Off centre prop or bent shaft?

[SanFran 12]

Just doing the annual haul out for maintenance etc.... Ive noticed that the shaft is wearing to the port side of the thru skin fitting... Im guessing that at some stage there has been lateral pressure on the shaft / skeg / prop (although theres no indication of this, other than the wearing on the thru skin fitting. Anyone had a problem like this? Its quite noticeable, ie the wearing on the tube is maybe full thickness on stbd, (couple of mm?) whears to port is less than .5mm.

Alternatively, could it be the engine mounts?

[Murky 1]

Dang. Been through a similar one myself. Apparently it is pretty easy to damage a shaft or skeg by tangling a marina line or a dinghy painter but that kind of incident doesn't necessarily leave a lot of clues for a prospective buyer to pick up. Other than what you have now seen, which sounds quite well advanced.

 

At least you are out of the water and I understand shafts can be straightened depending on the degree of damage. Call engineer, prepare chequebook, it is better sorted out and at least you have detected it with plenty of time to get it sorted out before summer.

[Murky 1]

I understand shafts can be straightened depending on the degree of damage.

Of course, by that I meant "uncoupled, removed from boat, driven off to a workshop with specialist equipment where I imagine an old guy mutters unkind words about the things people do to their equipment and then gently and lovingly proceeds to put it right". You knew what I meant...

[wheels 543]

A bent shaft will not cause wear to one side Only though. Nor will an out of balance prop and not to mention the horrible vibration you would have felt.

So that leaves some other possibles.

It is possible it is just normal wear. In some cases you can get a small amount of force in a lateral direction, which you will find is due to the compression of water between Hull and Blade tips. So the shaft can wear the bearing slightly egg shapped. If that is the cause, you will find the elongation on the same side as the blade approaching the Hull....if that made sense.

Or a worse possible is that the shaft goes out of align when under load and then comes back again when stopped. So check your engine mounts incase the engine and box is twisting when under load.

[SanFran 12]

Thanks wheels... thats what I hoped for... i think. Is there a way to measure any movement in the engine mounts.... are they on some sort of cam? Could it be theyve just worn out, and when replaced, will be brought back into alignment?

[SanFran 12]

I have to say that while there was some minor vibration when under power, it wasnt that bad... but definitely there as opposed to just revving in neutral. We're talking about a 30 year old girl here... (mmm... wish I was talking about the misssus, bit of vibration... 30 years, could be fun)

[w44vi 17]

I would check the engine alignment with the shaft, it sounds like it may be out

[wheels 543]

There can be two different ways your engine is mounted. Either solidly, or on a rubber (or possibly spring mount, but that is rare). If it is solidly mounted, then check the mounts have not cracked. With a bar under the edge of the engine, you should not be able to get any part of the engine to move. With a Rubber mount, you need to get a bar under around each mount area and check you don't have a broken rubber pad. Or give the engine a good push around. You should not be able to move the engine wildly.

What many don't realise is that the entire thrust created by the prop is coupled to your boat by those four mounts. Not only are they taking the thrust, but they are trying to stop the engine from turning over with opposing the torque.

Oh and broken mounts can alos be a cause of vribration.

[Atom Ant 0]

Ahem... it's very rare for me to post in tech talk 'cos I'm usually reading and learning myself. But Wheels what if the prop shaft is running through a thrust bearing? Doesn't that mean the thrust is "captured" at that bearing rather than the engine mounts? In this scenario the mounts are therefore simply holding the engine in place and dealing with any torsional load (and they stop the engine flopping around while you're sailing)

 

Not trying to be a smart arse or anything...

[wheels 543]

I have never come across a Thrust bearing on a prop shaft before. That is, a bearing that takes all the thrust of propulsion rather than that thrust being transmitted to the Engine.

A Thrust bearing is usually found at the end of shafts that do indeed see a Thrust load, but those shafts are normally found inside the Gearbox itself and transmit the thrust to the body of the box..... which is bolted to the motor and the motor is bolted to the mounts and the mounts are bolted to the Engine bed and the engine bed is fixed to the Hull and th.....

[Atom Ant 0]

My set up has a thrust bearing on the prop shaft between the back of the gearbox and where it exits the hull. Works quite well I might add!

[idlerboat 116]

Prop shaft thrust bearings are fairly common. Particually when there is a python drive (CV joint) in use.

It is a cure for situations where the engine angle and the final prop angle need to be differant. The thrust bearing is usualy captured in a frame prior to entering the stern tube...but there is a lot of variations. It is also done in some cases to take the load of the bronze thrust washer that is used in say ZF (Hurth) mechanical gearboxes (with a sliding spline shaft). It can dramaticaly extend the life of these boxes because it reduces both the floating material and movement. This in turn reduces the wear on the multy disc (ball and detent) clutch packs.

++++++++++++++++++++++++++++++++++++++++++

Another possibility for a one sided bearing wear is a bent stern tube....Sadly I have one on my other boat.

When I had the tang for the P bracket "professionaly welded" the heat introduced bent the tube. I didnt realise until I had epoxied the whole thing back in. This means that in practicle terms you simply cant line an in board and out board bearing up without line boring....something that I wasnt going to do. The bearings in this case are bronze, and chopped out in about two months........that was ten years ago and I "chose" to ignore the vibration....

Remember when a proffesional says "its ok..it will losen up in time"....

re dial your bullshit meter and check.. it will cost a heap of money to rectify it later....if they are wrong...

 

 

the cops wont understand it when they pick you up in tears with a damaged gentleman and you say....

 

"but when he welded my P bracket it bent my stern tube so the shaft would no longer fit straight and smooth"......"it has cost me my boat"

[Steve Pope 243]

Python and Aqua drives both use thrust bearings, it is the one system where you have to missalign the engine to thrust bearing linkage to cause the grease in the Cv's to do its job.

[Guest]

Yes thrust bearings are pretty common on larger yachts (more than 40ft) if not production models. Even without a CV drive (e.g. Aquadrive) a standard car/truck style prop shaft with UJs can be used to link gearbox to prop shaft at a thrust bearing (must meet minimum angle requirements). Makes for a very robust system with no issues on engine mount alignment and no vibration. We have had similar wear to that described at end of shaft log on our 42ft Ganley. Discovered that p-bracket was bent by about 30mm out of line, but prop shaft and prop had no damage. So internal end of prop shaft was just rubbing on outside edge of shaft log on one side. Being a steel boat problem was fixed by heating p-bracket to red heat and then banging straight with a big bronze hammer. Damaged had been caused by a 7m long log trunk floating just below the water that wedged between end of prop shaft and rudder (a common hazard of sailing in British Columbia) - the attempt to pull the log out with a high powered RIB (twin 350HPs) pulled us round in circles without dislodging the log, but caused the bent p-bracket.