I stated with a basic Type VIIB boat. Thew mechanics of the game are relatively simply - you roll to see which theater your next mission is - my first few where the British Isles. The sub marker is then placed on the right track and you roll for encounters which are varied and based on where you're patrolling. Earlier in the war (I started at Sep'39) you have a greater chance to catch single, unescorted ships. As the war progresses the allied escorts and air attacks become both more numerous and more effective.