Reasons for vibration in boat?

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Deleted User YDKXO

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I've had a vibration in my boat which has got steadily worse as the season has gone on. It's mainly apparent at slow speed and seems to disappear at high speed. As far as I'm aware I haven't dinged a prop and visually, I can see no damage to the props or any other sterngear bits. The drive system on my boat is a simple shaftdrive system with engine, gearbox and propshaft in line; there is no universal coupling or down angle gearbox in the drive system.
Could it be that one or both engines have settled on their mountings causing the propshaft to become non-concentric in the stern tube? How do I check for that? Are there are other possible reasons?
 
is the vibration synchronous with RPM ? (higher RPM, vibration is higher freq. .... )
is it coming from one engine / prop or both ?
what happens when you stop one engine ?
and when you stop the other ?
 
If it seems to disappear at high rpm then I would think that clears the shaft line etc as a failed bearing or misalignment would likely get more noisy at higher rpm. It could be an engine vibration which in which case as the rpm went up the vibration frequency harmonises thus reducing what you hear. This could be transmitted to the boat via a hardened mount etc and what you hear is actually likely to be another part of the boat (such as cabinetry etc) being vibrated. If it just appeared this season it is likely that its due to something wearing out which will only get worse. Of course all of this is guess work as I'm not there and can't hear it. My best advice would be to spend £10 and get yourself a mechanics stethoscope from a tool outlet and get down on your hands and knees and methodically work your way round the area listening to the boat through the stethoscope until you find whats vibrating then decipher whats causing the vibration.
 
Deleted User said:
I've had a vibration in my boat which has got steadily worse as the season has gone on. It's mainly apparent at slow speed and seems to disappear at high speed. As far as I'm aware I haven't dinged a prop and visually, I can see no damage to the props or any other sterngear bits. The drive system on my boat is a simple shaftdrive system with engine, gearbox and propshaft in line; there is no universal coupling or down angle gearbox in the drive system.
Could it be that one or both engines have settled on their mountings causing the propshaft to become non-concentric in the stern tube? How do I check for that? Are there are other possible reasons?
Click to expand...
Not familiar with shafts, but it doesnt make this aweful noise, does it?
http://www.youtube.com/watch?v=nC2gZMNkyJo
 
Hi,
Work out which engine it is .
First I would check the prop ,have you cleaned it evenly ?If its dinged these nowt you can do until lift out.
What state is the shaft anode -missing a bolt or eroded unevenly?
Does it come on in neutral or with spinning props?
engine bearers / mounts -are they tight ? Have you got a spanner big enough?
Gearbox -oil level or any way of checking ( using a inferred thermometer ) if one is hotter than t,other ie wearing out prematurely ?. Hope not
 
Probably cutlass bearing lift and pressure wash then check, it probably needs a wash off in the med anyway.

Alternatively, bent prop or shaft or a mount has softened or dropped.
 
Deleted User said:
I've had a vibration in my boat which has got steadily worse as the season has gone on. It's mainly apparent at slow speed and seems to disappear at high speed. As far as I'm aware I haven't dinged a prop and visually, I can see no damage to the props or any other sterngear bits. The drive system on my boat is a simple shaftdrive system with engine, gearbox and propshaft in line; there is no universal coupling or down angle gearbox in the drive system.
Could it be that one or both engines have settled on their mountings causing the propshaft to become non-concentric in the stern tube? How do I check for that? Are there are other possible reasons?
Click to expand...

What engines / gearbox / stern seal set up ?
 
Have a good look at the engine mounts; check if the rubber has perished or even delaminated from the metal. A large lever or crowbar is useful for this.
 
Thanks for all replies. BartW - good point but it is difficult to replicate on one engine only because much higher rpm needed to achieve same speed (8-9kts) on one engine compared to both engines. Cutless bearings changed winter 2012 so don't think they are the problem. Gearbox oil levels can be checked and are OK. Gearbox oil pressure and temperature are similar and within spec so don't think gearboxes are the problem. All anodes in expected condition for time of year and all are properly located (i have checked myself). Sterngear cleaned by diver recently and hull not badly fouled.
Engine mountings and shaft alignment seems to be favourite at present
 
Deleted User said:
BartW - good point but it is difficult to replicate on one engine only because much higher rpm needed to achieve same speed (8-9kts) on one engine compared to both engines.
Click to expand...
I don't think that matters a lot, M.
Whatever the source of vibration, it's very unlikely to be related to the water flow speed, which is the only thing which really changes between cruising at any given rpm with one or two engines.
I would definitely start with the checks which B suggested, that should restrict the field a lot.

Also, I wouldn't rule out cutlass bearings just because you changed them recently.
I've seen plenty of yards fitting bearings which weren't exactly the same, but "near enough", alas that wasn't actually enough, eventually...

PS: just for the sake of avoiding another potential difference between running with 1 or 2 engines, I would keep the rudders centered, even if that obviously means going in a circle, when only one screw is spinning.
 
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I got no vibrations at all, but complained much about the stern gland for years. Everything was quite ok, till it fell off, then wound a big hole in the stern. A hole in the stern, is not a big problem, long as you can go fast enough, unfortunately I could not, on only one engine. The other one looked a bit of a mess. Eight knots does slow down the leak and with two pumps from the life boat, the water was only nudging the air intake on the other engine and only 4 foot deep, but it did make the boat go slower and slower. It took only two fire engines and a crane to rectify the problem, by putting us on dry land. Six months later, mended. I would not worry about it a lot.
 
Take no notice of jiggery pockery. There is a simple test. Remove the coupling from shaft to gear box. Push the shaft back a bit. Get a local engineer to make you a collar, which fits snuggly over your gear box drive shaft, with say three inch remaining.. Now pull the shaft back, it will fit into the collar, or it wont. Many have tried other ways, but resorted to this, the shaft should glide straight in.
 
I got a small amount of vibration at low revs after I cracked an engine mount when I had to go astern very quickly (dinghy collision avoidance in Chichester)

Can you remember doing any emergency stops/360 pivot with excessive engine revs ?

It took a bit of finding as the casting had cracked but an easy fix, you can change one mount at a time using a vernier calliper to avoid shaft alignment issues.
 
maybe something else to check:
shaft anode position. Close to P bracket or somewhere rather in the middle? Latter can cause slight imballance and if wearing unevenly could cause some vibrations

V.
 
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