If you have RSRD that is supposed to have fleets and convoys show up at the "histiric" times. I dont like it myself though even in it the type of ships that appear will be random spawns.I see the randomess as a good thing because that means that you will never get the same patrol twice.Also it means that the number of targets will be high through out the war. In WWII by late 1944 the number of targets for subs began to shrink rapidly as the Japanese where being pushed closer and closer to home in last 2 years aircraft became the nubmer one ship killers as they where able to operate in larger numbers and much deeper into Jap zones. In 1945 it could be pretty slim pickings for a sub much of the time.For the first few years subs where the only thing that could strike at the heart of Japan as we got closer it became much more an aviators war though subs always played a very important role.