The physics of Thunderbird 4

Theo de Klerk

Thunderbird 4 is International Rescue's submarine and comes into play whenever an underwater rescue is needed. It is manned by aquanaut Gordon Tracy and is the smallest of the Thunderbirds, usually transported to the rescue zone in Pod 4 of Thunderbird 2. Surprisingly, despite its good looks, this submarine Thunderbird is the craft least likely to be feasible. It is all but hydrodynamic and far too small to allow for any type of engine or fuel to be stored on board. The way it is launched makes for exciting viewing but would do a lot of damage to both the submarine and its aquanaut. The pod is very susceptible to sinking and Thunderbird 2 cannot fly as easily with such a large hole in its middle.

Data on Thunderbird 4

Thunderbird 4 side
Side elevation
Thunderbird 4 plan
Thunderbird 4 reverse plan
Reverse plan
Thunderbird 4 front
Front Elevation
Thunderbird 4 rear
Rear elevation
Thunderbird 4 elevations

Most data on Thunderbird 4 has been summarized by Graham Bleathman and Sam Denham in their Haynes’ Agents Thunderbirds Technical Manual published in 2012. In the process they added some other data to explain their cutaway revelations. This article takes their effort as a starting point, applying scale factors where needed.


Length: 9.14m
Width: 3.35m
Height: 3.62m (incl fin), 2.39m (without fin)
Volume: approx.60m3


Mass: claimed to be 16,000 kg (realistically:only 9,800 kg dry, 60,000 kg with full ballast tank, 63,000 kg with full ballast and trim tanks)


Underwater speed: 160 knots (77.2m/s = 277.9km/h)
Surface cruising speed: 40 knots (20.6m/s = 74.1km/h)
Emergency launch speed: 30 knots (15.4m/s = 55.6km/h)
Main turbo drive: two axial-flow turbines providing forward thrust only
Emergency launch jets: 4 vertical thrust hover jets and 2 x 25 liquid fuel mini-rockets
Maximum operating depth: 9,144m (more realistic: less than 1,000m)
Power source: twin atomic fusion reactors (more realistic: nuclear fission reactor)
Forward and reverse drive: claimed to be 6 electrically driven reversible axial-flow turbine impellers (more likely: 6 reversible turbo engines)

Launch and return


Thunderbird 4 is generally launched by Thunderbird 2 dropping Pod 4 into the water. This occurs from a height of about 16 meters, causing an impact speed of 17.9m/s (64km/h) which is quite high considering that airbags in cars are triggered around 23km/h. The pod comes to an abrupt standstill within 1.6 seconds due to it hitting the water surface. This is guaranteed to give aquanaut Gordon Tracy a tremendous headache even before the rescue starts.

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Operation Crash-Dive drop — stock footage
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Man from MI.5 drop
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Launch in Atlantic Inferno
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Independent launch by ramp from Tracy Island
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Atlantic Inferno — mysteriously the pod is picked up
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Frank Bellamy — Blazing Danger and Operation Earthquake

It also means that everything (and everyone) inside the pod is subjected to a deceleration of 17.9/1.6 = 11.2 m/s2. Everything in the pod must be secured to keep it in place during the fall — but also during take-off, landing and other manœuvres Thunderbird 2 makes during its flight.

In The Man from MI.5 the effects team had to reshoot the launch as it occurs at night while stock footage was shot for daylight situations. In this case, they let Thunderbird 2 hover at sea level, carefully dropping the pod without much of a bump: the way it should always be done.

At times, when Thunderbird 2 is away on a mission or incapacitated, as in Terror in New York City, Thunderbird 4 is capable of launching from the island under its own steam. It is unclear how it leaves Pod 4 (or its hangar) and somehow drives or glides to the island's launch ramp.

Thunderbird 2's aerodynamics aren't getting any better with a big hole in its middle where the pod is usually situated, as is discussed in the article on Thunderbird 2.

However, in Atlantic Inferno, Thunderbird 2 mysteriously hovers above the Seascape rig with the pod back in place after having dropped it previously. Apparently, for once Virgil was wise enough to reload the pod to keep his transporter airplane in better aerodynamic shape. The question remains how he accomplished this feat.

Frank Bellamy goes so far in one of the comics (Blazing Danger) as to have Thunderbird 2 lower the pod to sea level on grabs and pull it back in once Thunderbird 4 is launched. In Operation Earthquake the pull back is done partly while flying. In any case, it is a much better concept than simply dropping the pod — however dramatic this may seem.


It is never shown how Thunderbird 4 returns to Pod 4 or straight into Tracy Island's Thunderbird 2 hangar. There must be some kind of winch inside Pod 4 by which the launch operation can be executed in reverse. How this is done when launched from the island is unclear to me. Perhaps Thunderbird 2 comes to the rescue lowering an empty Pod 4.

During Operation

Because the submarine is brought to the rescue site by Thunderbird 2 or under its own steam, its launch and return are covered by its normal launch and return procedures. Unlike Thunderbird 1 or 2 it does not relaunch from its rescue site.

Archimedes' Law violated by Pod 4?

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Thunderbird 4 floating on water

Pod 4 has the same size as all other five pods. Judging from the scale of the aircraft, pod size is approximately 17.1 meters wide, 29.8 meters long and 14.9 meters high.

When Pod 4 floats in the water, about 1/7th of its height is submerged. According to Archimedes’ Law on buoyancy, the total mass of the pod is the same as the amount of water it displaces. The water density is 1000 kg/m3 and the part of the pod that is submerged has a volume of L x W x H = 29.8 x 17.1 x (1/7 x 14.9) = 1,085m3. This leads to a pod mass of 1,085 x 1,000 = 1,085,000kg, which is about 12 times more than the oft quoted pod mass of 90,700kg. The "dry" mass of Thunderbird 4 at 9,800kg would fit well within the quoted 90,700 kg, being 1/10th of the total pod mass.

In this article we will use the quoted pod mass of 90,700kg, even if wrong. With such a mass the pod should be submersed only 0.18m or 18 centimeters which is also very unrealistic. In addition, with its centre of mass well above the water level, the pod will rock easily due to the waves. Being at least half submerged it would make the launch and retrieval of Thunderbird 4 a lot easier and the pod more stable. It begs the question how to realise this as the pod, with its open door, could easily take up water, lose its balance and sink entirely! The events of the ferry Herald of Free Enterprise on the night of March 6th, 1987 outside the Belgian port of Zeebrugge could easily happen to Pod 4.


Shape and stability

diagram illustrating forces
Forces on Thunderbird 4

Being a submarine, it is essential that Thunderbird 4 obeys the laws of fluid mechanics and has a shape that minimizes the water resistance for fastest speed. It also needs to be stable without unwanted rotations if some force operates on it.

Although somewhat streamlined to allow it to cut through the water, above and especially below the surface, Thunderbird 4 doesn’t match the shape of some of the fast fishlike sharks although it does better than Titan’s Terror Fish from Stingray.

This is compensated by some powerful turbo-engines on the top and sides of the submarine. These can work in both forward and backward thrust mode in case Thunderbird 4 must push or pull on (a part of) an object such as sunken aircraft, life-pods or boats. The two booster rockets normally used to launch Thunderbird 4 from its pod can also be used as additional towing engines. Pulling seems impossible as this would require reversing the working of these booster rockets.

If Thunderbird 4 had a cylindrical shape, any rotation around its length axis would cause no resistance as no water needs to make way for the cylinder's rotation. For a rectangular bar, or a somewhat fancier shaped Thunderbird 4, water must make way to allow the object to rotate. The more resistance the object encounters, the less likely it is that it will rotate under an external force. Hence boats have trims and fins: if they threaten to move sideways, the fins cause friction in the water, resisting the vessel's roll. The diagram above illustrates the external force (generated by TB4 or a current) applied to the water in red and the response of the water applied to Thunderbird 4 in blue. As long as the red forces are counterbalanced by equal blue ones, the vessel remains stable. If the red coloured forces win, rotation is inevitable. The blue forces either match the red forces or lose out: they are never bigger because they are reaction forces.

diagram illustrating stability
Stability of Thunderbird 4

The stabilizing fin on top of Thunderbird 4 is useful under water as it also functions to keep the submarine from rolling. There is a limit to the amount of reaction force and if the external force exceeds this amount, the vessel will start to roll. Just as with unstable highly manœuvrable aircraft, Thunderbird 2 in particular, Thunderbird 4 has a computer-controlled DFBW system (Digital Fly By Wire — or in this case Digital Float By Wire) that immediately reacts to such instabilities by moving Thunderbird 4 in the opposite direction. The DFBW system dates back to the Apollo lunar missions and was introduced by Neil Armstrong. It overrules pilot or aquanaut actions in situations where human response is too slow or unreliable.

Diving and floating capacities: Archimedes' Law does apply

Archimedes' Law of buoyancy applied
water line
water pushes up against the small part of TB4 submerged
the entire weight of TB4 pushes the submarine into the water
diagram illustrating Archimedes' Law of buoyancy
diagram illustrating Archimedes' Law of buoyancy
less heavy under water
diagram illustrating Archimedes' Law of buoyancy
float under water

The main question everyone asks about Thunderbird 4 is "does it dive?" Of course it does as its many rescue operations prove. But not by its own weight (as the special effects men in the studio found out — like Supercar and Stingray before it, Thunderbird 4 is made of balsa wood and that does not submerge easily). In fact, when floating, only a very small part of Thunderbird 4 is under water. About 0.32m as can be determined from the illustration below. With a width of 3.35m and length of 9.14m this makes for a submerged volume of 9.8m3.

We can use the same calculations as for a floating pod to see how likely it is for Thunderbird 4 to submerge by itself or what is needed to make it do so. As Thunderbird 4 displaces only 9.8m3 of water as it floats, the mass of this displaced water is equal to 9,800kg (weight about 98,000N). This implies that Thunderbird 4 has the same mass: only 9,800kg which is about as much as ten middle sized cars (and much less than the claimed 16,000kg).

It might be assumed that this low weight is a design decision. Thunderbird 2 carrying Thunderbird 4 in Pod 4 can fly faster if the pod cargo weighs less. The downside is that Thunderbird 4 by itself is very light and incapable of diving unless made a lot heavier, as we will see.

Because the submarine is so light, only a slight displacement of the water suffices to let it float. And it will never go under permanently. It acts just like a polystyrene ball or a piece of wood: push it down under water and it will return to the surface as soon as you stop pushing. To have it stay at any level under water (effectively "weightless") requires that the upward hydrostatic force and the gravitational downward force are equal and opposite so they cancel each other out. This can only be achieved if the object to be submerged is made much heavier. That can be achieved by filling it up with water. The extra weight of the water also pushes down whilst the upward force remains the same. This way an equilibrium can be obtained.

This mechanism of changing weight is also used by Thunderbird 4 and all other submarines. To submerge, it must become heavier than the volume of water it displaces. The easy way to do this is to flood part of the submarine with water. The flooded part adds to the weight of the vessel and acts against the hydrostatic upward force making this less effective. The extra water is stored in a large ballast tank.

When the tank fills up, Thunderbird 4 will submerge as its total weight ("dry" + intake of water) becomes larger than the hydrostatic upward force. How large must this tank be? The volume of Thunderbird 4 is approximately 60m3 which means it displaces a weight of 600,000N of water (1m3 water weighs 10,000N). With its own weight of 98,000N, this implies that to become "weightless" in water it must increase its weight with 512,000N by pumping water into the tank. This amount of water has a volume of 51,2m3, taking up 85% of the total volume of Thunderbird 4 (60m3). It remains a mystery where this large tank is positioned inside the submarine.

Archimedes' Law of buoyancy applied
diagram illustrating ballast tanks
floating on surface
diagram illustrating ballast tanks
submerged level
diagram illustrating ballast tanks
submerged moving slowly upwards
diagram illustrating ballast tanks
submerged moving slowly downwards
diagram illustrating ballast tanks
submerged rising fast
diagram illustrating ballast tanks
submerged diving fast
diving, surfing, manoeuvering through the water

To return to the surface, the water in the tank is pushed out by letting compressed air back in. The weight of Thunderbird 4 reduces, the hydrostatic force remains the same, exceeding the weight of Thunderbird 4 and hence it rises to the surface. To dive, the opposite is done: the tank takes in more water. Thunderbird 4 becomes heavier than the water it displaces and starts to sink.

We all know what a partially filled aquarium does: the water inside it will move if you tilt it and thereby the center of weight changes, toppling the aquarium. For this reason the ballast tank of Thunderbird 4 is completely filled once below the surface. This way it doesn’t matter how the tank is positioned: the water cannot move and neither does the center of weight. Thunderbird 4 cannot move in unexpected directions.

Under water Thunderbird 4 must be quick to move in any direction that Gordon Tracy navigates. Therefore, two additional trim tanks are present: one at each side of Thunderbird 4, positioned against the ballast tank. They are much smaller than the ballast tank but adjustable in size through a piston. Moving the piston forwards or backwards, Gordon can adjust the amount of water inside the trim tanks. In doing so, the hydrostatic upward force increases (trim tanks flooded less) or decreases (trim tanks flooded more). The additional weight of the completely flooded tank almost but not quite counterbalances the upward hydrostatic force. By filling or emptying the trim tanks, the total weight of Thunderbird 4 can be adjusted to a net downward or upward force. This way, Gordon can determine the speed of diving or rising. Emptying the main ballast tank fast will make Thunderbird 4 rise fast.

Deadweight: using torque (moment of mass)

diagram illustrating torque forces diagram illustrating torque forces
tilting through torque forces
diagram illustrating torque forces diagram illustrating torque forces diagram illustrating torque forces
torque applied to Thunderbird 4
old center of gravity
new center of gravity

The use of the torque of a force is also important for Thunderbird 4’s orientation to dive or surface. Another expression for "torque" is "moment of force". A torque is a force applied to an object at some distance from its centre of gravity, making it turn around an axis of rotation in that centre of gravity. This distance to the axis is generally called a lever arm. This principle can be demonstrated using a kid’s seesaw whose centre of gravity is in the middle where its axis is perpendicular to the fulcrum. If you press on the fulcrum nothing happens: the lever arm is zero. But press next to the fulcrum point and the seesaw starts to tilt: the lever arm is larger than zero. This principle works "for free" in conjunction with the fins. For this to work, deadweight is present in Thunderbird 4. It is part of the 9,800kg of the "dry" Thunderbird 4 and can move along the length axis in the middle of Thunderbird 4. In neutral position, the weight and center of gravity of Thunderbird 4 are along the same vertical line and there is no torque.

For ordinary ships it is important to load a ship evenly so that torque remains zero and the ship remains level and does not roll or yaw. Move the weight forward and the center of gravity moves in the same direction. Now a rotation is possible around the axis pointing sideways through the center of gravity. Moving the weight forward causes a rotation that dips the front of Thunderbird 4 downwards. Moving the weight to the back will produce a rotation in the opposite direction: the front will point upwards. Note that this rotation does not make Thunderbird 4 dive or surface. For that to happen, it must become heavier or lighter using the water in the trim tanks.


Because of the high pressure conditions found at great depths, most of Thunderbird 4 is divided into compartments that can each be sealed to prevent the submarine from flooding completely. This includes the ballast tank which is actually divided into many compartments: a leak in one compartment will not jeopardise the mission.

The hull can withstand the pressures of the deep but inside the cockpit and other human accessible areas the pressure is kept at approximately 1 bar, the same as the open atmosphere. This way Gordon can function inside the submarine under "normal" air pressure. Without these precautions, exposure to increased pressures would require decompression facilities to avoid caisson or decompression sickness (DCS).

Critical depth

Thunderbird 4 under water
Looking for an entrance in Terror in New York City

One aspect important to all submarines is the depth it can reach. The lower you go, the higher the pressure of the water. All the layers of water between the surface and the position of Thunderbird 4 are pressing upon the hull. The pressure at the bottom of the sea is more than the pressure at the surface. This pressure attempts to squeeze Thunderbird 4 from all sides into a smaller volume – even squash it.

The hull must be able to withstand this pressure but can only do so up to a certain point. Go any deeper, the hull will crack and the submarine will break apart. The shape of a submerged object is an important factor in withstanding pressure by better distributing the forces. The more even the forces are, the better. A spot where the force is a lot stronger is a prime candidate for cracks to start.

The ideal shape is spherical or at least cigar-shaped. Round forms distribute pressure most evenly along the hull. Thunderbird 4 is far from cigar-shaped. It needs to be employed in rescues, imposing conditions of high manœuverability (lightweight), high speed (powerful engines) and good sight for the aquanaut (cockpit). Television monitors can be useful but nothing equals 3-dimensional 180° visual sight.

Rescue attempts are estimated to occur mostly in the 200-500 metre depth region. Apparently, Thunderbird 4 has a hull made of a special alloy that can compete with regular, cigar-shaped submarines up to a maximum of a 1,000 metres.

This is small compared to the depth of the Mariana Trench in the Pacific Ocean (10km). At such depths the pressure is over a 1,000 times the atmospheric pressure (1 atm = 1,086 bar, 1.1 x 108Pa). Only small bathyscaphes like Jacques Piccard's 1960 "Trieste" or James Cameron in his 2009 Deepsea Challenger ever went that deep.

International Rescue must admit defeat at these depths – fortunately, all rescue attempts requiring Thunderbird 4 were well within reach of the submarine. Supersub Stingray is of an entirely different breed and can reach much larger depths (although in that case some artistic license is required as well).

Stealth mode and protection

Thunderbird 4 launching missile
Missile launch in Desperate Intruder

Thunderbird 4 does not employ propellers that give away the underwater presence of a typical submarine. Other than that, it has no special protection mechanisms to remain unseen – the aquatic environment makes discovery difficult to accomplish beyond measures also employed by military submarines. Also, it is unnecessary in most cases, as Thunderbird 2 is nearby as a dead giveaway for International Rescue’s presence at the rescue scene.

With Thunderbird 2 nearby, Thunderbird 4 does not have extensive armoury on board. This would reduce its flexibility, speed and increase its weight. For "ordinary" protection it can fire missiles from the front rescue equipment area.


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Launch from Tracy Island in Terror in New York City
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Launch from land in Desperate Intruder

For Thunderbird 4 to move quickly, turbo engines are used. Ordinary boats may use propellers that push against the water and, in return, the water pushes the boat and makes it move forward (Newton’s Third Law). Propellers won’t accomplish what is required of Thunderbird 4, though. With a speed of 160 knots (279km/h) and a less than perfect hydrodynamic shape, three turbo-engines are installed inside the nacelles. One nacelle is placed at each side of Thunderbird 4 and one is placed on top. By and large, they surround the submarine's main ballast tank.

The engines can work in both directions. This allows Thunderbird 4 to move forwards or backwards. In a forward direction, water enters at the front of the engine and is thrust out at the rear. As explained above, the force the engines exert on the passing water results in a counterforce, making the submarine move forwards. It depends entirely on the shape of the rudders or propellers how efficient the power of the engine is used to convert it into kinetic energy.

The rudder of Thunderbird 4 is cleverly put right in front of the exhaust stream: each nacelle has such a set of rudders or steering vanes at each end. They look like a grill, but each vane can rotate along an axis, to position it in a left-right or up-down orientation. The top nacelle vanes will cause a left or right movement, the side nacelles cause up or down movement. For this to work, the vanes need be almost at the edge of the nacelles and not half hidden inside as seems to be the case. Half hidden means that the nacelle walls produce a counterforce that opposes the movement Gordon wants to accomplish.

To sail at the surface as well as to launch from the Thunderbird 2 Pod 4 launch ramp (on water or on land) or the Tracy Island launch ramp, two powerful thruster engines are mounted at the bottom sides of Thunderbird 4. They deliver a strong forward thrust.

The vanes in the nacelles tilt Thunderbird 4 upwards, downwards or turn it sideways in combination with the torque produced by the deadweight. Together with its forward movement this allows Thunderbird 4 to swiftly move forwards (or backwards) at any angle.

Power requirements

Thunderbird 4 front elevation
up/down steering vanes
up/down steering vanes
left/right steering vanes
turbo-engines for surface sailing
Engine placement on Thunderbird 4

For a 60,000kg submarine (9,800 kg "dry" plus filled ballast tanks of 51,200kg) that must travel underwater at a speed of 77m/s (278km/h), powerful engines are required to make this possible. Additional power is required to operate rescue equipment or in cases where the object of rescue needs to be pushed or pulled through the water.

According to specifications, the maximum speed is obtained within 10 seconds. Once this speed is reached, power is still needed to compensate for the resistance (drag) of the water while moving through it. To produce the maximum speed of 77m/s, ignoring water drag, requires a power level (energy per second) of 17.8MW to give it its final kinetic energy (½mv2) within the 10 seconds specified.

To overcome drag, the engine must produce a force of a magnitude equal to the drag but directed in the opposite direction. This is similar to a situation where a car engine must provide power to overcome the air resistance that would otherwise slow it down to a final standstill.

To cruise at a fixed speed of vm/s it required that P = Fdrag⋅v. The drag force Fdrag is found to be proportional to the square of the speed: three times as fast gives a 3 * 3 = 9 times larger drag force. This is the same in air and water: it is the energy needed to move the water or air aside to make way for the moving object. The drag force is found to be Fdrag = 0,5pwaterCAv2 where Pwater is the density of water (1000kg/m3) and C⋅A a shape-specific constant that is unique for Thunderbird 4, composed of "drag coefficient" C and frontal cross sectional area A. The drag coefficient is assumed to be 0.5 (resembling a pointed cone — a rather idealized shape for Thunderbird 4) and the frontal area is approximately 4m2. These figures give a power requirement of P = F ⋅ v = 0.5 ⋅ 1,000 ⋅ (0.5 ⋅ 4) ⋅ (77)2 = 457MW to travel through the water at a constant speed of 77m/s.

Power generation

Thunderbird 4 is powered in a way that is common for submarines. Diesel engines and other combustion engines require lots of fuel and oxygen to work properly — not stuff that is available for extended periods of time. For prolonged submerged actions as well as power needed during rescue attempts, Thunderbird 4 uses nuclear reaction engines. These are also less noisy which helps to prevent its discovery while cruising the seas on its own without Thunderbird 2 nearby, it also helps in rescue situations where sound clues are important.

The nuclear engine is a fission reactor as there is not enough room to accommodate a fusion reactor like in Thunderbird 5. It is positioned between the top nacelle and the ballast tank in the rear part of Thunderbird 4. It produces 600MW power — sufficient for Thunderbird 4 to travel at cruising speed or use rescue equipment.

A scenario for the disposal of nuclear waste from Thunderbird 4 is that it is brought into space by Thunderbird 3 on its routine journeys to Thunderbird 5. During this flight the waste is shot towards the Sun for final disposal there. This operation is not without risk as an unexpected explosion of Thunderbird 3 in mid-air would distribute this nuclear waste over a large area. It is one reason why this disposal method is not in use for current nuclear reactor power plants. But perhaps all Thunderbird machines are 100% safe.

Updated cutaway

This is the original Thunderbird 4 cutaway. Keeping the numbering as is, some text has been updated (set in bold italic font) to reflect a more realistic description.
Thunderbird 4 cutaway drawing
Steering vanes in side nacelle control diving and surfacing.
Vanes in top nacelle work as rudder.
Twin atomic generators provide motive power for
main drive; the smaller 'mini-pile' at the rear
works auxiliaries and superheats air for the port
and starboard jet engines
Small jet engines, port and starboard, normally
watertight sealed, are used for surface cruising.
Battery of rocket tubes used to launch Thunderbird 4.
Main turbo drive.
One of six drive turbos that are used when submerged;
these nacelles can be sealed, pumped dry and used as
flotation chambers in an emergency.
Intake for surface jets – retracts when not in use.
Grille systems prevent intake
of anything that might damage
Auxiliary controls mounted on the rear bulkhead.
Telescopic 'Autolock' provides underwater exit.
Mechanism controlling position of lighting trough.
Lighting trough.
Cable gear for tilting trough to required angle
Electronic bar' light source set in parabolic reflector trough
Lighting trough in raised position
Solenoid ram loads missiles from
Conveyor into
Firing tube; missiles are mainly used for underwater demolition
Hydraulic ram in extended position
Central ram carrying interchangeable tool heads
Port ram works independently of starboard if required
Physics 9th ed – Cutnell, Johnson
Ship stability for Masters and Mates, 6th ed – Bryan Barrass, D. Derrett (Butterworth-Heinemann, 2006) Thunderbirds Annual 1966
Thunderbirds Code Geel – AH
International Rescue (Haynes) – Graham Bleathman, Sam Denham
text ©2014 Theo de Klerk
article originally appeared in fab #79