Stern tube weardown & rudder jumping clearance

[Pengze]

I would be happy if somebody can enlighten me on the following details:
1. What is the allowed maximum weardown for stern tube tailshaft bearings? Could I use vernier caliper (depth gauge) instead of poker gauge for this
weardown measurement?

2. What is the allowed maximum jumping clearance for a semi-balance rudder?

Appreciate your kind response.

Thank you very much.

[Madzng]

1.The poker gauge supplied when the vessel is new should always been used. This is normally supplied in a box, with the critical dimensions recorded inside. The actual distance measured is less important than the change in distance. This can only be determined acurately by using the same tool everytime.

Actual bearing clearance will depend upon shaft diameter, lubrication type (oil/water) bearing material (white metal/synthetic or Lignum Vitae), and loading. Bearing wear is very rare when looked after correctly, although a water lubricated bearing will be subject to wear particularly when operated in sandy water.

Dependant on the factors mentioned above and Class requirements the maximum bearing clearance will vary. Oil lubricated bearings will have a maximum recommended by the maker - controlled by design limitations, stress, alignment and loading.

Water lubricated bearing (other than rubber) maximum clearances under ABS Classifications for unrestricted service with machinery aft are as follows:

Shaft diameter equal to or less than 230mm (9") in diameter, maximum 4.8mm (0.19")
Shaft diameter greater than 230mm but less than or equal to 305mm (12"), maximum 6.4mm (0.25")
Shaft diameter greater than 305mm, maximum 8.0mm (0.31")

A water lubricated synthetic shaft bearing for a shaft diameter of 615mm (24") has a maximum recommended clearance of 5.5mm, and an expected life of 20 years.

2. The jumping clearance is required to prevent damage to the steering gear, regardless of rudder type. Therefore the maximum jumping clearance should always be less than the clearance between tiller and steering gear. An increasing jumping clearance can indicate that the bearing wear is excessive.

[Pengze]

Thank you, Madzng.
As for oil lubricated bearings, if the weardown is excessive, could I shift the liner or the seal seating positions. What is the procedure & how to do it ?
Looking forward to your reply.
Have a nice day.

[Madzng]

Hi Pengze,

We are now talking about two different things.

If the weardown is excessive then the stern tube bearing needs to be changed.

If grooves have been worn on the shaft liner from the stern tube seals, then the whole stern seal assembly can be moved.

The liner is bolted to the back of the propellor boss, and the seals to the stern frame. Split seals have been around for a few years, but split liners have never been successful, although the manufacturering technology is getting better, Blohm & Voss make a split types liner which is said to work well.

A split liner means that it is not necessary to remove the propellor, for exchange.

The location of the seal can be moved away from the worn grooves by the addition of a spacer before the forward most seal.