Why are water lubricated stern tubes preferred to oil?

Malcolm Latarche
Malcolm Latarche

17 July 2018


With some 90 percent of the world’s shipping fleet using oil as a propeller shaft lubricant, it might be hard to make the case that water lubrication is a better option. Nevertheless, in today’s operating climate water lubricated stern tube solutions are emerging as the sensible way to go.

It seems, in fact, that we have gone full circle as originally, seawater was used as the lubricant for the very first propeller shafts. However, because in those days the bearings were made out of a wood called Lignum Vitae, it was found to be unsuitable as the bearings were damaged much more easily, and needed to be changed very often. Oil therefore became the obvious alternative as it was readily available, inexpensive and environmental issues were less relevant at the time.

Since then, oil has largely been the lubricant of choice. The accepted attitude has been that oil works so why change, and the cost of oil has been considered acceptable. In the past, it is true that the short-term return-on-investment has indeed favoured oil. But when one considers the full lifecycle of the lubricating system, the fact is that today water offers a more economical option. In other words, in the long run water lubrication is less expensive than oil.

Water lubricated systems offer various cost advantages over and above the major reason of eliminating the need to purchase oil. The system’s life expectancy is often improved, there are reduced operational risks, less maintenance and potentially costly accidents are avoided as the risk of oil spills is non-existent. At the same time, the latest water lubricated systems offer equal or even better qualities for protecting the shaft, while providing an environmentally sound and reliable option. Furthermore, the new bearing materials and designs have outstanding tribology qualities that are at least on a par with oil lubricated systems.

With oil lubricated systems, the outboard propeller shaft seals present a risk in that they are expected to fulfil two functions, namely to prevent oil leaking out into the sea, and to prevent seawater from getting in and contaminating the oil. If the seal fails there could be either an operational or an environmental problem. According to Lloyds Register, 43% of all system failures are the result of damage caused to outboard seals. Water lubricated systems do not require an aft seal which therefore presents a far lower risk factor as there can be neither harmful leakage or damaging water ingress.

There is another huge factor that favours water lubrication, notably environmental legislation, and this affects all sectors of the maritime industry. By eliminating the risk of oil leaking into the sea, compliance is assured. This is a major reason why the main users of water lubricated solutions have, until now, been fishing vessels and smaller ferries operating relatively close to shore, as these waters are often more sensitive to pollution. It also explains the growing interest in switching from oil to water lubrication, most particularly from cruise ship operators, but also increasingly from the merchant shipping sector. Water lubricated systems are reliable and more suitable for shallower waters due to the minimal risk of operational failures that they present. Most importantly, there is no threat of oil spills into the sea which can result in hefty fines and clean-up costs, especially if operating in waters covered by environmental protection regulations.

It is worth noting that whereas earlier regulations required that ships using water lubricated systems undergo frequent shaft withdrawals and repairs every five years, this is no longer the case. Thanks to the reliability of today’s systems, regulators now allow shaft withdrawal intervals of up to fifteen years.

The stern tube bearings themselves have become the focus of continual technology development over the years. This work has culminated in superior, filament wound bearing materials for the marine industry. These materials provide outstanding composite shaft bearing properties and performance, as well as excellent tribology performance with the new designs that enable water to create a fluid film. One key feature is that composite bearings are water lubricated with all the long-term economic benefits that this implies.

These modern bearings are also extremely wear resistant, giving extended life expectancy and reduced maintenance. In fact, some manufacturers offer a 25-year extended warranty period against wear. They have high abrasion resistance making them suitable for operation in all climates, be it arctic exploration, tropical cruising or brown water applications. Above all, they are future-proof since they are fully compliant with the 2013 US Environmental Protection Agency’s (EPA) Vessel General Permit rules.

Wärtsilä, through its Seals & Bearings unit, has considerable experience and deep in-house competences in developing reliable, environmentally sustainable, water lubricated stern tubes. They offer a minimal risk of operational failure and, obviously, no threat of oil leakage. With their efficient flow control, the bearing temperatures within the system remain constant, while the fluid film between the shaft and bearing is maintained. Therefore, the bearing operates at all times in optimal conditions, thanks to this hydrodynamic lift. These factors clearly dispel common misconceptions that water lubricated solutions run less smoothly than oil alternatives.

The company offers two alternative versions. The CAPEX solution provides durability and high quality, but with a limited scope of equipment. The alternative OPEX solution provides high quality, reliability and fluid control. This version supports extended propeller shaft withdrawal periods and has optional enhanced monitoring.

Wärtsilä is a technology leader with more than 180 years of experience and a portfolio of products, systems and solutions that is unmatched in the marine industry.

This article has been supplied by Wärtsilä Seals and Bearings. Most of the content on the ShipInsight website is produced by our own team of writers, however, we do also accept contributions from other organisations. We check these contributions for obvious factual errors and objectionable or misleading statements but the views and opinions expressed within them are those of the contributing author or organisation.