Following on from the thread by Shotchas about prop shaft and coupling, it reminded me that I had wanted to remove my stern bearing at the next haul-out to inspect and potentially replace it.
I include two photos below - the first of the Strut near the propellor, and the second of the Stern Bearing.
Strut (below):
Stern bearing (below):
The Strut bearing was relatively straight forward to remove - you insert the new bearing and fixed it in place with two grub screws, one either side.
But, I request the views of forum members on how Stern Bearings are normally removed and replaced (fibreglass hull).
There are no grub screws hidden on the side of the stern tube, and the other end runs into a bronze stuffing box.
How would you pull it out to have a look without damaging the fibreglass stern tube itself, and when you re-insert it (original one or replacement) how do you make it stay in place?
i cut mine out carefully by cutting it out with a hacksaw blade in 3 segments, being careful not to score the tube.
the new bearing was sized up by the shipwright and ordered. it was frozen (to shrink it) and gently tapped into place where upon thawing sealed very tightly into place.
there was no way to remove it without damaging it, but had to be replaced due to too much play around the shaft because of another "proffessional" putting the prop together incorrectly and throwing out the balance.
Pulling the old one out can be done with a simple puller consisting of a steel tube, a threaded rod and a couple of washer plates. It depends on how the old stern bearing is fixed in the yacht as to whether the forced removal will cause much damage to the structure of the yacht.
This method is common with metal yachts but may be problematic with fibre glass yachts. Gently does it is the way to go. Sectional cutting it with a sabre saw is a good idea. It relieves the stresses of the fit of the bearing into the hull.
An accepted method of fitting a new stern bearing today is to fit the P bracket bearing first. The stern bearing is machined to be a close sliding fit in the stern tube.
The shaft is slid through the P bracket bearing and then the stern bearing is slid over the shaft. The shaft is then slid into the hull and the drive coupling fitted to it.
The drive coupling is then bolted to the gear box flange without the flexible coupling being in place between them. Then you attempt to slide the stern bearing into the stern tube.
If it slides in nice and easy, you are fairly much home and hosed. If not you need to adjust your engine mount bolts and do a precursory engine to prop shaft alignment to the point where your stern bearing will slide in nice and easy.
Once you have achieved the nice and easy slide in and out fit, you then slide the bearing out and coat it with adhesive resin and slide it back in and let the resin cure and set.
This method will result in a near perfect alignment of stern bracket (P or A bracket) bearing, stern bearing and engine coupling. A flexible mounted stern gland will complete the picture.
If you are prepared to go to these measures to get the love of your life just right, I further recommend you get rid of the flexible engine mounts you most likely have and install solid engine mounts.
It is flexible engine mounts and flexible engine couplings that flog out stern bearings and P bracket bearings. It is this area of yachts that causes a lot of grief, loss of horse power, low fuel efficiency and expensive high maintenance if not done correctly.
There have been more posts on this forum about this than just about anything else.
You are getting professional advice here. I am a Navy trained marine engineer and qualified MED I. If you think I am talking through my hat, tell me so. If you think I am giving you invaluable advice, tell me so.
I am happy to argue the toss with anybody on marine engineering topics but there are others here on the forum with other expert knowledge.
The "Brains Trust" here is a tad under rated I believe.
Cheers Cisco.
I pretty much agree with saltiest1 and Cisco. The thing to do is to decide what system you actually want or need. The bearing in the bracket is the critical one. If your engine is rubber mounted the shaft is supported by the rear [bracket] bearing and the flexible shaft coupling. The bearing in the rear of the hull does nothing or very little.
If the engine is bolted down with out flexible mountings then its a different story. The shaft is then supported by the rear bearing and the bearing in the stern of the hull and the engine is aligned to the flange. The second method is the common one for commercial fishing vessels. Unfortunately lots of yachts have a combination of the two methods!
Plenty of literature available on this. I have always used the Volvo Installation manual which explains it well. The Vetus site should also have good info.
Thank you for three seriously good posts gentlemen. If this forum had "sticky posts" that remain forever, your replies should become sticky.
Special thanks to Cisco for the extensive detail about re-installing a bearing. No doubts from me about your credentials on this topic.
It did, however occur to me that if the previous owner/builder had used the mentioned method of pushing the bearing in with a resin coating and left to "cure", there's no way I'm going to be able to pull it out with any sort of puller. The resin coating will bind it to the surrounding grp stern tube (esp epoxy resin) and I can't see any way other than using a grinder/burr of some sort to eat away the old bearing in situ. Sounds like a tricky job.
So let's think about whether I can just leave the current one there, even if it is no longer a close fit around the shaft.
It sounds like the shaft is being supported by the strut bearing one end, and the gearbox coupling (with flex disk) at the other, so the stern bearing is just there to stop the middle of the shaft from "whipping" (not a waterproof fit because the stuffing box provides that).
So if you had a clearance of say, 1mm, between the shaft and the stern bearing around the shaft, that would be ok?
Depends on what shaft size you have. The accepted tolerances will be on the web somewhere. Also the bearing material will make a difference as well.
Is the stern tube fibreglass?
Several saw cuts almost right through the old bearing and then force a blade screw driver between the tube should get it out. Once you get one strip out the rest will a lot easier. It seems you are on a hardstanding and should be be rushed. No local fishermen about for advice?
Sharp chisel to cut a V full length is another alternative. Once you have reduced the curcumferance it will come out OK. If its stuck in with araldite it will probably peel off.
Thanks Ramona. Photos are from last haulout - I'm not currently on the hard, I'm in planning stage.
Yes it's a fibreglass stern tube surrounding a 1-inch SS shaft.
Searched around for the Volvo manual you referred to Ramona but can only find stuff on Volvo "stern glands" and Vetus water-lubricated bearings rather than stern tubes and normal bearings. Wouldn't mind reading up a bit on stern bearings - can you remember where you saw those manuals ?
The bearings you will instal are water lubricated. The other type are white metal and you wont be going down that route. The installation tolerances will depend on what material the bearing is, some expand slightly in water.
If your in Sydney I would suggest wandering out to Porters in Parramatta for a chat.
The manuals I have are from the eighties when I was installing a new motor in a fishing vessel.
