Firefloat 81 was the larger of two prototype firefloats built by Vosper Ltd of Portsmouth for the RAF and designed primarily for combating fires in seaplanes or ships. The craft also has a subsidiary role of provding salvage pumping facilities to seaplanes or ships in emergency.
The craft was constructed of aluminium alloy to specificatio AW5. Probably the most original feature of Firefloat 81 was the fitting of Voith Schneider propellors which were the latest design at the time and had oil tanks incorporated in the propeller structure as opposed to seperate tanks previously used and synthetic rubber sealing rings for the main gearing and pinion shaft below the main gear wheel. They were intended to serve intended to serve the supplementary purpose of aiding the craft to maintain position when fire fighting, despite the reaction thrust of the jets. The main propelling machinery consisted of two General Motors 64-H.N.-5 marine diesels of 225 bhp each, fitted with combined neutral and ahead clutches and 2.04:1 reduction gears driving the Voith Schneider propellers. As the propellers were of variable pitch and could propel the firefloat both ahead and astern, the reverse gears of the engine gearboxes were removed, leaving only the ahead and neutral positions. The Voith Schneider propellers consisted of several aerofoil shaped blades installed virtically on a circular rotating runner, with blade shafts inside the runner and linked to an adjustable control point. When the point was central, the blades remained tangential when the propeller was rotated, and no thrust was produced. By moving the control point off centre, however, the firefloat could be propelled ahead or astern, the force of the thrust dependant on the degree of oscillation of the blades. The speed of the craft was thus infinately adjustable, from full speed ahead to full speed astern, without any change in the revolutions and direction of rotation of the engine and propeller.
The fire pumping set consisted of a General Motors diesel engine 64-H.N.-5, similar to the main propelling engines, and driving two fire pumps. The fire fighting equipment, which was the same as fitted to the smaller 55ft prototype, comprised four monitors. The two main monitors, specially designed by the Pyrene Company were each capable of delivering 350 gallons per minute of water or 1,800 gallons per minute of foam, at a pressure of 130psi. They were mounted on the main deck, one amidships and one aft. Two small monitors, of the standard RAF pattern, each delivered 150 gallons per minute of water or 900 gallons per minute of foam, at a pressure of 130psi, were mounted on the forward raised deck, one each side immediately forward of the wheelhouse. All four monitors operated through 360 degrees in the horizontal plane and 180 degrees in the vertical plane.
Two foam pourers were fitted, one to port and one to startboard, slightly forward of midships and approximately 1 inch above the waterline. They enabled a blanket of foam to be spread on the surface of the water, thus extinguishing burning oil or debris. Two 2.5 inch instantaneous double outlet type breeching connections were fitted on deck to provide a coupling for hose when firemen had to board a vessel. The monitors, pourers, breeching connections and reaction jets were served from a firemain fed by two Drysdale 5 fire pumps, fitted in the after end of the engine room. Specially designed, the pumps were of the water ring primer type, and each was capable of delivering 500 gpm against a head of 300 ft when running at a speed of 1,800 rpm.
Two four-jet foam inductors were mounted on the wheelhouse of the craft. Controlled by firemen, they drew foam compound from the foam tank in the forward compartment by means of water undr pressure. The mixture of water and compound was delivered to the forcing unit with which all monitors and pourers were fitted.
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