On the title screen hit 'F3' to bring up an options menu. The extra ship is the Pirate Mother ship. To reach it, collect the hidden Grafgold Key (usually found under crates, etc.) on every ship. Collecting a key keeps the Pirates at bay for an additional three minutes. A 5,000 point bonus is given for completing a ship with a Grafgold key). Complete the final ship with every Grafgold Key to be beamed aboard the Pirate Mother ship for a fight to the death.