Torpedo technology is giving old submarines a new lease of life.
Right: a modern torpedo can travel at 200 MPH
Naval planners must have been rocked on their heels when in 1998 it emerged that Russia had developed a torpedo that could travel at more than 200 miles per hour.
Russia’s ‘Shkval’ torpedo puts in jeopardy the safety of every billion dollar submarine and aircraft carrier. It made a mockery of the millions assiduously spent each year improving submarine protection and countermeasures.
- ” . . . [ the Shkval ] torpedo travels at a speed of 200 knots, or five to six times the speed of a normal torpedo, and is especially suited for attacking large ships such as aircraft carriers.” 
The speed of the airborne anti-ship Exocet missile that posed such a danger during the Falkland’s War, was in a stroke transferred to below the waves.
It is in fact rocket powered and a propeller would be superfluous. – they could never produce enough ‘push’ to reach the speeds claimed for the ‘Shkval’
In that scenario it would make more sense to swivel the thruster unit than reply on fins cutting through highly disturbed water.
The weakness of propellers – whether used in air or water – is ‘cavitation’, thatis, sucking through so much material (air or water) that a low pressure gap, or void, is produced in front of the propeller blades and the blades are unable to gain a useful purchase for the next rotation.
Hawker’s high speed Typhoon and Tempest of WWII were the first propeller driven aircraft to face cavitation. This has the effect in the case of an aircraft of limiting maximum level flight speed and to increase speed a dive is required.
This option is available to submarine but not to any other warships, however, if it dives it theoretically gains only a little more speed and will soon reach its maximum diving depth and it is distance not depth that is required .
The speed of an aircraft is limited by its inability to clear air molecules out of its path. A bullet fired from a pistol into water will rapidly slow down by water’s ‘resistance.’ This same resistance (sometimes termed ‘drag’), applies to ships and submarines.
Between the late 1980s and 2000s experiments to reduce the ‘drag’ of torpedoes and submarines were made by several naval nations as well as overcoming the properties of ‘cavitation’ (the US is known to have devised a highly efficient propeller for its nuclear powered submarines). 
By the end of the Cold War submarines and torpedoes had reached the maximum boundaries of their underwater speed potential. The British made Spearfish torpedo was one of the fastest of the time, reaching speeds of speed 80 kts.
These boundaries were set by the Law of Physics. An object can only be moved through a body of water or air to a point where ‘resistance’ overwhelms the propulsion. Speed cannot be increased any further due to the molecules compressing in front of the advancing object and finding themselves unable to get out of the way quickly enough (this makes the fuselage of supersonic aircraft heatup).
The ingenuity of the Russian design team was to invert the boundaries set by nature and the laws of physics and convert them from a detracting negative parameter into a positive one.
Since cavitation is unavoidable they used it to reduce drag. The new technique is called “supercavitation.”
The principle used owes something to phenomenon of the Bermuda Triangle where it is thought that massed bubbles released from the ocean bed render the surface water unable to support the weight of a passing ship and it sinks suddenly and without warning.
A stream of bubbles is produced inside the Shkval torpedo and pumped ahead of the torpedo warhead, water resistance then diminishes and the torpedo speed is increased. This is called ‘supercavitation.’
Left: the all important nose design.
However it should be noted that the 6,166 tons Canadian Coast Guard Ship (CCGS) Henry Larsen, an ice-breaker built in 1987, features high pressure side jets located along and below the water line. This ‘air bubbler’ system of jets reduces hull friction during ice-breaking operations, it is also used to break up the ice and can be used as side thrusters for manoeuvring (http://en.wikipedia.org/wiki/CCGS_Henry_Larsen).
To work at it’s most efficient the whole of the torpedo is surrounded in a jacket of air bubbles. The result is comparable to a sledge speeding along on snow. Pictured above is the silver nose of the Shkval which allows air to be exhausted forwards and vented around the sides.
With ‘supercavitation’ the torpedo is, in effect, flying in a gas bubble created by deflecting water away from the torpedo using its specially shaped nose cone. The gases for the bubbles come from its engine and a reserve tank.
Early designs may have relied solely on an inertial guidance system and or acoustic signatures, but later models are believed to have an auto-pilot guidance system or “control wires” from within the firing submarine. The choice of a ‘homing’ option as used on most torpedoes is unlikely as they are ‘blockable’, i.e. can be electronically jammed.
From Problem to Headache
At the speed that the Shkval travels, it could literally punch a hole in the hulls of most U.S. / NATO / ASEAN ships, with little need for an explosive warhead.
Originally the Shkval was designed as a rapid countermeasure against torpedoes launched by undetected enemy submarines (i.e. US). The problem for Russian naval forces was the quieteness, performance, stealth and sonar sophistcation of the West’s submarines. This disadvantaged Russian skippers and put them forever on the back foot.
The design requirement was for “a new weapon system capable of combating the threat posed by nuclear powered submarines. American boats had better sonar and were significantly quieter than Russian-made submarines and detection of an incoming stealth torpedo gave little time to take evasive action.
One solution was a very-high-speed torpedo to kill the in-coming ‘enemy’ torpedo. This solution was the one arrived atby the Research Institute of Applied Hydromechanics in Kiev, Ukraine – a weapon to counter any possible or potential incoming torpedo that was detected. By launching a very-high-speed torpedo at an enemy submarine’s torpedo (or the submarine itself), would force the enemy submarine to evade, and in the process obliging it to cut the guidance wire(s) to its own torpedo. 
Although announced in the West the 1990s the VA-111 Shkval is rumoured to have been in prototype form, or even operational, as early as 1977. The Russian Pacific Fleet held the first tests of the Shkval torpedo in the spring of 1998. In early 1999 Russia began marketing a conventionally armed version of the Shkval high-speed underwater rocket at the IDEX 99 exhibition in Abu Dhabi.
The following 2 cutaway diagrams show how the high speed Shkval torpedo is thought to be configured. The first shows the bubble flow emanating from the side of the nose but not directly in front of the torpedo body.
The second (below) differs in that externally riggers or skids are itemized and it appears to show the bubble jacket engulfing the nose immediately ahead of the torpedo body. The bubble jacket can be seen inthe artist’s impression at the begining of this article.
Richard Fisher, a defence analyst and senior fellow at the Jamestown Foundation believes China has already purchased the Shkval rocket torpedo: 
- “The Shkval was designed to give Soviet subs with less capable sonar the ability to kill U.S. submarines before U.S. wire-guided anti-sub torpedoes could reach their target.”
- “The Chinese navy would certainly want to have this kind of advantage over U.S.submarines in the future”
Surrounding the body of a torpedo with bubbles while increase its speed by lowering resistance renders the propeller inefficient as the sheer quantity of bubbles reaching the propeller would cancel out any gains.
Right: steering nozzles on the Shkval
Powered by a rocket motor the torpedo literally becomes an underwater missile, capable of reaching its target before the threatened ship has time to respond to the threat. .
Such a high velocity weapon would be well suited not only in its original guise for as an ‘anti-torpedo torpedo’ but close-range submarine encounters and also general anti-submarine and anti-surface ship warfare. It would also be well suited as a defence against high-speed surfaceattack craft.
America had been focusing its work on the ‘stealth’ capabilities of torpedoes. The MK48 Mod 6, while a quiet weapon, still alerts a target when it begins active pinging at the “enable” point. To solve this problem advanced passive homing techniques, covert active waveforms with LPI (Low Probability of Intercept) and LPR (Low Probability of Recognition) properties, and associated signal processing were being pursued.
One ‘stealth’ option is the Integrated Motor Propulsor (IMP). This is a closed-cycle propulsion motor which is quiet, wakeless, and depth-independent. The IMP has few moving parts and depends on a radial-field electric motor fitted not internally butatthe rear to propel water away.
Right: diagram of an IMP
The heart of the Integrated Motor Propulsor is a radial-field, rim-driven electric motor integrated directly into the tail-cone propulsor assembly. This eliminates the need for a separate internal electric motor, facilitates a simpler interface with the rest of the torpedo, and creates opportunities for reduced length, greater reliability, and lower noise.
Notwithstanding the above ingenuity, the ability to ‘kill’ a target before it can react provides a distinct advantage. Speed kills, and to date speed has the edge over stealth.
Supercavitation properties of the Russian Shkval are bringing forth countermeasures. The aim is to maximize the safety and survivability of the warship. Shkval demands that self-defence systems and anti-torpedo platforms are able to detect and then destroy the incoming torpedo.
Supercavitation is said to be a ‘noisy’ method of achieving high speeds. Arguably it is easily detectable but given the circumstances when it would be used, i.e. against an incoming torpedo, the enemy submarine must already knows of your presence so it is not a high price to pay.
Early versions are thought to have had a range of just over 1 mile (2 km). Newer versions are thought to have a range of around 7 km to 13 km (4 miles and 8 miles). 
If supercavitation increases ‘noise’ underwater there is a possibility of compromising its homing abilities. Early aircraft radar sets were found to interfere with their own ability to receive the ‘ping’ back and the progress of Tigerfish, the Mark 24 torpedo was delayed for many years because of the electronic contradictions of trying to use both digital and analogue systems in the same ‘fish.’
Based on technology reportedly under developmentat ONR (Office of Naval Research), a 6.25 inch-diameter self-protection weapon is under study for the defence of surface ships and submarines. The defence platform of this self-protection weapon also uses supercavitation technology.
In effect an underwater field piece, the Advanced High Speed Underwater Munition (AHSUM) programme has already demonstrated the effectiveness of such high-speed underwater bullets. Fired from an underwater gun, these projectiles have successfully broken the speed of sound in water (1,500 meters per second), bringing their future application much closer to reality. 
Supercavitation bullets are also being experimented with by the Navy for use in mine-clearance but fired from a helicopter. The Rapid Airborne Mine Clearance System (RAMICS) targets minesatshallow depths and delivers bursts of armour-piercing rounds from the air, through the intervening water, and into the mines. Merging RAMICS with AHSUM could provide the Navy with a multi-purpose round capable of engaging a range of mine-like targets from above or beneath the ocean surface.
Torpedo as Game Changer
The primary weapon for the Chinese Type 039 diesel-electric submarine (NATO Song class) is the 21 inch (533 mm) Yu-4 torpedo which is a development of the Russian SAET-50 passive acoustic homing torpedo capable of 40 knots. This, together with Yu-4 range of 15 km, is about the industry standard for ‘the average’ torpedo.
Torpedoes have been seen by many as unglamourous and something of a backwater , little changed since their invention in the 19th century. In part this is correct but in many other ways it is totally wrong. Torpedoes can now pick up and follow the wake of a ship. The 53-65KE wake-homing torpedo, designed to attack surface targets, is described as ‘unique.’ It weighs 2,200 kg with 200 kg explosive charge and has a range of up to 40 km. Anti-submarine torpedoes can have an active sonar system which homes in on an enemy submarine, e.g. TEST-71MKE. Torpedoes can be fitted with TV guidance systems which allow the operator to manually switch to an alternative target, and allows for manoeuvring in two axes.
High speed torpedoes are not new. The Japanese Type 93 of World War II, usually referred to as “Long Lance” was 24 inches in diameter, 27 ft long torpedo and had the incredible range of over 40,000 yards (22 miles)  and a speed 52 kts (approx 60 MPH).
During World War II torpedoes used byGermany and the Allies (US and UK) were comparable in speed and charge. The Germans developed electric powered torpedoes (G7e) in an effort to reduce noise and the tell-tale trace of bubbles from the compressed gas motor (G7a). However, the electric version was slower and had a shorter range.
Post war efforts saw hydrogen-peroxide introduced by many navies as a means of increasing a torpedo’s speed. Known in the RN as “fancies” they could be unpredicatable, ie explosive, while being stored or moved (ref. HMS Sidon, 1955).
With the advent of supercavitation a completely new benchmark has been set for high speed torpedoes. It has dramatically moved the goal posts in an environment more accustomed to incremental change. It has had an extraordinary effect ondiesel-electric submarines such as the Song class or similar on the cusp of obsolescence submarines of every nation, e.g. India, Iran. No longer can diesel-electric submarines be written-off as too noisy or not fast enough.
They are already able to carry the Yu-6 wire-guided torpedo which can be used for targeting submarines and can enhance that ability by adopting Shkva type torpedoes which can be fired from the existing 21 inch tubes.
From the same tube as the boat’s torpedoes it is possible to launch the YJ-8 (an anti-ship missile), and a subsonic Cruise-type missile with a 165 kg warhead.
Suddenly, being a noisy Foxtrot, Tango or Kilo class submarine is no longer the disadvantage it once was. Sophistication has lost out to quantity – and many NATO countries have only 4 to 6 ‘modern’ ie sophisticated, submarines (many other supposedly out-of-date boats having been scrapped due to their perceived obsolescence).
China’s nuclear powered attack submarines, Type 093-class (NATO code Shang) are similar to Russia’s ageing Victor III class first produced at the Leningrad shipyards in the 1970s. Each Chinese Type 093 weighs more than 6,000 tons and is over a football field in length. Chinese type 093 submarines are armed with eight 21-inch torpedo tubes that are large enough to fire the super-fast Shkval.
In the opinion of Richard Fisher, a defence analyst and senior fellow atthe Jamestown Foundation: 
- “The Type 093 is projected by the U.S. Office of Naval Intelligence to have a performance similar to the Russian Victor-III nuclearattack submarine. By one estimate, four to six Type 093s should enter service by 2012,”
Armed with the Shkval torpedo these leviathans could be as dangerous as any small and agile craft. Currently, there are no effective countermeasures to the Shkval in service, according to weapons experts. Therefore, concludes Fisher, its deployment by Russian and Chinese naval forces has placed the U.S. Navyat a considerable disadvantage.
One source (John Macneill on a ‘scientfic’ blog ist, (www. popsci, June 1st. 2004) writes of a US Navy water-tunnel tests where it is claimed the astonishing speed of Mach 1 was achieved for a submerged projectile (ie 5,082 feet per second). 
Life just gets faster
With no US version of a Shkval type supercavitation torpedo on the horizon, naval check and counter check is dead in the water. It will be somewhere after 2015 that a US Shkvaltype can be expected to become operational.
Regardless of whether the claim of a speed of Mach 1 is entirely true or not, the next step is patently obvious. Whatcan be done with the body of a torpedo to increase speed can also be done to the hull of a submarine.
The speed gap between the present day 200 mph Shkval torpedo and 700 mph torpedo will probably throw up all sorts of hydro-dynamic peculiarities.
The speed gap between conventional torpedo and a supercavitation torpedo is about 5 times, i.e. 40 knots vs. 200 knots
Imaginative means of propelling a torpedo had been devised and tested. The Spearfish (1992) was intended to catch high-speed, deep-diving threats such as the Soviet Alfa class submarine. Its high speed of 80 knots was achieved by the use of a gas turbine engine (21TP04) driven by a “Otto fuel II” with hydroxyl ammonium perchlorate as the oxidizer.
The American Mark 48 first became operational in 1972 and uses a ‘swashplate’ piston engine or barrel engine. The engines ‘radial’ layout makes it ideal for fitting to a tubular torpedo. For a brief explanation of how a barrel engine creates rotation to the propeller, see Appendix A.
Right: schematic of ‘swashplate’ piston engine
The Mark 48 is also driven by Otto fuel II – described as a smelly, reddish-orange, oily liquid. Otto fuel II is a monopropellant that decomposes into hot gas when ignited without the need of oxygen. Otto fuel II is therefore similar in some ways to its hydrogen-peroxide predecessor cited earlier but without the unstable and explosive characteristics. The thrust generated is conducted to a propulsor, i.e. ducted jets, giving the Mark 48 a top speed of about 55 kts.
China’s Yu-6 torpedo is based on the American Mark 46 lightweight ASW torpedo (and standard NATO issue since 1967 ! ), sold to China during the Bush administration of the 1980s. China has probably reverse engineered some of the imported batch (as it has done so often with Russian technology) and produced their own derivatives (see Project 109). The speed Yu-6 is 65 kts (inattack mode) based on US torpedo which is said to be good for 40 kts.
China’s Yu-7 torpedo is based on the American Mark 48 heavyweight submarine-launched torpedo. At least one Mark 48 torpedo was reportedly recovered by “Chinese fishermen” in the late 1970s or early 1980s (“Chinese fishermen” crop up in numerous international incidents making one wonder if they are naive / unlucky itinerant Chinese fishermen, or PLAN).
Torpedoes may one day be propelled by a magnetohydrodynamic drive (MHD) . The fundamental concept behind MHD is thatmagnetic fields can induce currents in a moving conductive fluid, which in turn creates forces on the fluid and also changes the magnetic field itself. Using membranes, an electric current is passed through seawater in the presence of an intense magnetic field. The seawater (as plasma or ions) would interact with the magnetic field of the current through the water. Compared with the position of the motor, the seawater is then the moving, conductive part of the engine pushing water out the back and accelerating the vehicle.
The significant advantage is that there are no mechanical moving parts although it has to e accepted thatatthis point in time only some ‘working prototypes’ exist.  Its stealth capabilities are known but the penalty is low speeds and critics point to the huge electro-magnetic field created that would make the drive easily detectable. To date only the Mitsubishi Group of Japan in the 1990s have built a MHD powered ship, the Yamato 1, a small craft capable of 8 knots.
Re-wrting Future Order of Battle
Every recent shooting war or conflict has involved the transportation of men and material over great distances. In the past paratroopers would trail blaze, followed up by conventional ground troops, but then paratroopers found they needed more mobility, Jeeps, Land Rovers etc. Still later light tanks, compact and light enough to be parachuted in to consolidate and protect ground forces evolved, examples include the T92 Light Tank at 18 tons, the M551 Sheridan at 15 tons and most recently the LAV-25, an eight-wheeled amphibious reconnaissance vehicle.
“Air assault” was the evolution of this trend where helicopters would ferry in ground-based forces and their hardware to seize and hold key terrain. The philosophy behind Air Assault and its predecessor, paratroopers of World War II, was ‘surprise’ and hopefully in large enough numbers to succeed.
Weight, and the restrictions it imposes on transport aircraft and helicopters, means thatair assault forces are usually lightly armed, though some may have an occasional armoured fighting vehicle as reinforcement. Most heavy lift helicopters able to lift Light Tanks are linited in their range.
Invariably, assaulting troops are highly dependent on aerial re-supply and aerial fire support provided by the armed helicopters or fixed-wing aircraft. The opposing side, meanwhile, have the ability to shoot down any aircraft providing ‘cover’ or re-supplying troops.
The ‘surprise’ element could equally be achieved silently by the use of ‘cargo’ submarines.
A non-military submarine cargo vessel has already been proposed by the Rubin Design Bureau (Saint Petersburg,Russia). The proposal would utilise a laid up Typhoon class submarine (48,000 tons) having its missile removed and replaced with cargo holds. The projected cargo capacity of this configuration is 15,000 ton). The arithmetic is simple; ; adequate quantities of 60 ton main battle tanks, huge supplies of food water plus ammunition and infantry could be transported unseen to the coastline under dispute.
Right: Typhoon class
The Lockheed C-130 Hercules is the ubiquitous military transport aircraft used throughout the West. However, it is limited on what weight it can carry, is highly visible on radar and its four-engine turboprop give it a top speed of around 320 knots.
The idea of moving huge supplies is not new; in the World War I Germany built two submarines as ‘blockade runners’ to the USA, the Deutschland and Bremen, to acquire key resources.
The disadvantage, of course, is thatunderwater troop deployment does not get fighting forces into a country likeAfghanistan. The limitation is one of coastal or deep river landings only. However, submarine troop deployment could be done so quickly and in such strength (15,000 tons of materiel) as to be overwhelming.
If we assume the submerged speed of most nuclear powered submarines is 25 knots, a four or 5 fold increase would make them capable of 100 knots. Hydro-dynamic peculiarities would make this theoretical speed unlikely due to the need for a conning tower (sail) and periscope. A submarine, while it might have clean, aerodynamic lines is not quite as aerodynamic as a torpedo. A submarine without a periscope or conning tower could, theoretically, have a speed comparable to the “Shkval” torpedo (230 MPH).
However, assuming these drawbacks could be overcome and do not vanquish the advantages posed by supercavitation, our perceptions of troop mobility/deployment might have to be revisited. Even without further advances and the realisation of Mach 1 and Mach 2 underwater speeds, slow vulnerable air transport might lose its allure as the universal choice.
The ramifications might be played out in the “order of battle”, ie identifying the strength and disposition of personnel, equipment, and units of an armed force participating in the field.
“Air-portable” may become a relic.
Russian foreign policy has either to be influenced by its Trade Ministry actively selling arms to countries despite the danger thatthey might become future rivals to Russia or its enemy, or Russia’s foreign and diplomatic service is prompting its Trade Ministry to export to whoever has the money.
This can only indicate that Russia has deliberately renounced militant action and military force as a means of resolving international differences of opinion. It no longer sees itself as a world power or a contender for superpower status or a counter to the influence of the US in the political or military spheres.
Perhaps the memories of the economic strain placed on the entire economy during the Soviet era have reshaped their perspective. Russia seems anxious to become the West’s supplier of oil and gas – a very strategic weapon to be able to wield. It is bent on building up it technical excellence and importing expertise where it is lacking. The aim, one suspects, is to make Russia self-sufficient, a net exporter and to build-up gold and foreign currency reserves that will enable it to ride out any economic storm – or even replace the dollar.
The lesson for Germany has been that wealth, power and influence does not have to be bought in blood or paid for by years of deprivation. Germany in the latter part of the 20th century has achieved as much power and influence by economic strength as was ever envisaged under the Third Reich using military might. Has Russia drawn it own conclusions about this ?
Russia’s attitude towards NATO is not one of passive disliking it but is one of continually carping, finding fault and denigrating its involvement in places such as Libya.
It currently advocates disengagement to the extent that it mimics the US between the two World Wars eschewing “all foreign entanglements.” The cost of ‘engagement’ and moderating volatile situations, e.g. Libya, is certainly an expensive exercise.
One suspects that Russia would have preferred to let Kaddafi win in Libya as its position at the UN was clear and on Syria it is one of disengaging and ensuring other nations keep out of Syria’s “internal affairs.”
One has to wonders how successful this policy will be in the long run ? As the death toll continues to mount in Syria the numbers of deaths in Libya (whatever they may be) have at least ‘peaked’ – we hope. Yet Russia preaches non-involvement.
Instead, it is happy to sell the bullets to those who want to settle dispute by conflict and violence. Putin’s Russia may prove to be not so very different from the Stalinist era when wars were fought by surrogates, organised and made possible by Moscow.
Swash plate drive
Trying to understand how a swash plate engine works is very difficult. Swash plate engines are sometimes referred to as Stirling engines, barrel or radial engines.
Most internal combustion engines with which people are familiar are to be found in motor cars. They can be 4 cylinders in-line driving a single crankshaft, or in a Vee configuration with two banks of cylinders driving a single crankshaft. The crankshaft in these engines is always linear, i.e. in a straight line and links piston 1 through to piston 4 (or piston 8 in the case of a V8 engine).
In a swash plate engine the crankshaft is circular – not linear. The pistons are attached to swash plate engine with load-bearing shells, i.e. bearings, just as in a normal car engine where pistons are attached to the crankshaft.
With the help of this illustration it is hoped to show the similarities and the diffennrce and thus how its works. The piston stokes are reciprocal, i.e. up and down successively – just as they would be in a car engine.
2. The green arrow points to the set of pistons that are fully extended (the power stroke).
3. The blue arrow points to pistons that are at their fully shortened, i.e. the exhaust stroke, in readiness for the compression stroke (see green arrow).
4. The bronze coloured circular plate oscillates as the pistons extend and shorten (green and blue arrows).
5. The lilac coloured arrow (verticle dotted line) points to the disk which rotates with the shaft. It glides over the bronze coloured disk which is fixed in its position by the pistons.
The position of the cylinder bores marked by the green and blue arrows remain static within the engine while the pistons thrust up and down. In this illustration the pistons move left to right inducing a circular motion on the swash plate and shaft (red arrow).
 Torpedoes – lightweight and shallow water, ‘smart’ technology etc http://www.navy.mil/navydata/cno/n87/usw/issue_14/torpedoes.html
 The GSKB-47 was ordered to merge with NII-24 Research Institute
 Richard Fisher, a defense analyst and senior fellow at the Jamestown Foundation. http://archive.newsmax.com/archives/articles/2001/4/23/220813.shtml
 In 2000 Edmond Pope a former U.S. Naval intelligence officer was tried, and convicted of espionage related to information he obtained about the Shkval weapon system. President Vladimir Putin pardoned Pope on humanitarian grounds because he had bone cancer in Dec 2000 (Ref. Lockerbie bomber release on humanitarian grounds).
 Bernard Myers, Deputy Technical Director at the Naval Undersea Warfare Center (NUWC) and Jontay Jeong, from the Torpedo Countermeasure, http://www.navy.mil/navydata/cno/n87/usw/issue_14/torpedoes.html
 At sea level the horizon is appprox 20 miles away.
 Richard Fisher. http://forum.woodenboat.com/archive/index.php/t-78027.html
 US Engineer Steward Way, “Run Silent, Run Electromagnetic”. Time. Sept 23rd 1966. http://www.time.com/time/magazine/article/0,9171,842848-1,00.html . See also film “The Hunt for Red October” ref “caterpillar drive”, an undetectable “silent drive” intended to achieve stealth in submarine warfare.
- http://forum.woodenboat.com/archive/index.php/t-78027.html http://www.militaryperiscope.com/mdb-smpl/weapons/minetorp/torpedo/w0004768.shtml