A metre- (3.25-feet) long prototype, built by Texas A and M University at College Station, can flap its tail like a fish, the British science weekly reports in next Saturday's issue.

Propellors are submarines' weak spot as they cause noise and a wake of disturbed water, leaving the vessels vulnerable to enemy sonars or aircraft.

Fish, however, have evolved in favour of efficiency, using muscles to move their tail and fins and propel themselves sleekly through the water.

The Texas prototype comprises a hull divided into six sections like vertebrae. They are rigid but each can be deflected with respect to their neighbouring sections.

They are pushed in and out by wire "muscles" made from shape-memory alloys.

These alloys are a novel mixture of strong, resilient metals that contracts when it is heated beyond a certain temperature and then expands, recovering its original shape, when it cools back down below that point.

The prototype's wires, made from nitinol, an alloy of nickel and titanium, are electrically heated, which causes them to shorten and thus pull the section in.

• Chart by Fluid Dynamics Research Group at A&M.

By carefully controlling the heating and cooling and coordinating the movements of each section, the prototype can be made to wriggle forward, fish-like.

The top rate, so far, is five tail-flaps a second, although the researchers are tight-lipped about what speed that gives, New Scientist says.

"Initially, we are focussing on unmanned water vehicles," it quotes one of the development team, Othon Rediniotis, as saying, adding that the potential is there to make a bigger version that would be manned.

One of the biggest challenges is finding a power source. Rediniotis is interested in clean, light and silent sources such as fuel cells and hydrogen to drive the alloy actuators.