We are often asked at this time of year about propeller shaft seals the differences in design and what would best suit your boat and the type of sailing you are doing. In this article we will explain the common types of propeller shaft seal and the differences between them, the pros and cons to each type, their serviceability and life expectancy.
Stuﬃng Box / Gland Packed Shaft Seals
The traditional approach and what many boats are still using. Commonly they use strips of a waxy rope around the propeller shaft and as they compress, they seal around the shaft and into the body of the gland shaft seal, minimising water ingress.
They are designed to minimise water ingress and do drip every 30 seconds or so, meaning you will get deposits into the boats bilge that could work there way into the ocean, some consider this environmentally questionable.
When fitting stuffing box gland packed shaft seals, it is extremely important to get the tension correct, if they’re too loose they will leak far too much water and if too tight they won’t drip, and the internal packing will overheat and eventually burn through resulting in a poor seal which will leak far more than it should. If you are unsure, you have the correct tension touch the body of the gland when the shaft is rotating in use and if it is warm to touch then that is fine, if you cannot touch it because it’s too hot then they are too tight and in danger of burning out. If you have access to an infrared temperature gun, then use this and if below 40°C it should be safe from burning out.
Regular inspection and maintenance of these seals is required, and you can expect to replace or service comprehensively every 5-7 years depending on use.
Mechanical Face Shaft Seals
Mechanical Face Shaft Seals superseded the Stuﬃng Box / Gland Packed Shaft Seal. These seals are dripless and therefore eliminate unnecessary discharge into the bilge area and then potentially into the sea.
They consist of two flat faces that press against each other, creating a watertight seal. A thin ﬁlm of water sits between the stationary carbon/graphite face plate and the stainless-steel rotational plate that is clamped to the yacht’s propeller shaft. The seal attaches to the stern tube using a high-quality silicon hose. The Lasdrop Gen2 mechanical face shaft seal creates compression via an innovative pressure housing, eliminating the need for a bellows hose that can crack and wear with time. This pressure housing is far longer lasting, resulting in less maintenance over the life of the seal.
These seals require an engine water feed and the Lasdrop Gen2 mechanical face shaft seal has two inputs in case you have twin engines leading to one shaft guaranteeing the water feed even if you are only running one engine.
Mechanical face seals are an extremely effective solution for boats that are used regularly and/or kept dry land stored for long periods of non-use, this is due to the two face materials being of different composition and if allowed to sit for long periods in salt water you may find that the stainless steel face suffers from galvanic corrosion (minor pitting of the face surface) reducing its effectiveness. When servicing and replacing the internal ‘rod’ rubber seal it will need to be out of the water and removed from the shaft. You can expect around 10,000 engine hours from each replaceable internal ‘rod’ rubber seal.
Rotary Lip Shaft Seal
Rotary Lip Seal Shaft Seals are also dripless and use a durable nitrile lip seal that creates a seal around the shaft. They consist of a nitrile rubber ring, with a lipped profile contained in a housing. The rubber lip seal runs on the shaft to form the seal. These seals are easy to fit and require little ongoing maintenance as long as the engine water feed is maintained during operation.
The Lasdrop Elite lip shaft seal comes complete with an innovative spare housing and quick changing split face plate that can hold up to 2 spare nitrile lip seals, allowing you to renew the lip seal without the need for removing the shaft seal, speeding up lip seal replacement, minimising maintenance and costly out of water servicing time. Each nitrile lip seal will give around 5,000 engines hours of use, so you can carry around 15,000 engine hours of use on the shaft.
As with the mechanical face seal these seals require an engine water feed and come with two inputs.
Rotary Lip Shaft Seals are commonly used in leisure craft, yachts, and power boats as they are easy to install and maintain, whether the boat is in the water or out. It is however important to ensure the shaft is in as good a condition as possible when initially fitting and to check all retaining clamps and hoses regularly as you would with all should all types of shaft seal.
Ultimately the choice of yacht shaft seal depends on various factors, including the type and size of the yacht, the shaft’s angle and alignment, the cruising conditions, frequency of use and the owner’s preferences. Be sure to regularly inspect all components and maintain hose clamps, hoses, bellows, fittings and shafts and if unsure get it professionally inspected.