When I explain this game I say that a new age of seaborne trade has opened in the Greek world and all the kings want a taste of it. Each player is the head of a trading clan and by being the major investor in a kingdoms trading economy you (or a relative) will be named that kingdoms' Master of Trade and can direct that kingdoms trading efforts for maximum profit to the kingdom and you. The Exploration ship I call the Trade Embassy as the map seems pretty explored. Saying the ships rot is a suitable substitution for "rust" and just as handy. Ships' hulls could last for decades but would eventually decay so a new generation of ships makes sense.