PART I: PREPARATIONS CONSTRUCTION HAS BEGUN of tWO prototypes of the RNLI's new 47ft fast slipway lifeboat. The boats are to be built by Fairey Marine at Cowes, Isle of Wight, but the start of building only comes as the culmination of much preparatory work.

For the proposed new lifeboat, intended to replace the Institution's 46ft 9in and 47ft Watson housed lifeboats when they reach the end of their station lives, it was specified that she should retain the traditional shallow draught, long straight keel and protected propellers which make possible housing ashore and rescue work in shoal waters; that she should attain a speed of at least 15 knots; that she should have a self-righting capability; and that her overall dimensions and weight should be such that she would fit into existing boathouses and could be launched from existing slipways.

As a starting point, a suitable basic lines plan was provided by the National Maritime Institute. These lines were for a semi-planing hull which, when she accelerates, will help the boat to rise clear of the water's drag. Protection for the propellers is given by partial tunnels, by very deep bilge keels, port and starboard, and by a straight, wide keel ending in a hauling shoe aft.

The next stage was an exhaustive programme of model testing. First there were tank tests at the National Maritime Institute to establish resistance, flow and powering data; then,with the co-operation of the British Hovercraft Company, trials of a radiocontrolled model were run at sea to obtain information about the proposed hull's manoeuvering, sea keeping and course keeping capabilities.

Work had, by now, begun in the RNLI's own design office. Resulting from model trials, modifications were made to the bows and stern; the watertight aluminium superstructure which will give the self-righting capability was designed; while work was going ahead on the layout and all the many details of vital importance to a boat from which so much is to be asked. The general arrangement reproduced below, justone drawing of many, gives some idea of the immense amount of work demanded at the planning stage.

The fast slipway lifeboat will be built in steel. Although steel may not give such a good surface finish as wood or GRP, it is a material which has much to recommend it. As a steel hull can be thinner, with the scantlings generally smaller, than would be possible with either wood or GRP, and as special seating for fittings do not present such a problem, there is an immediate saving of both internal space and weight. In the design of a boat which has to contain a considerable amount of machinery and equipment, but which has specific dimension limitations, internal space is of great importance; and weight is critical in the design of a boat from which high speed is required.

The fast slipway lifeboat's hull will be plated in 4 and 5mm Cor-Ten steel, the same material used in the building of 44ft Waveney lifeboats for the past 16 years. Regular ultra-sonic checks made by district surveyors of lifeboats during that time have shown only negligible reduction in plate thickness. So, rusting presents no serious problem and maintenance is not costly.

(to be continued).