India to place $11bn order for AIP hi-tech submarines
Nazi style super-U-boat kit finally spreading
Not so much an underwater Prius as an underwater Chevy Volt
AIP submarines operating fully underwater are more like so-called "range extended" fully electric drive cars. They generate relatively small amounts of electricity for supply to the electric motor, rather than being coupled straight to the props like the ship's diesels (AIP boats still have diesels for surface running and snorting, avoiding the need to top up with exotic fuel every time you want to go somewhere).
Germany's Type 212 boats, for instance, can deliver only 250 kilowatts or so from their PEM fuel cells. Their electric drive, by contrast, requires 1700 kW to deliver maximum submerged speed - which is still only 20 knots, noticeably slower than a modern container ship.
Exact details aren't publicly available, but a Type 212 plainly can't go much faster than a few knots underwater for any length of time, just like a normal diesel-electric boat. Sweden's Gotland class, with Stirling engines instead of fuel cells, are thought to be able to make a sustained five knots underwater: the difference is that they can keep this up for 14 days instead of three.
AIP boats still can't intercept a fleet or convoy at sea, then, except by being lucky or very numerous (even assuming someone on their side knows where an enemy fleet is, which would normally require long-range air reconnaissance or satellites). But they could, perhaps, sneak slowly across a thousand miles of ocean to mount a surprise attack - for instance torpedoing important ships as they pass some obvious chokepoint or strait.
That could have been bad news in the old days, when navies usually tried to find enemy subs simply by listening for the noises they made using "passive" sonar. Electric drive allows AIP machinery to have no mechanical links to the sea - it is very quiet.
But so were the final generation of Soviet nuke boats, in fact. This has led the first-rank navies to shift in recent times away from passive sonar to long-ranging low frequency active "pinging" sonar for hunting subs.
Just two Royal Navy Merlin HM1 helicopters, equipped with active dipping sonar, are said to be able to monitor the entire English Channel for submarines. That's probably a bit overhyped, but they could surely monitor, say, the Straits of Hormuz. These choppers can easily be based ashore, aboard warships, or aboard merchant vessels with suitable helipads added. Once they detect a sub they can close in fast at well over 100 knots and sink it by dropping homing torpedoes into the sea.
Any fleet anchorage or chokepoint or convoy guarded by a handful of Merlins or equivalent helicopters is pretty much safe from attack by AIP submarines. AIP boats would be highly effective against countries without modern ASW helicopters, but not against first- or second-ranking powers.
Third- and fourth-rank militaries usually don't have very good seagoing radar/infrared coverage either, though. So in fact a normal sub without AIP would probably do fine against them: or even a still-simpler surface ship.
The joker in the pack is of course the fact that subs can also launch long-ranging cruise missiles instead of torpedoes. In some cases, such as Israel, this could be a big deal. One of these days India too will have nukes small enough to put into a sub-launched missile along the same lines as the Israeli "Popeye Turbo".
You'd need an lot of Merlins or equivalent to stop an AIP boat getting into missile range of an entire coastline, true. But the big deal here is the nuke, not the delivery method. (Most countries able to make a nuke can also make ballistic missiles anyway, far more difficult to stop than sub-launched cruise. Certainly India can.)
You can see why it's taken a while for AIP to actually happen, then. And why it's possible to suggest that India could, perhaps, find better uses for its $11bn. ®
Lewis Page is a former Royal Navy officer.