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
Lift and Propulsion
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.
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.
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.
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
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.
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.
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
Demand signals for pylon and fin angles were transmitted
electrically from the commander's controls.
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
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.
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).
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.
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.
Double curvature was avoided other than in the region of
the air intakes and bow.
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
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.
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.
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.
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.
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
Powerplant: 4 x 3,400 shaft hp Rolls Royce Marine
Proteus Gas Turbines
Overall Length: 39.68 m
Overall Height on landing pads: 11.48
Skirt Depth: 2.44 m
Passenger / Vehicle Floor Area: 539
Vehicle Deck Headroom - centreline: 3.43
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
adaptation to loads up to 75 tonnes
Normal Gross: 203
Fuel Capacity: 20,456 litres (4,500 imperial
(at normal gross weight at 15 degrees
Max. water speed over calm water, zero wind (continuous
power rating): 70 knots
Average service water speed: 40 - 60
Operation: Up to gale force 8
stopping distance from 50 knots: 480m
Endurance at maximum
continuous power on 2,800 Imperial Gallons: 4 hours
Gradient from standing start: 1 in 11