Combustion chamber of a jet engine

Drive system of the SRN4

Lift system of the SRN4

Rudders and pylons

Cutaway image, adapted from Blunden & Witcomb, 1985. Whilst expressely focusing on the SRN4, most of the concepts and features mentioned on this page apply to nearly all hovercraft. Those features which do not apply will be immediately apparent

Bow ramp in the down position, as seen from the front of the craft. Cars, vans and coaches all load from here, as do the supplies trolleys for in-flight service such as those carrying the duty-free. The skirt in this picture is still slightly inflated as the craft had just arrived from France. Note the cars all facing away from the shot, ready to exit through the rear doors (pictured later).

Also note that the skirt appears to have a hole in it on both sides of the ramp. For correct alignment with the car deck, the ramp had to go through the level of the skirt (on Mk II and Mk III craft). For this, a closeable seam had to be added to the skirt to allow it to fold down whenever the ramp was lowered, yet seal up upon raising of the ramp.

The bowramp, lowered, as seen from inside

Side view of the bow ramp. Notice two things: One, the panel in the middle - this would normally be laid down by ground crew in the gap between the ramp's end and the ground, to allow smoother transition for embarking cars, especially those with a lower wheel base. Secondly, the yellow hydraulic ram in the foreground at the edge of the ramp. This was used to retract and extend the lip of the ramp. During flight, this lip is folded 90° to the ramp for the whole mechanism to fit snugly into the superstructure.

Close-up of the bowramp front edge dampers

The bow ramp in its closed position, just before departure of the hovercraft. The ramp would normally have been raised after all four main engines were brought to idle during startup procedures, and lowered again upon engine shutdown by means of a mechanical winch and cable system and the craft's hydraulics.

The car-deck main control cabin. This photo is looking toward the forward starboard side of the car deck. The yellow cockpit access ladder is visible, as is the open bow door. The ladder was initially intended to be a retractable feature, allowing crew access and then the sealing of the cockpit hatch (climbing the ladder leaves you at floor level upon entering head-first into the rear of the cockpit, so a bit of fearless gymnastics is required to get up into the flight deck!). It was however decided to bolt down and strengthen the ladder, making it a permanent fixture. This added extra strength to the roof, as well as a quick access route to the cockpit at any point during flight.

Looking aft from the port forward car deck access door. Note the cars all pointing aft, and the cockpit ladder locked in the down position. Further along in the shot on the far wall notice a change in height of the car deck roof into what appears to be an alcove. This is the area originally designated extra passenger space in the Mk I version of the SRN4, and is also the area where the 19m extension of the two Super-4 craft was inserted during their expansion in the '80s.

Here a view of the control cabin as seen from the pilot's seat, you can see the control column - with four degrees of motion: left & right controlling pylon movement (+/- 30° from normal), in and out controlling hoverheight and propeller pitch amount interchangeably.

Also seen are eight red/brown levers. These control, from left to right: Propeller pitch on each propeller: Positive, Zero and Negative; Engine throttle from idle to full power. A selection switch also exists to control whether all four pylons would move synchronously, or whether just the rear two pylons would swivel.

At the bottom of the two foot wells are the rudder pedals, controlling the angles of the two rear fins to the airflow.

The pilot and first officer's control column and rudder pedals are mechanically linked to each other, guaranteeing synchronous operation at all times.

More detail on each of these controls is on the Cockpit Tour page.

The main instrument panel of the cockpit. More detail of the various instruments is available in the Cockpit Tour page which comes next in the tour, but a brief run through of the systems from top left to bottom right, starting with the overhead panel, is:

Fuel system, Electronic System; Flight control position indicators, Directional equipment, Propeller pitch, Main Engine instruments (turbine RPM, compressor RPM, exhaust gas temperature), Hydraulic system, Gearbox statuses.

A combustion chamber from the Rolls-Royce Proteus engines

A view through to No. 3 (inboard) engine on the port side of the craft, showing the connection of the main engine drive shaft to the craft's main drive shaft (green). The two inboard drive shaftWednesday, August 15, 2012ift and thrust fans. The rear two drive shafts are basically direct linkages to the aft gearboxes, directly linking the rear thrust and lift fans.

From this picture of the same bay, but this time on the forward bulkhead of it, you can see that the craft's systems operate on AC 3-Phase power, that is the red, yellow and blue 'channels' seen. Each of the APUs generate electricity for the craft via integrated drive generators (IDG) and constant speed drives (CSD), similar to the systems found on most jet aircraft.

The power generated from these IDGs was then fed, via isolator switches controlled from the cockpit, into the main high voltage AC busbar, and from there via transformer rectifier units (TRU) onto the low voltage DC busbar. The AC busbar is used to power the flight control instruments, hydraulic actuators, and the loading doors/ramps. The DC busbar provides power for domestic systems such as lighting, radio and intercom.

I'm not sure as to the operating frequencies and voltages of the electronics system but they will probably be in the range of 415v (AC) at 200Hz and 30v (DC).

The port electronics bay of The Princess Anne. Mirrored on the starboard side of the craft, these two bays really show the age of the SRN4s in terms of the technology they use. Many of the components in this bay and on the craft were no longer manufactured by the end of their running lives, and so had to be hand crafted by Hoverspeed engineers.

A view looking toward the drive end of the engine, showing (partially) three of the eight inter-linked combustion chambers. Fuel and air mix flows right-to-left in this picture, igniting by means of igniters in a few of the cylinders, and the flame allowed to propagate through to non-igniter cylinders by means of two flame ducts on each cylinder. You can see these in the form of grey pipes connecting each combustion cylinder.

Fuel is injected at the head of each cylinder at high pressure and mixed with high pressure air from the compressor by means of a fan-shaped device at the intake. Combustion occurs and the fuel/air mix expands down the cylinder (toward the left here), and is output onto the high pressure turbine at its end. It is the output from this turbine which drives the compressor, and provides useable power to the drive shaft, hydraulic systems and electrical generators. The engine exhausts via an exhaust pipe out of the back of the craft.

Main drive-shaft output from a Proteus engine

The two main engine exhausts and (smaller) auxilliary power unit exhaust. During startup procedures, large fireballs can be seen emitted from these exhausts as fuel is ignited by each engine in turn. In order from port to starboard of the craft, each exhaust covers engines 1, 3, 2 and 4.

During startup, the main rear doors are closed, auxhilliary doors closed, then the engines are individually ignited and brought to idle in numerical order, checked, then the bow ramp is raised. The craft is then brought to full power whereupon the skirt is allowed to fully inflate and hovering is achieved.

The Rolls Royce Proteus marine gas turbine engine used on the SRN4s. Mk I and II craft used a less powerful version of the Mk III craft but the basic engine is the same. Engine details can be found in the SRN4 section.

This photograph shows the various fuel pipes (red) and their servo control valves leading to the eight combustion chambers on this gas turbine, as well as the output spindle onto which could be attached the main drive shafts of the craft. Nominal speed of the engine output shaft during cruise was 11,500rpm, with 10,750rpm on the turbine and 11,100rpm on the compressor.

Engine output and main bearings

Engines in-situ on the starboard (right) side of the craft, as seen from Number 2 engine

An engine seen ready and waiting to be installed on a craft, in the engineering department of Hoverspeed, Dover.

Here is visible the port fin/rudder assembly. An all-moving fin, this is moved hydraulically according to the inputs of the rudder pedals in the cockpit. You can just about see one of the control linkages toward the bottom of the fin between it and the main fuselage.

Drive shaft leading to the gearbox, on the starboard rear pylon

A gearbox being transported. Note its size compared to the two full-sized fire extinguishers. The input drive shaft to the gearbox would be connected to the left protrusion, the propeller shaft to the top (capped) and the lift fan to a protrusion on the bottom. An oil scavenge pump would be connected to the lowermost part of the flat plate on the right hand side, redistributing oil throughout the craft's systems.

Base of the gearbox showing the connector to the shaft of the lift fan (shown here disconnected for maintenance).

Inside the skirt, looking toward the bow of the starboard side. You can see the structure and supports of the plenum chamber in green at the top of the photograph, and the perforations in the skirt material (about 1cm diameter, 5cm regular matrix spacing) allowing airflow under the craft and down to the fingers below.

A rare opportunity. Climbing into the skirt via an access panel in the plenum chamber and a small rope ladder whilst the craft was on jacks for maintenance. The jacks when raised put the craft an extra 3m into the air, and, walking along the narrow sill of the plenum chamber alongside the passenger cabin is not for the feint hearted!

A view down the air intake of the forward port engine, showing a lift fan in position at the bottom of the shaft. Note also the hinged door in the centre of the shot. This would slam shut in the event of engine failure by means of the air pressure behind it, preventing the escape of air through the intakes and thus allowing the air input to the skirt from the remaining engines to keep the skirt inflated. The SRN4 is capable of hovering and moving on just one engine, although not very fast or efficiently.

The SRN4 Mk III had a total of 16 life rafts for use by passengers and crew in case of emergency. Whilst naturally buoyant, even without the use of the main engines, due to its 50 or so buoyancy tanks, life rafts are required in case of emergency such as fire or collision or any other good reason to abandon ship.

Life raft drills were carried out a few times each year by Hoverspeed during crew training, in the anchorage area of Dover Harbour. These involved taking a craft into the middle of the harbour, dropping anchors (two located on the bows of the craft either side of the loading ramp) and releasing the life rafts.

The world's biggest lightweight centrifugal lift fan. One of the four seen here in position on The Princess Margaret during maintenance.

The access door to the left of the fan pictured in the lower image allows engineering access to the drive shaft and gearbox assemblies, as shown previously on this page.

Note the clearance between the lift fan and the deck: Very small. The tolerances involved in the positioning, alignment and balancing of all fans on the craft were essential to smooth operation. A fan throwing a blade, or even just being improperly balanced could have had very catastrophic consequences considering their revolution rate!

Main passenger stairs affixed to the plenum chamber at the main entrance. One of these flights of stairs was wheeled by tractor to each side of the craft during the unloading/loading period between flights, to let the foot passengers and crew off.

Looking forward in the starboard passenger cabin. The SRN4 Super-4 was capable of holding 424 passengers and 60 cars in the configuration used by Seaspeed/Hoverspeed. Here is shown the final layout of the cabin before the craft's retirement in 2000. Note the forward galley (blue box like structure), in the approximate position of the front pylon and lift fan assembly, and the pull-down life-jackets for passengers in the overhead compartments.

Access panel in the plenum chamber showing the stringers and ribs used to construct the light frame

A view toward the rear propeller and fin of this Mk II craft at Dover. The propellers of the Mk II craft were smaller than those of the Mk III's (the Super 4's), giving the Mk III's propellers the status of largest in the world! Just about visible, to the left and right of the base of the pylon are two holes in the roof, these are the air intakes for the lift fan.

You can see the mast and pylon here in this photograph, just behind the control cabin. The mast houses both a strobe light and two VHF radio antennae for ship-to-shore and ship-to-ship communication. Directly forward of the mast are the two RADAR antennae, used to supply RADAR information to the navigator (see Cockpit tour page).

A pylon fitted with a trimmed propeller, following a new paint job and testing while in the engineering department of Hoverspeed. Note the complex nut in the centre of the propeller, attaching it to the main prop shaft; this nut provided the variable pitch capabilities of the craft, that is to say, forward, zero, and reverse pitch. Zero pitch was used for stationary hovering, reverse pitch for manoeuvring or slowing down, and the varying degrees of forward pitch could be used to vary speed.

The rear doors seen open here, during unloading of cars. A hoverspeed tractor would approach the craft after each arrival with the moveable ramp shown, which would then be fixed to the craft and vehicles then allowed to drive down it, off the pad, and through to immigration. The APU exhaust can clearly be seen here also.

The rear doors of the craft, seen closed here. Notice just beneath the left hand visible starboard exhaust (of No. 2 engine), a small protruding grey pipe? This is the exhaust for one of two APU's (Auxilliary Power Units), themselves Rover gas turbine engines, providing power to the craft's systems such as electricity and hydraulics, allowing operation of doors, latches, lights, etc.

Rear cargo doors opened, with ramp attached for vehicles

Rudder control pedals of the captain's seat

Underneath the craft, whilst on jacks. Again a rare experience, this shows one of the seven hydraulic jack rams raised. This is at the bow of the craft. A few things to note here -

The rubber pad onto which the jack ram (rusty) is placed. This is one of the seven landing pads underneath the SRN4 - on landing it didnt sit on the skirt or even have a metal-to-ground contact. Instead it was placed down on these giant rubber biscuits, engineered to be the strongest points of the craft's structure.

The buoyancy tanks, seperated by a rusty coloured criss-cross grid and riveted together. These insulated tanks ensured the ability of the hovercraft to float, no matter how much feasible damage.

The fingers. Notice the skirt is not a tube all the way to its base, especially at the front, but is indeed made up of ripples of cone-shaped fabric toward the bottom. These allow better ride quality, and less wear on the main bag. The skirt is at its biggest at the bow, giving the craft its wave-riding characteristic nose-high attitude in flight.