Friction and Wear Performance of Lifeboat Launch Slipways.

Abstract

The Royal National Lifeboat Institution provides a marine search and rescue service using lifeboat stations sited along the coast of the UK and Ireland. In locations where there is no natural harbour or where there is a large tidal range it is necessary to use an inclined slipway to launch a large lifeboat. Lifeboat slipway stations consist of an initial section where the boat is held on rollers followed by an inclined keelway of nickel/chromium coated steel, the lifeboat is released from the top of the slipway and proceeds under its own weight into the water. The lifeboat is subsequently recovered to the top of the slipway using a winch line. With the introduction of the new, larger Tamar class lifeboat existing boathouses are being upgraded and existing low friction coated steel slipway lining materials replaced with a low-friction jute fibre/phenolic resin composite. The composite slipway lining material was selected in part because it was able to run unlubricated or water lubricated. However the friction problems have been such that it is usual to line the slipway with grease before every launch and recovery. This adds to the number of operations involved in a launch and has safety implications. The use of grease to line the slipway results in the grease being washed out to sea with effects on the surrounding area, it is likely that there is some environmental impact due to this as the grease is non-biodegradeable and not recommended for open water use according to the material data sheet. Because of these issues it is desirable to develop a set of working guidelines for crews to reduce both these risks by setting appropriate conditions for the manual application of grease along the slipway. These guidelines will also feature a method of assessing the wear of slipway panels so that panels can be replaced before they present a hazard to lifeboat operation. This thesis describes a method for assessing slipway lining materials and lubricants. Appropriate tribometer test machines are selected to assess slipway lining materials performance, the TE57 reciprocating tribometer and the TE92 rotary tribometer are used in conjunction to ascertain friction and wear performance respectively. These results are combined with detailed slipway panel surveys and case studies, and with Finite Element models to develop a method for assessing and predicting the friction and wear along a panel lined slipway. These results are used to develop slipway performance monitoring techniques for lifeboat crews and to develop design modification to combat high friction and wear on slipway panels. The adoption of a modified slipway panel and water lubrication system is proposed, this arrangement reduces panel misalignment contributions to slipway friction and wear resulting in more reliable slipway performance and is also projected to save the RNLI up to £195k annually compared with current practice.