Australië: ”A pump-jet for the future submarine? Not so fast (or slow)”

A couple of months ago Andrew Davies offered some comments on the pump-jet propulsion system that’s proposed as a key feature of the Shortfin Barracuda. As he observed, there seems a clear case for propellers being more efficient at low speeds—which is very important for conventional submarines. Comments from Naval Group’s director of the future submarine program seemingly confirmed this.

Andrew raised two important questions that might help resolve the apparent contradiction between Defence’s confidence in the pump-jet solution and seemingly credible evidence that its low-speed inefficiency makes a pump-jet an odd choice for a diesel-electric submarine.

The first was that the cross-over point—where a pump-jet becomes more efficient than a propeller—was unknown. If the pump-jet was actually more efficient at plausible higher transit speeds, on balance that may offer a benefit over the entire mission profile, even if the pump-jet is marginally worse at very low patrol speeds. The second was the effect of the ‘hotel load’. Since the square law for drag means that the energy required for propulsion becomes extremely small at very low speeds, underwater endurance might be more affected by the power required to keep the lights, sonars and air conditioning running than by the power necessary to move forward.

Both of these arguments really require some better understanding of the likely efficiency curves in order to be answered with any confidence, as Andrew notes. But it’s quite possible that sufficient evidence does lie in the public domain to rule some possibilities in or out.

Lees verder op aspistrategist.org.au