Stern-Tube Bearings, Hold the Oil

Builders of small recreational craft would probably be surprised to learn that lubrication of stern-tube bearings on ships is an environmental issue. An estimated 10.3% of oil escaping into the sea is caused by lubrication leakage, which includes oil used to lubricate stern-tube bearings. It’s the third highest cause of marine oil spills, behind vessel discharge at 51% and tank ship spills at 18.9%, according to a 2003 National Research Council report.

A paper recently published by the Society of Naval Architects and Marine Engineers (SNAME) cites those findings and suggests more “sustainable” solutions, including plastic/composite bearings that are self- or water-lubricating. These are presented as the most attractive solution, because there’s no risk of oil being introduced into the marine environment. Not coincidentally, the paper’s author, John Thornhill, is the U.K. general manager of seals for Wärtsilä, which offers water-lubricated composite stern-tube bearings.

Where white metal (babbitt) tubes are commonly employed on ships, most of us are more familiar with stern tubes made of naval brass, bronze, and stainless steel (see www.pacificmarine.net for a wide variety), and lined with a softer material such as nitrile rubber, which the propeller shaft actually touches. (For a wood stern bearing, see “Saved by the Wood of Life,” on page 40 of the April/May issue of Professional BoatBuilder, No. 166) The development of nylon products decades ago introduced economical alternatives for some marine bearings, such as rudderstocks, though the evolution of plastics has produced improved products.

Vesconite Hilube stern-tube bearingsCourtesy VEscoPlastics Sales

For recreational craft, thermoplastic Vesconite Hilube stern-tube bearings incorporate a PTFE (Teflon) lubricant.

Another such material is Vesconite Hilube, described by the manufacturer as “an advanced engineering thermoplastic incorporating a PTFE internal lubricant.” (PTFE is the initialism for polytetrafluoroethylene, perhaps the best-known brand of which is Teflon.)

VescoPlastics Sales, the distributor, says Vesconite Hilube’s dynamic friction coefficient does not significantly increase over time or due to rising temperatures. It is also said to not absorb water (nonhygroscopic), which means it won’t swell; and with low creep rates, it is superior in handling high loads. The specifications sheet lists maximum design load for static and intermittent motion as 30 MPa (4,250 psi).

VescoPlastics Sales, P.O. Box 40647, Cleveland, Johannesburg, 2022 ZA, tel. +27 11 616 1111, website www.vesconite.com. In the U.S., tel. 866–635–7596, fax 212–937–3184. See also “The Challenge of Stern Tube Bearings and Seals” on sname.org.