Shaft misalignment

Shiver Metimbers

Shiver Metimbers

Well-Known Member
Joined
16 Aug 2008
Messages
816
Location
Newcastle upon Tyne
Visit site
Refitting my shaft with new Stuffing Box Cutlass bearing etc I have come across a big problem. With the cutlass fitted in the "P" bracket and the shaft pushed through to the engine coupling, the shaft is hard against the stern tube at the top. To fit the stuffing box and rubber tube, considerable downward force has to be used on the shaft to get it central enough in the stern tube for the rubber hose and stuffing box to fit. At this stage the engine end of the shaft is lined up.

Another approach I tried was to build the shaft up first and fit the Cutlass Bearing last. Without the cutlass in the "P" bracket, the shaft is central in the bracket at the prop end but is 1/4" out at the other, (1/4" out over 3"). To take this sort of misalignment up using the engine mount adjusters would be 1 1/4" this would put far too much stress on the stuffing box and stern tube. Any ideas?

Thanks in advance.
 
Obviously the ideal would be for the shaft to go through the centre of the stern tube and the cutless bearing and be aligned with the engine without any pressure required.

So what has happened? Is this a new boat to you that has knocked out the cutless bearing and you just think it is normal wear replacement? Did it all work last season with no vibration?

Logically something has changed - either the shaft is bent or you have whacked the P bracket (on land or sea) and bent it. I think you need an engineer to look at it.
 
not quite clear on your description, the shaft is pushed through the P bracket then enters the stern tube still in line, and is then in line against the gearbox flange, how then is the force required?? if all in line. Sounds like the p bracket has taken a knock some time, was the original cutlass bearing worn at any point or was it even? and the same question for the shaft.
 
What has been moved during the job? Everything you have written suggests that the engine is too high, unless the P-bracket (or the stern tube) has moved. Most times that I read procedures for aligning engine and shaft, the critical point that is omitted is that the shaft must be central in the tube. Even if the two are not touching but there is not an equal space all around, there will be contact at some stage, maybe reversing or hard rudder movements.

Assuming that you have not carried out any major surgery it sounds as though something has caused the engine to be higher than normal.
 
sounds like as others have said, poor layup putting stress on the hull or p bracket
Just like a car when jacked up the door shuts close up


Have you tried relieving pressure on any given point of the hull to see what difference it makes ?

cheers
Mick
 
The boat is 20 years old. I have owned her for about a month now and just doing the work arising from the survey

The misalignment is as if the P bracket has been bent forward as if the boat during a lift out has had the strop over the shaft.
 
Did you take the shaft out? Presumably it must have been hard against the stern tube when you did it? Unless it's just a bent shaft?

If the P-bracket has moved it must have ripped itself out of the hull, as it won't bend in the fore and aft. If the survey didn't pick this up I would be having stern words with the surveyor.
 
The shaft is not bent, this is the first thing I checked. The shaft was hard against the stern tube as it come out. The "P" bracket does have a very slight play on it in all directions, vey very slight.
 
mtb's point is interesting. We know the hull distorts when out of the water and final shaft alignment has to be done whilst afloat. Is the distortion likely to be of that magnitude? I suppose it depends how the boat is supported. First call to surveyor.
 
from your description it sounds as if the P bracket is out of line. What I dont understand is what the situation was before your efforts. Was it out of line before? Was the old cutlass bearing badly worn at the bottom looking forward and the top looking aft?

there is no way that the P bracket should be 1/4 inch out of line at the front. So you need to investigate why. P brackets can come loose (my last one did) so grab hold of it by hand and try to waggle it about.

either way you cant put the shaft back with that sort of mis alignment even if you can force it all back together.
 
Hi Sharkbait, best to tackle this head on and , like others have said, the engine alignment is the last thing in the chain. I really recommend getting inside the hull and grinding off (if encapsulated) the grp to allow the P bracket to be removed and checked in its entirety. Is it Aluminium? if so it could be fractured just inside the hull or maybe made up with fillers epoxy etc. Is there a bump where it enters the hull. this might indicate a bodged repair. Easier to do the right job now, and make sure all is aligned to the centre of the tube where it accepts the stuffing box. Support it central whilst you bond in the p- bracket afresh and you are then certain you have done a good job especially as you say the shaft has checked straight.The repair will ensure that your engine will not be high on its mounts, they more commonly dip out of alignment where one foot has been contaminated with diesel fuel oil and sag at one quadrant. This must be corrected if it is an issue. Best of luck, been there, snagged my own dinghy painter once and wrecked everything! good sailing to all.(p.s. if hull is stressed leave the final alignment until afloat, and backstay tension at normal with mast in place, as hulls can "banana")
 
how heavy is the propellor ? as when i changed my cutlass the shaft which is almost 7ft long was touching the top of the cutless but with the prop on it spun round all ok.
 
Before you start tearing the P bracket apart, if there is no obvious signs of severe movement, start from the principle that the P bracket should be concentric with the stern tube.

Fit just the shaft and then wedge the shaft central with the inboard end of the stern tube. If the shaft runs free at this point then they are concentric. The engine then needs to be aligned to the shaft.

Presumably the engine was running satisfactorily beforehand and you have not altered the P bracket, which suggests that there is not an inherent problem.
 
The old cutlass has worn evenly and not in common with the misalignment. I am going to build the stern gear back up bar mating up with the engine flange and see what happens when she is lowered into the water. Hopefully when the hull is supported by the water like it's designed to do and not sitting on her keel everything will line up, or at least any minor adjustments can be made at the engine mounts.
 
There is something not right here, seriously, as I read it you have the shaft free in the stern tube and the p bracket is misaligned over 3", you had to force the shaft down to get the stuffing box on?
If these are the facts then engine alignment is nothing to do with the prob. It sounds as if the p bracket has been "moved/bent" Think, as someone else said, strop over shaft to do this, or forklift/tractor has bashed p bracket, I cannot see hull movement out of water making the p bracket out of line 1/4" over 3". Prob is that if you splash and it is still out of line then several hundred notes for another lift? I would pay a surveyor to give you re assurance, probably cheaper than a lift in and a lift out!
Stu
 
Hi
I would be tempted to pull the shaft as far as possible and check if there are any polished areas on the shaft indicating the shaft HAS been running against the tube.

If you do find polished areas on the shaft then obviously you do have considerable misalignment that is occurring with the boat afloat and ashore.

Personally I suspect that the boat has changed shape slightly as it was shored up, causing the misalignment. This is not at all uncommon.

I f there is no indication of contact between the tube and shaft I would reassemble everything , but leave the shaft coupling off the gearbox until the boat is re launched and you can check the alignment.
 
I think that if my boat was changing shape by as much as the OP says I would be investigating rather more than shaft alignment. From the information we have been given it would seem that something is considerably not right.
 
I agree 0.25” over 3” does sound like a dramatic amount of misalignment.
But if we assume the shaft was central in the stern tube when this measurement was taken, the angle of the P bracket relative to the shaft has altered by only 4 degrees.
It is surprising just how much some boats change shape , I re engined a French production boat a few years ago and could get a 0.020” misalignment between the gearbox and coupling by tensioning the back stay!.
 
Hull distortion caused by the way it is sittin gin the cradle (hull weakened??) and/or a heavy knock on the P-bracket.
 
You must log in or register to reply here.

Share this page

Top