Mobil Stern Tube Lubricant

Mobil Stern Tube Lubricant is a high performance, high viscosity lubricant developed especially for the lubrication of Cedervall-type stern tube bearings used in ships and for certain bearings of ships' fin stabilizers. Mobil Stern Tube Lubricant is formulated from relatively high Viscosity Index (VI) base oils combined with emulsifiers, structure modifiers, surface active agents and corrosion preventives. It emulsifies readily with seawater to form a stable water-in-oil emulsion that is adhesive and an excellent lubricant. It provides effective rust protection in the presence of seawater and the lubricating film is resistant to water washing so that corrosion protection is maintained for extended periods under severe operating conditions.

Where the propeller shaft passes through the hull of a ship, it is supported by a bearing that is housed in the stern tube. Three distinct types of bearings are employed in this application: 1) The lignum vitae bearing, which has a stuffing box on the inboard side and no seal on the outboard side, is lubricated by seawater. The stuffing box may be lubricated with grease or oil, if this is deemed necessary. 2) Bronze or white metal bearings having oil-tight rubber seals on the inboard and outboard ends normally are filled with a lubricating oil that readily separates from water. Turbine oils, engine oils or hydraulic oils of the required viscosity generally are recommended. 3) The Cedervall type, which is a bronze or white metal bearing having an oil-tight inboard seal or stuffing box and metallic outboard seal that is not fully oil-tight, requires a lubricant that readily emulsifies with water. Mobil Stern Tube Lubricant has been formulated to meet this need. With the Cedervall type bearing a high viscosity lubricant generally is supplied to the bearing from a gravity tank located above the waterline so that the lubricant is under sufficient pressure to help exclude water from the bearing.

A recent development in ship stabilizers is the Denny-Brown-AEG stabilizer developed jointly by Brown Brothers and Co. Ltd, William Denny and Brothers Ltd., AEG, and Deutsche Weft. In these newer designs, the fins are mounted on trunnion bearings which permit them to be folded forward into storage boxes in the hull of the ship when not in use. A fixed shaft extends from the crux trunnion and the fin is mounted on this shaft on two bearings, which permit the fin to be tilted by a hydraulic mechanism to provide the righting movement to reduce roll of the ship. The interior of the fin and the fin bearings are lubricated by gravity from a header tank located above the waterline and the crux trunnion bearings are lubricated by pressure feed from a lubricator pump.

Features and Benefits

The requirements for lubrication of these three sets of bearings in the stabilizers and stern tubes are sufficiently similar so that a single, carefully formulated lubricant can be used for all of them. To perform satisfactorily, however, the lubricant must have the correct viscosity to flow properly under gravity head without excessive rapid flow that would result in high consumption; must minimize wear under the heavy loads encountered in the bearings; must protect the bearings and other components from corrosion and must emulsify with any seawater that passes the seals in order to maintain lubricating characteristics and minimize corrosion.

In stern tube applications, consumption of the lubricant is a major concern, both from the standpoint of the cost of the lubricant and the cost of labor required to fill the tanks. In two ship trials conducted over an extended period, Mobil Stern tube Lubricant gave significantly reduced consumption compared to the previous formulation, and at least as low as the best competitive product. Throughout the tests, lubrication was entirely satisfactory. These results indicate that Mobil Stern Tube Lubricant provides effective and low cost lubrication of stern tube bearings.

Key features and potential benefits include:

Applications

Mobil Stern Tube Lubricant is recommended for the lubrication of Cedervall-type stern tube bearings on ships where a high viscosity, readily-emulsifying lubricant is recommended by the manufacturer. It is also recommended for the lubrication of fin tilting bearings, as well as the crux trunnion bearings of Denny-Brown-AEG ship stabilizers and is approved by the manufacturer for use in these applications.