Tail shaft bearing. Questions

[Theo]

I need to check the tail shaft bearing for play to see if it needs replacing. It this something that I can do while the boat is floating? I imagine that I need to rattle the propeller from side to side and up and down to see how much play there is. I don't mind getting wet.

 

How much lateral play is acceptable?

What are the consequences of leaving it too long?

What is involved in replacing the bearing? Engine out? (Bearing in mind that on Theodora the shaft runs right under the BMC 1.5 which has the gear box at the front and a belt drive down to the shaft.)

 

I am sure that you knowledgeable people will come up with some sensible ideas.

 

TIA

 

Nick

[Dalslandia]

play up to 2% of shaft diameter, but max 2 mm (will be on a 100 mm shaft then)

[BEngo]

I need to check the tail shaft bearing for play to see if it needs replacing. It this something that I can do while the boat is floating? I imagine that I need to rattle the propeller from side to side and up and down to see how much play there is. I don't mind getting wet.

 

How much lateral play is acceptable?

What are the consequences of leaving it too long?

What is involved in replacing the bearing? Engine out? (Bearing in mind that on Theodora the shaft runs right under the BMC 1.5 which has the gear box at the front and a belt drive down to the shaft.)

 

I am sure that you knowledgeable people will come up with some sensible ideas.

 

TIA

 

Nick

What Dalslandia said for play limits. You can do it in the water, but it's awkward doing it through a weed hatch. If you can get into the water it will be a bit easier. The hard part is measuring how much movement there is. You may be able to cut-down a feeler gauge or two and use those from the outside. for a 1.5 in shaft I'd want a narrow ( max 1/8 wide) strip of 25 thou feeler gauge to not go in anywhere round the shaft with it pushed away from where the feeler is far as possible.

 

The bearing itself usually screws into a boss welded (sometimes bolted) into the stern post. You need the boat out of water and the rudder out, then undo the stern shaft coupling and draw it and the propellor out backwards. Dismantle the greaser and dismantle and unscrew the stuffing box (if fitted) from the front of the bearing. Ditch the packing. Unscrew the bearing from the boss.

 

Replacement is the reverse of removal. Grease the inside of the new bearing before fitting it. Use a really good thread locker/sealant between the bearing and the boss and between the bearing and the stuffing box. Grease the bearing part of the shaft before fitting it. Renew the gland packing once its all together and grease it all up thoroughly. Check it turns smoothly before the final connections to the gearbox.

 

You may find that the new bearing is not in all respects absolutely identical to the previous one so that the stuffing box bolt holes end up in a different place. Drilling new holes to suit is always a job for a specially trained, double jointed primate with eyeballs on stalks and which likes to hang upside down for long periods whilst various bits of engine or boat make holes in the anatomy. If ypou don't have, or know one, it's easier to add more thread seal/gas PTFE tape and adjust the position of the tube to make the existing holes suit! You may also need some packing between the stuffing box and the mounting bar if the new bearing is a bit shorter.

 

You may find that the wear in the stern tube is accompanied by wear on the shaft where the gland packing runs so that you need a new tail shaft as well.

 

N

[RLWP]

Some shaft tubes have an inner bronze bearing, so replacing a shaft and bearing is relatively easy

 

Richard

[Timleech]

All the above assume a bronze stern bearing. If it's a 'cutless' synthetic rubber type of bearing there will be much more play even when new.

 

Symptoms of worn stern bearing are generally rumbling sound or feeling, and difficulty keeping the gland watertight. You can actually go on until there is far more wear than the figures suggested, but it will get more and more annoying and troublesome as the wear increases.

 

Tim

[RLWP]

And you end up with a very interesting shaped prop shaft

 

Richard