SRN4 Cutaways

From Blunden & Witcomb, 1985, cutaway labeled views of the SRN4 Super-4 hovercraft, shown here in generic livery but very similar to the Seaspeed livery of the day. Working clockwise around the image from the bow ramp, the series of photographs below attempts to show the craft from various locations around the cutaway. Items highlighted in yellow on the main cutaway diagram are those which have associated photographs below.

The second image shows how air is blown into the skirt from the lift fans and shows a cross-sectional view of this process, and the third image shows airflow through the skirt from the plenum chamber. Note that the holes in the inside lining of the bag skirt in image 3 are exaggerated, these are really about 1cm in diameter and placed about 5cm apart in a standard grid fashion.

Photographs

Bow ramp, Front pylon & mast, Cockpit, Drive shaft & gearbox, Fan Air Intake & Rear Pylon, Fins & rudder, APU and rear doors, Engines & Exhausts, Electronics bay, Life-raft, Car Deck, Plenum chamber and Skirt, Main entrance and forward passenger cabin, Lift Fan.

Bow ramp [top]

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.

Front pylon and mast [top]

Cockpit [top]

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.

Here a different 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.

Drive shaft and gearbox [top]

Fan air intake and rear pylon [top]

Fin & rudder, APU and rear doors [top]

Engines and exhausts [top]

The two main engine exhausts. 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.

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.

Electronics bay [top]

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 engine, and the APUs, generate electricity for the craft via integrated drive generators (IDG) and constant speed drives (CSD), identical 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).

Life rafts [top]

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 Hovercraft Museum has a downloadable video of this drill somewhere on its website, so do take a look.

Car Deck [top]

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.

Plenum chamber and skirt [top]

Looking down through to the plenum chamber through an access hatch in the floor of the passenger cabin of The Princess Margaret. Notice the amount of struts and supports down there - these are actually flexible and give the craft its strength by allowing it to move with the undulations of the water. A rigid craft would experience forces close to its structure's elastic limit, so that would be a bad idea.

The plenum chamber is basically, as the name implies, a plenum (opposite to a vacuum). It is through here that lift air is channeled, allowing hovering by the inflation of the skirt. As detailed more in Concept and Theory , high pressure air is forced underneath the hovercraft by means of this chamber, and with the aid of the skirt, creates a "momentum curtain", lifting it off the ground.

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:

1. 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.
2. 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.
3. 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.

Main entrance and front passenger cabin [top]

Lift fan [top]

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!