The Spearfish Torpedo is used by the Royal Navy (Image: BAE Systems/MoD).
By David Hambling, technology expert
Everything moves slowly underwater because it's a thousand times denser than air and produces far more drag.
But now researchers have found ways of overcoming this drag to create high-speed, and highly lethal, underwater weapons.
Standard torpedoes, like the American Mk 48, run at about thirty knots (50 kph).
The drag of the water seemed adequate until the 1980s when the Russians tested the K-222, a submarine which was clocked at over forty knots (74 mph).
In theory, such a submarine could simply turn around and outrun any torpedo fired at it.
The K-222 never made it beyond the prototype stage, but the threat was one of the reasons that the UK fielded the Spearfish torpedo.
This is the fastest in NATO; rather than a propeller, the Spearfish is driven by a gas turbine engine driving a pump jet, giving it a speed of 80 knots (148 kph) – "faster than a cheetah" as the makers say.
Going still faster requires a special type of technology known as supercavitation.
Normally, torpedoes are designed to be as streamlined as possible, but Russian researchers took the opposite approach with their Shkval (Squall) torpedo.
This has a blunt nose which pushes the water aside, creating a low-pressure zone around the torpedo.
At high enough speeds, the torpedo is surrounded by a bubble of water vapour, enclosing everything except the steering fins. The bubble is sustained by venting exhaust gases into it and it greatly reduces the friction on the torpedo, allowing it to reach an incredible 200 knots (370 kph).
Propulsion is via a hydrojet which burns magnesium, using seawater as the oxidiser. Its main limitation is a short range of only about 15 km, compared to 50 km for Spearfish.
The Russians used the same supercavitating design for a range of underwater rifles including the APS assault rifle.
Water friction stops most bullets within about three metres at most but the blunt-tipped projectiles fired by the APS are deadly at thirty metres, giving Russian combat divers a distinct advantage. The APS has now been fitted to a new underwater robot for harbour patrols.
The physics of supercavitation are complex and others have struggled to match the Russian design.
German company Diehl announced a program for Barracuda - a supercavitating torpedo in 2004 but the project was never completed.
US Navy researchers have worked with underwater projectiles and have achieved extreme speeds of over 1.5 km a second - but are yet to produce a system which would be operational.
A mine-busting projectile which could be fired into the water from a helicopter was cancelled in 2011, and the latest US torpedo is a slightly upgraded version of the Mk 48.
Others claim to have developed their own version of the technology. China acquired a batch of Russian Shkval torpedoes in the mid-90s and in 2015 Chinese media announced that their scientists were working on a high-speed submarine but these claims were not taken seriously in the West.
Perhaps more significantly, Iran acquired and test-fired a Shkval torpedo in 2004.
In 2006 the Iranians claimed to have test-launched their own prototype supercavitating torpedo, the Hoot or al-Houth (‘Whale’) and released video footage of it hitting a derelict target ship.
Iran is well-known for making overstated claims about its new weapons but other test firings have followed including one in 2017.
After 11 years, the Iranians may be close to an operational copy of the Russian design.
A high-speed torpedo could be significant in combination with Iran’s new Fateh attack submarine.
Fateh weighs just over 500 tons (compared to over 7,000 tons for a Royal Navy Astute-class submarine) and is designed for the coastal waters of the Gulf - it's big enough to have four full-size 21-inch torpedo tubes.
The Shkval has been described as a 'suicide weapon' which is so noisy it would give away the location of the submarine which launched it.
This is not an issue for a country which celebrated the martyrdom of volunteers who sacrificed themselves in the Iran-Iraq war of the 80s.
High-speed supercavitating weapons may be fast and furious carrier-killers, or, as critics suggest, they may be easy to spot and counter. Until they are used in action, the subject is very much open for debate.