Shaft Seals on a Boat

Shaft seals are arguably one of the most important parts of the boat, if it fails the boat could sink. On today’s boats these tend to fall into two main types, the traditional stern gland with grease and packing rope or the more modern packless mechanical seal such as the PSS seal. 

Looking after your stern gear might not be the most exciting of jobs, but it is extremely important. Good quality packing and greasing is often all that stops your boat from filling up with water. If you don’t have a maintenance free shaft seal (like a PSS seal), then you need to ensure that you regularly follow these maintenance steps:

Inspect 

Regular inspection of the gland is crucial, take note of the amount of water that it’s letting in. It is designed to allow a small amount of drips in to lubricate and cool the packing around the propeller shaft. 

When the propeller shaft is rotating, the gland should drip about 1–2 drops per minute, if too much water is dripping, packing may be worn or the gland may be loose. If no water is dripping, the gland may be too tight and could cause overheating.

You should also look at the packing condition making sure it is not frayed or worn and doesn't have any dust or debris.

Be sure to also check the nuts have become loose due to vibration or corrosion. 

Lubricate 

The packing rings need to be lubricated, so you need to ensure the stern tube is always topped up with grease by either using a grease gun, or, a more convenient way is to put a turn or two of grease from 
a remote greaser. Doing so improves sealing, lubrication, protection and extends it life by preventing it from drying out.

They should be lightly lubricated and not over greased to still allow the small amount of seawater in to cool.

To lubricate the packing rings you should turn the top of the grease cup clockwise to force grease down the pipe into the stern gland, usually 1–2 turns of the grease cap is enough. Typically doing this every few hours of engine running time.

Compression

The packing rings must always be compressed so they make a seal against the shaft, once the packing feels solid this means it’s at the end of it’s life and you must then replace the packing rings. Packing ring joints must be staggered to minimise water ingress. 

The non-grease mechanical type seals such as the PSS need less maintenance but do need regular inspections to ensure the seal is in good condition and compression is maintained. This type of seal can also make your engine more efficient as there is virtually no resistance to the shaft unlike on the traditional stern tube seal.

Pros of Traditional Stern Glands:

1. Centuries of use, reliable if maintained properly.

2. Easy to understand and repair, parts widely available.

3. Can regulate water leakage by tightening the packing.

4. Generally lower initial purchase cost compared to PSS.

Cons of Traditional Stern Glands:

1. Requires regular adjustment and replacement of packing material.

2. Even when properly adjusted, a small trickle of water is normal, although it can be messy.

3. Shaft and packing material can wear over time.

4. Slight friction between shaft and packing can reduce efficiency.

Pros of Packless Sealing Systems (PSS)

1. No packing to replace, generally a “fit-and-forget” system.

2. Virtually no water leakage when installed correctly.

3. Minimal friction on the shaft, potentially improving fuel efficiency and reducing wear.

4. Usually made of stainless steel, ceramics, or other long-lasting materials.

Cons of Packless Sealing Systems (PSS)

1. Higher upfront cost than a traditional stuffing box.

2. Requires precise shaft alignment and sometimes professional installation.

3. If it fails, replacement can be more expensive or complicated than a simple packing change.

4. Misalignment or debris can cause rapid wear or leakage.