6 actionable troubleshooting tips to identify the cause of shaft seal leakage

Manouk van Tintelen

Before taking measures against shaft seal leakage, you would want to know the cause of the leakage in your stern tube or thruster. We speak of leakage when there is an excessive amount of water ingress to the stern tube, or oil leakage to the engine room or the marine environment of a vessel. A leakage is defined as any loss of oil beyond this tolerated value which can be found in the manual of a given seal. There are several reasons why a leak can occur within a seal.

Six quick checks: what can you do to identify the cause of shaft seal leakage?

No one likes to discover a leak within the propulsion system. These quick checks below may help you to determine the cause of the leakage.


1. What happened right before you discovered the leakage?
This sounds like an open door, but too often this question remains unasked. It may well be that the vessel has been in heavy weather, has hit something, or had to make a special manoeuver. This may explain what happened.


2. Did the leakage start suddenly?
If so, chances are that external factors – such as fishing lines or ropes – have caused damage.


3. What colour is the oil? – oil sampling
By taking an oil sample you can learn a lot about what kind of leakage is taking place. It is important that the ship is stationary when taking a sample. In normal conditions, the oil will darken over time due to rubber wear. If the oil is mixed with water, it will become cloudy and will have a milky colour. This indicates that water is able to enter through the water-repellent and oil-repellent lip seals of the aft seal. They may be damaged or something may be wrong with the stern tube pressure.


4. How big is the leakage, in terms of litres per 24 hrs?
When you have discovered the leak, you first want to know where the leak is located. If you have discovered this, it is important to know how big the leak is. It can be measured in terms of litres in 24 hours. You can measure the leakage of the Supreme® shaft seal based on the amount of oil you need to add to the header tank every day/week. The oil you have used to refill is the amount of leakage.


5. What are the prevailing pressures?
Lip seals need the right pressure to perform. What are the prevailing pressures inside the stern tube, thruster, water, etc.? Identifying the pressures may unveil what is happening.


6. What are the possible leakage paths?
Often the seals are the main suspect in case of oil loss. But CPP systems, especially with combined lubrication for the stern tube and the propeller, may have alternative leakage paths. It is worthwhile investigating all possible leakage paths.


Please note!
All our products are provided with a manual. Within this manual, there is a troubleshooting chapter. Here you can find how to solve common problems and what to do about certain issues. If you have a problem or a leak, take a look and easily find the necessary steps to solve the problem or get in touch with our specialist via +31 (0)880 216300.

Haven’t got access to the manual? Get in touch via to receive the required manual. Please note that we need the serial or IMO number in order to supply the right information.

Solutions: temporary and long term

Now that you have identified where the leak is coming from and how big the leak is, you can contact our sealing experts for a service kit with the right spare parts

Root cause analysis

When a leak occurs, Lagersmit will always try to find the fundamental problem; material research could be done and Lagersmit will always provide product advice to customers.

Replacing worn or compromised seals with new, original spare parts is key to ensuring that all parts are perfectly attuned to each other. As mentioned earlier, it is always better to stop the leak as soon as possible by replacing worn parts, like lip seals or bearings. Our sealing experts are happy to advise on the best maintenance schedule for your vessel and situation.

Spare parts

Is your seal experiencing leakage or are the wearing parts due for replacement? Our spare parts are in stock and can often be sent the same day. Request a spare parts quotation via

Prevent leakages with our Supreme® propulsion shaft seals

Lagersmit has several excellent products, each of which has its own advantages to prevent leaks. Products that need additional explanation here are: the Supreme Ventus®, the Supreme SeaGuard®, and the combination of these two products, the Supreme SeaGuard Ventus®.

Supreme Ventus

The Supreme Ventus is an intelligent, durable, and self-regulating sealing solution that guarantees zero emissions of oil for seagoing vessels. It has a constant airflow that prevents an oil-to-water interface and drains any leakage into a monitoring tank. The airflow within the Supreme Ventus system collects the water in a tank in case of leakage, and this system can collect no less than 288 litres of water. This guarantees zero emissions of oil and allows you to safely postpone a dry-dock.

Supreme SeaGuard

The Supreme SeaGuard gives you extra security against oil leakage into the marine environment. The chamber between the two oil-repellent lip seals is pressurized by the stern tube header tank. This way the pressure on both sides of the fourth lip seal remains equal. The extra oil-repellent lip seal can be activated in the event of excessive oil leakage. In this way, an unnecessary dry-dock can be prevented, and maintenance can be postponed safely.

Supreme Seaguard Ventus

Supreme SeaGuard Ventus® combines the extra security against oil leakage from the Supreme SeaGuard with guaranteed zero emissions of the Supreme Ventus. This powerful combination of seals is the most chosen option for retrofit.


There are also a few additional options that can be important in preventing leaks. Examples are a dirt barrier, HML coating, and the correct choice of lip seals. There are two types of lip seals: Supreme NBR lip seals and Supreme FKM lip seals. Supreme NBR lip seals are more resistant to abrasive particles and FKM lip seals are more resistant to higher temperatures. Do you have questions about shaft seal leakage? Contact the experts at our service department via +31 (0)88 021 6300 or

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