Tribology of Composite for Offshore Applications: Lifeboats

In order to achieve reliable launch conditions current lifeboat designs rely on a controlled, low coefficient of friction between lifeboat and slipway and a well-defined slipway geometry. The coefficient of friction is critical in that it dictates the forces opposing motion of the lifeboat down the slipway and can hence determine whether the boat can be launched on a given slipway. The choice of appropriate materials for slipway and keel, plus the use of greases for lubrication, is thus an essential part of lifeboat design.

Most of the contact between lifeboat and slipway occurs between the keel and keelway and it is the friction in these regions which controls launch behaviour. Current designs consist of a steel or cast iron keel sliding against a galvanised steel keelway and a grease is required to ensure that the coefficient of friction is low enough to ensure a viable launch. This grease is applied manually and can easily be washed away particularly in the splash zone near to the water surface. It is therefore difficult to guarantee that the friction coefficient will be low enough in these regions. Since the lifeboat is moving relatively fast at this point the effect of poor lubrication is generally only to reduce launch speed but in extremis it could lead to the lifeboat stopping on the end of the slipway and requiring further assistance to achieve a correct launch. Clearly an alternative material system which does not suffer from this problem could be an advantage.

At low sliding speeds the coefficient of friction between a range of polymer and composite materials and steel is very low both in lubricated and unlubricated conditions. However, at the high launch speeds anticipated for the next generation of fast-launch lifeboat frictional heating leads to degradation of the polymer matrix and a rapid increase in the friction coefficient. In order to avoid this problem we are adopting the following approaches:-

• Use matrix materials with better thermal stability or less sticky degradation products
• Use matrix additives designed to reduce friction
• Use coatings to reduce friction and thermal degradation

This page was last modified on 18th January 2002.

S.J.Bull@ncl.ac.uk

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