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.

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.

Materials for Sealing Rings and Propeller Shaft Liner:

• Sealing Rings: Nitrile rubber (NBR) is a commonly used material for stern tube sealing rings due to its good oil resistance, elasticity, and relatively low cost.
• Shaft Liner: Chrome-plated steel is a common material for stern tube liners. The chrome plating provides a hard, smooth, and corrosion-resistant surface, minimizing wear and improving the life of the sealing rings.