Airspace sterntube seals ensure low total cost of ownership and clean oceans

Saving money, serving the oceans

The best sterntube seal should protect a ship’s uptime, the investment, and the environment. An especially vulnerable part of a vessel’s sterntube system is the aft seal. If it is damaged, water can enter; as the amount of water increases, the load carrying capacity of the oil is gradually reduced. In the end, the propeller shaft bearing will fail and expensive repairs are inevitable. A single incident of this nature can mean a reduction in yield numbering in the millions.

Economic criteria aside, a sterntube seal’s ability to ensure sustainability and a “greener sea” is equally crucial, as the following statistics illustrate. Worldwide oil pollution from standard sterntube operations is estimated at over 80 million liters (21m US gal) per year – and that’s excluding oil lost from damaged seals. In comparison, oil pollution from 1989’s notorious Exxon Valdez oil tanker spill was 41.6m liters (11m US gal).

A crewmember performs maintenance work on a propeller shaft

Air chamber provides security

Since the US Coast Guard’s Vessel General Permit (VGP 2013) came into effect, it has been mandatory to use Environmental Acceptable Lubricants (EALs) in various systems when navigating US coastal and inland waters. Said systems include sterntubes, thrusters, pod drives, controllable pitch propellers, and stabilizers. Any vessel that has had its keel laid since December 2013 and sails in US waters must comply with the VGP 2013 regulations. The same applies to ships following their first scheduled dry docking after this date. So what does this mean for ship operators? Which lubricants can they use? And what do they have to change technically?

Against this backdrop, the “airspace” aft seal represents a purely technical solution. It separates oil and water through a chamber filled with compressed air, making it impossible for oil to leak out of, and seawater to seep into, the stern. This in turn means that the VGP 2013 regulations can be fully adhered to. Furthermore, the US Environmental Protection Agency (EPA) does not class the airspace aft seal as an oil-to-sea interface, due to its void chamber. Consequently, no Environmentally Acceptable Lubricant (EAL) is necessary.

“Cutting”-edge design for extra protection

While a vessel is at sea, there is a constant risk of fishing nets or lines floating across its path and causing damage to the sterntube seal. After analyzing a huge number of service incidents, SKF discovered that damage to the outer sealing ring can be avoided by simply mounting add-ons to the seal, providing protection from dirt, nets, and rope.
These add-ons include an extra net protector ring equipped with “net cutters” to protect the aft seal. There is also an additional net pick-up ring, which rotates with the propeller. It catches net and rope before they can cause an obstruction.

Figure 1: How an airspace aft seal helps ships comply with VGP 2013