Your patrol score is based on Tonnage, type of vessel sunk/aircraft shot down, damage to the U-boat or personnel and realism setting.. I started out at 29% realism and on my next career, moved it up to 47%.. It made a huge difference in renown but the patrols are more difficult..
Good Luck and here's a helpful site. It'll show costs & renown earned for different things:
http://www.communitymanuals.com/shii...itle=Main_Page