This week's challenge is to torpedo the opposition. Each team must build re-usable torpedoes that run just under the surface of the water, like the torpedoes used to attack boats (rather than those used to attack another submarine). The teams then have to try and hit wooden 'battleships' in front of each other's launch platforms, taking it in turn to fire. The platforms are 150 feet apart, and there's a rock in the middle which they have to steer their torpedoes around. They will have to hit each other's battleship three times. In the event of both teams being on target with their first two shots, the third and final shot will be a simultaneous shoot-out.