(b) Common forms of seal failure in a stern tube

1. Loss of elasticity in seal material – Nitrile rubber seals may lose their elastic properties over time, reducing their sealing effectiveness.
2. Surface damage to chrome liner – Grooving or scoring of the chrome liner can impair sealing surfaces, leading to leakage.
3. Excessive shaft vibration – Heavy vibration of the propeller shaft can cause uneven wear and seal deformation.
4. Insufficient cooling – Inadequate cooling can cause rubber sealing elements to harden and eventually fail.
5. Exceeding running hour limits – Operating beyond the manufacturer’s recommended service life increases the risk of seal failure.
6. Deterioration of oil quality – Contaminated or degraded oil reduces lubrication and protection, accelerating seal wear.
7. Incorrect header tank level adjustment – Failure to adjust the header tank according to vessel draft can cause oil loss, leading to inadequate lubrication and seal damage.

(c) Restricting oil loss due to seal failure whilst on passage

1. Use of high-viscosity oil – Recharge the system with a thicker oil to reduce leakage rate through damaged seals.
2. Temporary oil supply arrangement –
   • Disconnect the existing oil supply line.
   • Connect a 45-gallon drum supported by a block and tackle arrangement.
   • Adjust the drum height to vary the oil head pressure, matching it to the surrounding water pressure and minimizing leakage.
3. Fresh water introduction – Supply fresh water to the gravity tank to emulsify with any leaked oil. The resulting emulsion helps coagulate around the damaged seal area while the oil is circulated to maintain lubrication and sealing.

The aft stern tube bearing is designed to distribute the propeller load uniformly and prevent excessive pressure from being concentrated at one location. The main design features are:

1. Slope Boring (Taper Boring) (Most Important Exam Point)

The aft bearing is machined with a slight taper (slope bore) to match the natural deflection of the propeller shaft caused by the weight of the propeller.

Reason:

This ensures that the load is distributed over the entire length of the bearing instead of being concentrated at the aft end, thereby reducing wear and increasing bearing life.

2. Long Bearing Length

The aft bearing is made longer than the forward bearing, providing a larger contact area between the shaft and the bearing.

Reason:

The increased bearing area reduces the unit bearing pressure and distributes the load more evenly.

3. Large Bearing Surface Area

The bearing is provided with a large diameter and a long white-metal or composite bearing surface.

Reason:

The larger bearing surface spreads the propeller load over a greater area, reducing localised stresses and wear.

4. Proper Bearing Clearance and Hydrodynamic Oil Film

The bearing is designed with the correct clearance to maintain a continuous hydrodynamic oil film between the shaft and the bearing during operation.

Reason:

The oil film prevents metal-to-metal contact and supports the shaft hydraulically, ensuring uniform load distribution and reducing friction and wear.