AT Dover is stationed the only motor life-boat of this type, specially designed for the special conditions of the Straits, across which there is not only the heavy passenger steamer traffic, but a con- siderable daily traffic by aeroplanes, maintained in all but the worst weather.

To meet these special conditions this type of motor life-boat was built in 1930.

The ami of the Institution in the design and construction of motor life- boats and their engines, suitable for work under all conditions, is not high speed, but a great reserve of power, enabling the life-boat to maintain her speed in the worst conditions of weather.

Since an aeroplane which has come down on the sea will remain afloat for only a short time in anything but a calm sea, the aim of this special boat was to obtain the greatest speed with- out undue sacrifice of the essential qualities of a life-boat, of which the chief are buoyancy and stability.

This special type has a speed of between seventeen and eighteen knots, nearly twice as great as that of any other life-boat, but this speed could only be obtained by making a number of modifications in the design of the boat, as compared with the standard types. She is built of lighter timber.

Her beam is smaller in proportion to her length, which means some sacrifice of stability, and she has fewer air- cases. Her engines are not water- tight. On account of these modifi- cations she is intended to deal only with casualties at sea, and is not suitable for work inshore or on the Goodwin Sands.

This life-boat is not only the fastest but the largest in the Institution's fleet. She is 64 feet long and 14 feet broad, with a mean draught of 4 feet 2 inches. Her displacement on service conditions is 27 tons. She is divided into eight water-tight compartments with steel bulkheads, and is fitted with eighty-two air-cases. She is pro- vided with two cabins and can take about 100 people on board in rough weather.

She is built with a double skin of mahogany, ribs of Canadian rock-elm, and stem and stern posts of English oak. Her keel also is of English oak, 12 inches deep and 7 inches thick.

For the sake of lightness it is of wood only, without the usual 7- or 8-inch iron keel beneath it. The ribs are very close together, the space between them being from 5 to 9 inches instead of the usual 17 to 19 inches. The result is an unusually strong and elastic boat.

Such a boat required engines of much higher power, and lighter in proportion to their power, than any that the Institution had designed for its ordinary boats, of which the largest is a 6-cylinder engine of 80 h.p. When she was built the only well-tried engine suitable was the Thornycroft Y 12- cylinder engine, largely used in naval launches during the war, and sub- sequently adopted by the Royal Air Force for its fast launches. The Dover life-beat has two of these engines, of 375 h.p. each, and is driven by twin screws. These engines give her a maximum speed of 17J knots. She carries 350 gallons of petrol, in four tanks, and can travel seventy-eight miles at full speed without refuelling, so that while having a much greater speed she has a smaller radius of action than other life-boats. She carries no sails, but is fitted with a light signal mast.

She has a crew of seven men, and is fitted with a line-throwing gun, an oil- spray in the bows for pouring oil on the waves, and an electric searchlight.

She is lighted by electricity. To save weight she has a hand instead of a mechanical capstan, and hand fire- extinguishers instead of the , fire- extinguishing plant which is used on the large motor life-boats of other types.

She is provided with a Marconi receiving and transmitting wireless telephony set, with a range of fifty miles, by means of which she can keep in touch with the wireless stations at Lympne and Ramsgate, and with the light-vessels in the area of the Goodwin Sands..