bad, bad, bad , bad vibrations

[Birdseye]

to misquote the beachbots.

just finished my winter program and have been running up the engine on the hard. Ok to 2500 rpm and after that real bad vibrations causing the shaft to whip around. stuck as to where to start.

the work program has involved removing the cylinder head, lapping in valves, checking injectors, replacing the head and checking tappet clearances. the P bracket had to be removed and re-installed so the engine alignment had to be re- done. the engine now starts OK and runs smoothly up to 2500 rpm out of gear but shortly afterwards there starts a vibration that is for all the world like a jack hammer. the shaft between the stern tube and P bracket vibrates - it might even be hitting the sides of the tube though it was centralised when doing the alignment.

nothing seems to be loose.

any ideas? I cant see alignment causing this problem when the shaft isnt turning. up to 2500 rpm the engine is smooth and seems to be running on all 3.

 

[fisherman]

Stupid question, have you got a cutless bearing, in the P bracket and/or stern tube? Water lubricated, see? No, now I notice you're out of gear, sorry.

 

[Birdseye]

not stupid at all. it didnt vibrate before I did my work so the first supposition is that I've done something daft.

 

[Poignard]

[ QUOTE ]
first supposition is that I've done something daft.


[/ QUOTE ]

Well that's a sensible attitude. First thing would be to carefully re-check all you have done.

Have you ever run the engine at that speed out of the water before you worked on it? . Point I'm getting at[without knowing what sort of boat you have] is that if your hull is of light construction it may be flexing because it is not being supported by the surrounding water and the whole set-up may start to shake around. I wouldn't like to run my engine at high speed off-load.

Is there any unusual noise or smoke. Any fuel leaks in the high pressure part of the system

Are the engine mounts ok?

 

[vyv_cox]

The fact that the engine starts and runs smoothly to 2500 rpm suggests that the engine is mechanically OK and things like injector faults, tappet adjustment, fuel supply problems can be ruled out. A problem coming in at certain revs sounds like a balance problem. Were rubber mountings checked, changed? It sounds rather like a condition called 'soft foot', where one mounting is not as high as the other three. Check by loosening bolts between mountings and engine, try to judge if each pair is similar at front and back.

 

[Searush]

Try feeling the mounts when running to see if one or more vibrate more than the others.

Is there any play/ flex in the shaft when not running?

 

[billskip]

sounds that you are running the engine at a critical speed. all engines have a critical speed at wihch they will go into "resenance" this simply means that natural vibration starts at a cirtain speed and this ampifies itself if not supported or damped.
(see twisterowner) your hull canot support this out of the water ..also the critical speed/frequency changes under load so when in the water and in gear it may not happen..only a guess..hope you sort it..please excuse my spelling I am dyslexic.

bill

 

[Pasarell]

May be totally irrelevant but I've just had a new engine put in a VW Passat diesel.
When collected from the dealer they said it had been tested and all was fine but on the road it vibrated terribly at 2300RPM. Turns out the flywheel was loose. Dealer had only tested at tickover speed so not found the problem!

 

[Birdseye]

[ QUOTE ]
The fact that the engine starts and runs smoothly to 2500 rpm suggests that the engine is mechanically OK and things like injector faults, tappet adjustment, fuel supply problems can be ruled out. A problem coming in at certain revs sounds like a balance problem. Were rubber mountings checked, changed? It sounds rather like a condition called 'soft foot', where one mounting is not as high as the other three. Check by loosening bolts between mountings and engine, try to judge if each pair is similar at front and back.

[/ QUOTE ]

Did wonder about exactly this.

I disconnected the shaft and discovered that the noise was the shaft hitting the side of the stern tube due to excess engine vibration and despite the shaft being central ar rest and with 6mm all round clearance.. The mounts look OK with no obvious unbonding etc, but since I have had to adjust the height and lateral alignment, it could be that the mounts are not supporting the same weight.

So tomorrow, they come off for inspection and then carefuly height setting.

 

[Anonymous]

[ QUOTE ]

So tomorrow, they come off for inspection and then carefuly height setting.

[/ QUOTE ]

Is there any reason why you shouldn't do this after she is launched? That way you remove doubts about flexing of the hull. It is not only the engine that will vibrate; the stern of the craft, and the propshaft will also vibrate and all of these characteristics will be different in the water.

 

[rhumlady]

Unless the boat is in the water you cannot realy align the engine/propshaft as the boat is not sitting correctly as has been said above.

 

[colvic987]

what sort of flexible coupling do you have..

www.boatsandoutboards.co.uk/view/F138056/

if it is a volvo one with doughnuts, then the old doughnuts are prone to spliting the rubber when removed/ replaced.

I had the same problem when i had to remove my gearbox and refit it, the flexible coupling looked ok when i reconnected it up, but my engine would vibrate and the shaft would vibrate. once i replaced the 4 doughnuts and inspected the old ones, problem solved, the threads which are sitting in a moulded rubber had become dislodged and the rubber split, i only had 1 and 1/2 doughnuts holding the flexible coupling together...