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At 240 tons the N500 is by far the world's largest amphibious hovercraft.

The basic civilian version is a mixed traffic vehicle with a payload of 85 tons. Up to 400 passengers and 65 cars can be carried at a top speed of 75 knots (140 km/h). The large deck areas offer innumerable other load combinations and possibilities for both civilian and military use.

The N500-01 (Cote d'Argent) is scheduled to 'roll out' in October 1976 and to become operational the following April, after six months of severe testing. A second craft (Ingenieur Jean Bertin) will be ready shortly after for SNCF Channel crossings. Two more options have been placed by Compagnie Transatlantique for a fast link between Nice and Corsica.

SEDAM designed the N500 with simplicity in mind. Five independent engines are used for lift and propulsion; control of the craft is ensured by rudders and variable pitch propellers. No transmission or intricate mechanics are involved.

The N500 benefits from the wide experience SEDAM has acquired with smaller hovercraft. The result ? Fuel Consumption is lower, maintenance is easier and comfort is improved as compared with earlier craft. The N500 can genuinely be called a hovercraft of the new generation.

SPECIFICATIONS

MAIN DIMENSIONS OF THE N500

Length overall: 50 m
Width overall: 23 m
Height overall on cushion: 17 m
Length of cargo deck (inside): 46 m
Width of cargo deck (inside): 22 m
Cargo deck area: 960 m²
Passenger deck area (total): 320 m²
Side and rear skirt height: 2.5 m
Bow skirt height: 3.6 m
Cushion area: 340 m²
Cushion length: 45 m
Cushion width: 22 m
Average air cushion pressure (at weight of 230 tons): 2.8m.Bar
Vehicle entry door width: 10 m
Vehicle clearance height:
central: 3.8 m
sides: 2.1 m
Passenger entry door height: 2 m
Buoyancy: 450 m³

WEIGHTS (metric tonnes)

Hull: 93 tons
Powerplants 19.5 tons
Equipment 6.75 tons
Empty Weight 118.25 tons
Commercial equipment for 385 passengers and 65 cars: 15 tons
Crew 0.25 tons
Fuel and oil 20.50 tons
Payload 85 tons
Total gross weight 240 tons

PERFORMANCE

Maximum speed on calm water 75 knots
Endurance 5 hours

THE STRUCTURE

The main hull structure is made up of welded longitudinal girder boxes which also function as buoyancy chambers - giving 100 % more buoyancy than required. The main longitudinal girder is the coach passage, with a cross section of 4.50m by 3.20 m. Welded trellis lateral beams support the car deck. The floating freeboard of the craft is 75 cm at the sides. On the ground the craft rests on small, cylindrical pads, and lift jacks can be fitted under the structure for easy maintenance. All structural units are of simple modular light alloy construction. Both front and rear doors can accommodate three vehicles abreast - and are watertight. The control cabin, on top of the craft, is supported by the main girder box and can be reached by stairs at the bottom of the starboard lift engine compartment.

LIFT SYSTEM AND SKIRTS

The fans and lift engine compartments are located in the forward part of the craft on each side of the coach passage. The fans are 4 m. - diameter axial types, with a hub diameter of 2 m. and 43 blades. The nominal speed is 900 rpm, which is derived from the engine speed via a two-stage reduction system. The second stage is a bevel gear which alters the shaft line through 90º. The fans are regulated at a constant speed around an output which can be varied according to flight conditions. Cushion air from the fans goes through the trellis between the car deck and the hull bottom, which has openings at the skirt level. Air valves fixed to the structure are used to control the flow to each group of skirts

PROPULSION SYSTEM

Three propellers - 6.5m in diameter, stern mounted - provide propulsion. They have variable and reverse pitch, four blades and a maximum rotation speed of 640 rpm. Three reduction units reduce the nominal engine speed of 15,400 rpm to 622 rpm at the propeller. The second of these units is equipped with a brake to stop propeller rotation in case of engine failure.

POWERPLANT

Five identical Avco Lycoming Tf 40 modular marinised gas turbines are used to power the N500, two for lift and three for propulsion - with the propulsion engines mounted in pods behind their respective props. Maximum power of the Tf 40 is 3,000 hp with maximum intermittent power of 3,400 hp and maximum continuous of 3,200 (at sea level in a temperature of 26.7 ºC. Specific fuel consumption is 247 gall/hr.). Auxiliary power for electricity generation is provided by two turbo alternators operating on independent circuits.

RESEARCH AND DEVELOPMENT ON THE N500

SEDAM has engaged in a tremendous research and testing effort to insure the success of the N500. Several small models were first tried out at the French Navy's towing tank in Paris; larger operating models were then tested on the Etang de Berre. When the project was sufficiently advanced. SIKKIM - an exact 1/7th scale model of the N500 - was built to confirm estimated performance, and results have been highly satisfactory in all respects. Heavily instrumented, SIKKIM can accommodate two pilots with double controls (it will be used later to train the commercial pilots). The craft is used to study dynamic stability, behaviour on rough water, passenger comfort and manoeuvrability of the N500. Conditions within small hoverports with strong winds have been simulated. The ambitious project of designing 'The World's Largest Amphibious Hovercraft' requires the gathering and processing of monumental amounts of technical data and the mastering of advanced technologies. SEDAM has put special emphasis on research concerning air cushion generation and behaviour, the aim being to minimise both the necessary hoverheight (the gap between the bottom of the skirt and the water) and skirt drag. The result is a significant reduction in operating costs.
The power needed to obtain the same performance is much less than that of similar existing craft:
'188 hp per ton of payload as against 246 hp.'

These figures alone amply illustrate SEDAM's progress in air cushion technology. The skirt system developed by SEDAM / BERTIN is partly responsible for this result. It consists of 48 independent skirts, with half arranged around the periphery; the rest form an inner envelope. The two volumes thus contained are at different pressures. The primary advantage of this solution lies in the independence of the skirts: each can 'react' to local obstacles independently, and each can be controlled separately through regulation of the air flow feeding it. A two-way 'stick' can be used by the pilot to produce any combination of pitch and roll moments - an extremely valuable control in aiding ground manoeuvres or 'trimming' the craft in flight with strong cross winds. The design of the craft provides good operating conditions for the skirts - and thus additional advantages: a relatively thin material can be used, resulting in reduced weight, cost and maintenance requirements.

Thin, flexible and independent skirts also result in vastly improved passenger comfort through elimination of high frequency vertical accelerations. The passenger cabins are also located close to the centre of gravity of the craft, so that pitch and roll motions are minimal.

And so - the N500 in brief: advanced technology but simple design, more comfort, lower operating costs.

MILITARY VERSIONS OF THE N500 NAVIPLANE

Military versions of the N500 NAVIPLANE can be equipped with armaments system comparable to those found on modern, fast - but conventional - patrol boats. The N500 then easily outclasses the more 'standard' vessels because of its speed and adaptability to shallow water. The NAVIPLANE offers the additional advantage of being able to use any sheltered beach as its attack or withdrawal base, where it can be camouflaged against enemy detection.

1. The proposed armaments system would include: