a line around the shaft.

To me it sounds like the shaft is hitting the tube.

thanks
 
You describe the classic symptoms of shaft alignment problems. The seal will leak oil because you are squashing the lip seals to the point where there is an air gap.

If the systemn appears to be in line then stick a crowbar under the oil pan and lift the engine at each corner. Check that none of the mounts have separated, and that all corners lift at the same time (ie you arent carrying the engine on three mounts only.

Then its back to the alignment again. The knocking is most likely the shaft hitting the side of the prop tunnel. It needs to be central at the seal end as well as the bearing end.

Had very similar probs with a 2003 in my last boat - turned out to be a sheared engine mount bolt that was not at all visible when running the engine ashore.
 
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