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

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Technical plans of the SR.N4 Mk II craft.
Comparison details of the SR.N4 Mk I and MK II (right) show the increased capacity

TYPE

200 Tonne amphibious passenger / car transport
Powerplant: 4 x 3,400shp 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)

PERFORMANCE
(at normal gross weight at 15 ºC)

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

CRAFT CONVERTED / BUILT

(Yard No 002) GH-2004 Swift
Rolled out as Mk I craft at East Cowes 10 December 1968
First commercial service Ramsgate to Calais 2 April 1969
Converted to Mk II craft 1972 / 73
Last Service Calais to Dover 29 September 1991
Total Hours Logged to end of 1989 (few hours between 1989 and 1991): 22,419
Laid up 1993 and on 25 June 1994 donated to the trustees of the Hovercraft Museum
Currently preserved at the Hovercraft Museum Lee-on-Solent

(Yard No 003) GH-2005 Sure
Rolled out as Mk I craft at East Cowes 1968
Officially named & first commercial service Ramsgate to Calais 3 June 1969
Converted to Mk II craft 1974
Last Service Calais to Dover 1983
Total Hours Logged to 1983: 17,852
Broken up at Pegwell Bay in 1983 for use as spares for the other craft

(Yard No 005) GH-2008 Sir Christopher
Rolled out as MK I craft at East Cowes in May 1972
First commercial service Ramsgate to Calais 3 July 1972
Converted to Mk II craft 1974
Last Service Calais to Dover 29 September 1991
Total Hours Logged to end of 1989 (few hours between 1989 and 1991): 19,116
Laid up early 1993 pending possible sale
Broken up at Dover in 1998 for use as spares for the two remaining Mk III craft

(Yard No 006) GH-2054 The Prince of Wales
First craft built as Mk II from the outset
First commercial service Ramsgate to Calais 18 June 1977
Last Service Calais to Dover 29 September 1991
Total Hours Logged to end of 1989 (few hours between 1989 and 1991): 11,755
Laid up early 1993 pending possible sale
Electrical fire on 2 April 1993 destroyed port cabin
Broken up at Dover in 1993 for use as spares for the two remaining Mk III craft (insurance write off)

LIFT AND PROPULSION

Power is supplied by four 3,400 shp Rolls Royce Marine Proteus free turbine, turboshaft engines located in pairs at the rear of the craft on either side of the vehicle deck. Each has a maximum rating of 4,250 shp but usually operates at 3,400 shp when cruising. Each engine is connected to one of four identical propeller / fan units, two forward and two aft. The propulsion propellers, made by Hawker Siddeley Dynamics (now part of British Aerospace). are of the four-bladed, variable and reversible pitch type, 5.79m in diameter. The lift fans, made by BHC, are of the 12-bladed centrifugal type, 3.5m in diameter. Since the gear ratios between the engine, fan and propeller are fixed, the power distribution can be altered by varying the propeller pitch and hence changing the speed of the system, which accordingly alters the power absorbed by the fixed pitch fan. The power absorbed by the fan can be varied from almost zero shp (i.e. boating with minimum power) to 2,100 shp, within the propeller and engine speed limitations. A typical division on maximum cruise power would be 2,000 shp to the propeller and 1,150 shp to the fan; the remaining 250 shp can be accounted for by engine power fall-off due to the turbine rpm drop, transmission losses and auxiliary drives. 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 is comparatively short, but to the forward units is 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 is similar to the main transmission shafts, except that the shafts above the main gearbox are of steel instead of light alloy to transmit the much greater torque loads to the propeller. This gearbox is 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. The upper gearbox, mounted on top of the pylon, turns the propeller drive though 90 degrees and has a gear ratio of 1.16:1. This gearbox has its own self-contained lubricating system. Engines and auxiliaries are readily accessible for maintenance from inside the craft, while engine, propellers, pylons and all gearboxes can be removed for overhaul without disturbing the main structure. The fan rotates on a pintle which is attached to the main structure. The assembly may be detached and removed inboard onto the car deck without disturbing the major structure.

CONTROLS

The craft control systems enables the thrust lines and pitch angles of the propellers to be varied either collectively or differentially. The fins and rudders move in step with the aft pylons. The pylons, fins and rudders move through +/- 35 degrees, +/- 30 degrees and +/- 40 degrees respectively. Demand signals for pylon and fin angles are transmitted electrically from the commander's controls. These are compared with the pylon or fin feed-back signals and the differences are then amplified to actuate the hydraulic jacks mounted at the base of the pylon or fin structure. Similar electro-hydraulic signalling and feed-back signals are used to control propeller pitches. The commanders controls include a rudder bar which steers the craft by pivoting the propeller pylons differentially. For example, if the right foot is moved forward, the forward pylons move clockwise, 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 is applied, as in the case of using the propellers for braking, the pylons and fins are automatically turned to opposing angles, thus maintaining the turn. A wheel mounted on a control column allows 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 is by propeller pitch lever. The pitch of all four propellers can be adjusted collectively over a limited range by a fore and aft movement of the control wheel.

HULL

Construction is primarily of high strength, aluminium clad, aluminium alloy, suitably protected against the corrosive effects of sea water. The basic structure is the buoyancy chamber, built around a grid of longitudinal and transverse frames, which form 24 watertight sub-divisions for safety. The design ensures that even a rip from end-to-end would not cause the craft to sink or overturn. The reserve buoyancy is 175%, the total available buoyancy amounting to more than 550 tons. Top and bottom surfaces of the buoyancy chamber are 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 are reinforced to carry unladen coaches, or commercial vehicles up to 9 tons gross weight (maximum axle load 5,900kg), while the remainder are designed solely to carry cars and light vehicles (maximum axle load 2,040kg). An articulated loading ramp, 5.5m wide, which can be lowered to ground level, is built in to the bows, while doors extending the full width of the centre deck are provided at the aft end. Similar grid construction is used on the elevated passenger-carrying decks and the roof, where the panels are supported by deep transverse and longitudinal frames. The buoyancy chamber is joined to the roof by longitudinal walls to form a stiff fore-and-aft structure. Lateral bending is taken mainly by the buoyancy tanks. All horizontal surfaces are of pre-fabricated sandwich panels with the exception of the roof, which is of skin and stringer panels. Double curvature has been avoided other than in the region of the air intakes and bow. Each fan air intake is bifurcated and has 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 pivot on pintles just ahead of the rear bulkhead. The fans deliver air to the cushion via a peripheral fingered bag skirt. The material used for both bags and fingers is nylon, coated with neoprene and / or natural rubber, the fingers and cones being made from a heavier weight material than the trunks.

ACCOMMODATION

The basic manning requirement is for a commander, and engineer radio operator and a radar operator / navigator. A seat is provided for a fourth crew member or a crew member in training. The remainder of the crew i.e. those concerned with passenger service or car handling, are accommodated in the main cabins. This arrangement may be modified to suit individual operator's requirements. The control cabin, which provides nearly 360 degree vision is entered by one of two ways. The normal method, when the cars are arranged in four lanes, is by a hatch in the cabin floor, reached by a ladder from the car deck. When heavy vehicles are carried on the centre section, or if for some other reason the ladder has to be retracted, a door in the side of the port forward passenger cabin gives access to a ladder leading on to the main cabin roof. From the roof a door gives access into the control cabin. The craft as configured for Channel service carry 282 passengers and 37 cars. The car deck occupies 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 give access to the passenger cabins which flank the car deck. The outer cabins have large windows which extend over the full length of the craft. The control cabin is sited centrally and forward on top of the superstructure to give maximum view.

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