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The Incredible Hovering Craft, Part 3

The anatomy of the wonderful SR.N4 hovercraft

Courtesy of the Hovercraft Museum Trust*

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This article was first published in 2008.

The SR.N4 was one of the world's largest commercial hovercraft and was designed for passenger/vehicle ferry operations on stage lengths up to 185km (100 n miles) on coastal water routes. Here’s its technical make-up.

Lift and Propulsion

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Power was supplied by four 3,400 shaft hp Rolls Royce Marine Proteus free turbine, turboshaft engines located in pairs at the rear of the craft on either side of the vehicle deck.

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Each had a maximum rating of 4,250 shaft hp but usually operated at 3,400 shaft hp when cruising. Each engine was connected to one of four identical propeller/fan units, two forward and two aft.

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The propulsion propellers, made by Hawker Siddeley Dynamics (now part of British Aerospace) were of the four-bladed, variable and reversible pitch type, 5.79m in diameter.

The lift fans, made by BHC, were of the 12-bladed centrifugal type, 3.5m in diameter. Since the gear ratios between the engine, fan and propeller were fixed, the power distribution could be altered by varying the propeller pitch and hence changing the speed of the system, which accordingly altered the power absorbed by the fixed-pitch fan.

The power absorbed by the fan could be varied from almost zero shaft hp (ie boating with minimum power) to 2,100 shaft hp, within the propeller and engine speed limitations. A typical division on maximum cruise power would be 2,000 shaft hp to the propeller and 1,150 shaft hp to the fan; the remaining 250 shaft hp could be accounted for by engine power fall-off due to the turbine rpm drop, transmission losses and auxiliary drives.

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The drive shafts from the engine consist of flanged light-alloy tubes approximately 2.28m long, supported by steady bearings and connected by self-aligning couplings. Shafting to the rear propeller/fan units was comparatively short, but to the forward units was approximately 18.27m. The main gearbox of each unit comprises a spiral bevel reduction gear, with outputs at the top and bottom of the box to the vertical propeller and fan drive shafts respectively.

The design of the vertical shafts and couplings was similar to the main transmission shafts, except that the shafts above the main gearbox were of steel instead of light alloy to transmit the much greater torque loads to the propeller. This gearbox was equipped with a power take-off for an auxiliary gearbox with drives for pressure and scavenge lubricating oil pumps, and also a hydraulic pump for the pylon and fin steering control.

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The upper gearbox, mounted on top of the pylon, turned the propeller drive though 90 degrees and had a gear ratio of 1.16:1. This gearbox had its own self-contained lubricating system. Engines and auxiliaries were readily accessible for maintenance from inside the craft, while engine, propellers, pylons and all gearboxes could be removed for overhaul without disturbing the main structure. The fan rotated on a pintle which was attached to the main structure. The assembly could be detached and removed inboard onto the car deck without disturbing the major structure.


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The craft control systems enabled the thrust lines and pitch angles of the propellers to be varied either collectively or differentially. The fins and rudders moved in step with the aft pylons. The pylons, fins and rudders moved through +/- 35 degrees, +/- 30 degrees and +/- 40 degrees respectively.

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Demand signals for pylon and fin angles were transmitted electrically from the commander's controls.

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These were compared with the pylon or fin feedback signals and the differences were then amplified to actuate the hydraulic jacks mounted at the base of the pylon or fin structure. Similar electro-hydraulic signalling and feedback signals were used to control propeller pitches. The commander’s controls include a rudder bar which steered the craft by pivoting the propeller pylons differentially. For example, if the right foot was moved forward, the forward pylons moved clockwwase, viewed from above, and the aft pylons and fins move anti-clockwise, thus producing a tuning movement to starboard.

The foregoing applies with positive thrust on the propellers, but if negative thrust was applied, as in the case of using the propellers for braking, the pylons and fins were automatically turned to opposing angles, thus maintaining the turn. A wheel mounted on a control column allowed the commander to move the pylons and fins in unison to provide a drift to port or starboard as required.

The control of the distribution of power between each propeller and fan was by propeller pitch lever. The pitch of all four propellers could be adjusted collectively over a limited range by a fore and aft movement of the control wheel.


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Construction was primarily of high strength, aluminium clad, aluminium alloy, suitably protected against the corrosive effects of seawater. The basic structure was the buoyancy chamber, built around a grid of longitudinal and transverse frames, which formed 24 watertight sub-divisions for safety. The design ensured that even a rip from end-to-end would not cause the craft to sink or overturn.

The reserve buoyancy was 175 per cent, the total available buoyancy amounting to more than 550 tons.

Top and bottom surfaces of the buoyancy chamber were formed by sandwich construction panels bolted onto the frames, the top surface being the vehicle deck. Panels covering the central 4.9m section of the deck were reinforced to carry unladen coaches, or commercial vehicles up to 9 tons gross weight (maximum axle load 5,900kg), while the remainder were designed solely to carry cars and light vehicles (maximum axle load 2,040kg).

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An articulated loading ramp, 5.5m wide, which could be lowered to ground level, was built into the bows, while doors extending the full width of the centre deck were provided at the aft end.

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Similar grid construction was used on the elevated passenger-carrying decks and the roof, where the panels were supported by deep transverse and longitudinal frames. The buoyancy chamber was joined to the roof by longitudinal walls to form a stiff fore-and-aft structure. Lateral bending was taken mainly by the buoyancy tanks. All horizontal surfaces were of pre-fabricated sandwich panels with the exception of the roof, which was of skin and stringer panels.

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Double curvature was avoided other than in the region of the air intakes and bow.

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Each fan air intake was bifurcated and had an athwartships bulkhead at both front and rear, supporting a beam carrying the transmission main gearbox and the propeller pylon. The all-moving fins and rudders behind the aft pylons pivoted on pintles just ahead of the rear bulkhead.

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The fans delivered air to the cushion via a peripheral fingered bag skirt. The material used for both bags and fingers was nylon, coated with neoprene and/or natural rubber, the fingers and cones being made from a heavier weight material than the trunks.


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The basic manning requirement was for a commander, and engineer radio operator and a radar operator/navigator. A seat was provided for a fourth crew member or a crew member in training.

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The remainder of the crew i.e. those concerned with passenger service or car handling, were accommodated in the main cabins. This arrangement could be modified to suit individual operator's requirements. The control cabin, which provided nearly 360 degree vision, was entered by one of two ways. The normal method, when the cars were arranged in four lanes, was by a hatch in the cabin floor, reached by a ladder from the car deck.

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When heavy vehicles were carried on the centre section, or if for some other reason the ladder had to be retracted, a door in the side of the port forward passenger cabin gave access to a ladder leading on to the main cabin roof. From the roof a door gave access into the control cabin. The craft, as configured for Channel service, carried 282 passengers and 37 cars.

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The car deck occupied the large central area of the craft, with large stern doors and a bow ramp providing a drive-on drive-off facility. Separate side doors gave access to the passenger cabins which flanked the car deck.

The outer cabins had large windows which extended the full length of the craft. The control cabin was sited centrally and forward on top of the superstructure to give maximum view.

*Hovercraft Museum Trust – material used with permission



200 Tonne amphibious passenger / car transport
Powerplant: 4 x 3,400 shaft hp Rolls Royce Marine Proteus Gas Turbines

External Dimensions

Overall Length: 39.68 m
Overall Beam: 23.77 m
Overall Height on landing pads: 11.48 m
Skirt Depth: 2.44 m

Internal Dimensions

Passenger / Vehicle Floor Area: 539 m²
Vehicle Deck Headroom - centreline: 3.43 m
Bow Ramp Door aperture size (h x w): 3.51 m x 5.48 m
Stern Door aperture size (h x w): 3.51 m x 9.45 m

Weight / Capacity

36 cars & 278 passengers
Capable of adaptation to loads up to 75 tonnes
Normal Gross: 203 tons
Fuel Capacity: 20,456 litres (4,500 imperial gallons)


(at normal gross weight at 15 degrees C)

Max. water speed over calm water, zero wind (continuous power rating): 70 knots
Average service water speed: 40 - 60 knots
Operation: Up to gale force 8
Normal stopping distance from 50 knots: 480m
Endurance at maximum continuous power on 2,800 Imperial Gallons: 4 hours
Negotiable Gradient from standing start: 1 in 11

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