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Aviation History
1964
1964 - 1180.PDF
Air-Cushion Vehicles FifGHT International supplement, 23 April 1964 Skating on a Skin A radical new approach to the old but largely unrealized idea of air lubri- cation of a boat's hull, married to an advanced type of hydrojet propulsion unit, has taken shape in the last two years in the unlikely setting of an asbestos shed behind a small flintstone cottage in the Hampshire village of Chalton. Though not strictly an air- cushion vehicle, it qualifies for inclusion here in being designed to achieve the same objectives as over-water ACVs— high speed, negligible draught and divorcement of the craft from the re- tarding friction of water—while its mode of propulsion is equally applicable to side-wall ACVs. The craft, the AMD-X1, has been designed and built by a retired Royal Navy test pilot and bush operator Cdr Basil Hurle- Hobbs, who has been assist- ed by the village plumber, Mr Tom Rabbets. Cdr Hurle-Hobbs claims to have spent about £8,000 on the project which is undergoing its initial floating trials this month. He expects to achieve a speed of about 60kt from the 45 b.h.p. of a modified Ford 100E car engine with the single-seat test vehicle, which has been built to prove the concept but which is merely the forerunner of much larger projected commercial craft. The designer-builder has arrived at the AMD-X1 in three stages. Initially he began experiments in achieving laminar flow with planing, outboard runabout hulls—and a pile of several now discarded hulls, inverted one atop the other in his garden, each subtly different from the others, testifies to this work. Having achieved major reductions in the normal turbulence over the after portion of these hulls, he turned to developing a more efficient form of water jet unit, relying on volume efficiency rather than the pressure effi- ciency of hydrojet units then commer- cially available. It was during initial water jet experiments that he hit upon the idea for air lubrication, and work on the two concepts has proceeded in parallel on the AMD-X1. It is a small planing hull, of laminar flow form over the after part of the wetted area, of stressed skin plywood construction, reinforced with bonded and riveted (belt and braces?) alumin- ium sheet over the deck. But the major part of the undersides is of two- skin construction—the outer skin being of tightly stretched, permeable nylon fabric, prepared for the craft by Fother- gil! and Harvey Ltd, with myriad holes of no greater than one ten-thousandth WATER COOLING OUTLET FUEL SUPPLY LINE KATE" COOLING SUPPLY BLOWER UNIT •JET DRIVE BELT FORD IOOE ENGINE WATER OUTLET BLOWER DRIVE IV BELT) WATER BLEED LINE (PORT I STARBD) AIR SUPPLY LINE (HAND OPERATEDI U ^" (HAND OPERATED) A diagram of the engine-blower-hydrojet installation of the AMD-XI of an inch in diameter. The permeable skin is bonded to Jin longitudinal stringers outside the structural skin, forming a Jin inter-skin airspace, or bag, into which air from an engine- driven centrifugal blower, by Keith Blackman Ltd, is passed through two non-return valves. Port and starboard bags, which ex- tend from a few inches outboard of the keel line to the chines, are each divided into four longitudinal channels by the stringers, which are drilled with varying diameter holes along their length, cal- culated to achieve an equal pressure distribution over the entire blown sur- face. The present blower belt driven by the engine on a 1 : 1 ratio, is calculated to give a pressure of 40in WG over the entire blown surface at 5,500 r.p.m. and maximum blower r.p.m. are 7,200. A new light alloy blower with a 1 : 1.75 ratio and approximately 50 per cent greater throughput is being prepared and will be installed when the craft is re- engined with a Nordec Bond flat-four two-stroke engine, delivering 55 b.h.p., after completion of its early trials. Installation of the more efficient en- gine and the new blower will reduce the present 7721b fuelled-but-pilotless weight of the craft by about 2001b. At the aft end of the engine is a second power take-off which drives by means of a toothed belt the six-stage hydrojet. The bore of the jet unit is constant—the inventor will not say what it is—but there is a slightly convergent-divergent efflux nozzle, and changing incidence and varying numbers of blades on the six bronze rotors, which are mounted on an EN58 stainless steel shaft. A statically balanced steering vane oper- ates in the mild steel jet nozzle, which is soon to be remade in AW6 light alloy, from which the casing is made. The length of the jet unit is 28in, weight 451b complete and throughput at 3,500 r.p.m. l,25Ogal/min. Cdr Hurle-Hobbs calculates that his craft will ride ona skin of air 1/64in thick, with only part of the unblown surfaces flanking the keel in contact with the water. For directional stability at speeds of over about 30kt, a cantilevered stressed-skin aerodynamic fin, easily detachable for quick modification, is fitted but can probably be reduced. The entire concept, the inventor em- phasizes, is likely to prove too expensive to be exploited in the form of personal runabouts; his ideas for it are wholly for larger, commercial craft. As a first stage he intends to build a 42ft long 30-seater with a 19ft 6in beam. This will be built entirely on aircraft prin- ciples with a stressed-skin, resin-bonded plywood hull and a stressed light alloy superstructure. The engine and blower installation will be aft with two blowers and the electrical generator run by a 60 b.h.p. engine and propulsion by two hydrojet units, each driven by a 125 b.h.p. diesel engine. Steering will be by means of two nozzle vanes work- ing in phase. The bags covering about four-fifths of the underside are again Jin deep and the air skin the same 1/64in. Lest one regards with suspicion the idea of a thin nylon membrane forming the outer skin of a high-speed craft, it is worth noting that the bearing strength of the drum-tight fabric on the test vehicle is no less than 35Olb/sq in, while Fothergill and Harvey have signified their ability to produce permeable nylon fabrics up to |in thick. The cruising speed of the first Hurle-Hobbs com- mercial air-lubricated hydrojet craft will be in the region of 47kt. It will, in the inventor's opinion, prove a far better sea-goer than ACVs, craft about whose controllability he expresses serious doubts. He intends to build a prototype as soon as he can obtain a lease on a suitable boatyard. Near the Solent and the Isle of Wight, he is in the thick of ACV country and ACV waters. It will be more than interesting if he proves himself right. R. R. R. 53
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