French shipbuilder DCNS raised eyebrows at last October's Euronaval show with its extraordinary SMX-25 concept for a submarine like no contemporary design - sharp-prowed, with a big but low sail and a jet-like stern. At the IMDEX show in Singapore last week, DCNS gave a few more details of the design.
It doesn't look like other submarines because it's designed for a different job - primarily, anti-surface warfare, rather than anti-submarine warfare. That changes everything. The submarine needs mobility to catch and stay up with surface ships. It will be optimized for survival against active, not passive sonar. It needs a heavy missile load, ready to fire, because surface ships travel in groups and multiple shots can overwhelm the defenses.
Non-nuclear submarines can't run fast for long: batteries don't store enough energy, their circular-section hulls aren't efficient on the surface, and wave effects limit snorkeling speed to 11 knots or so. SMX-25 is therefore designed for high speed on the surface, with a wave-piercing hull and retractable air inlets for three 16 MW gas turbines driving waterjets. Top speed is 38 knots and range at 14-20 knots is 8000 nm.
In action, SMX-25 can ballast to a semi-submerged position with the sail above the surface. It can still run on turbines, the sail is a small visual or radar target, and all 16 missile tubes are ready to fire. Finally, the boat can submerge completely and run on diesel (via a snorkel) or batteries, driving retractable motor-propeller units.
It's a big submarine for a non-nuke, 360 feet long with a surface displacement of 2850 tonnes and 5460 tonnes submerged. under the water, the hull is faceted rather than rounded - like a stealthy aircraft, faceting weakens back-scattered echo from an active sonar.
DCNS argues that the SMX-25 could be remarkably survivable. It is a small target for surface attack - and if engaged by a missile it can dive. Its speed makes it a tough target for a torpedo, and it has sensors above and below the water. And although it looks like something out of Thunderbirds, it is designed entirely around existing technology.
If the SMX-25 has a predecessor in history, it's one that DCNS would rather people forgot about.
In World War 1, the Royal Navy ordered K-Class submarines, powered by steam turbines on the surface, in search of a submarine that could keep up with the battle fleet. The Ks were problematic and unlucky: six of the 18 built were lost to accidents, with one officer expressing the view that they had "too many damn holes."