I am still a novice player, but it seems to me that u-boats, at least for the Germans, are an "All or Nothing" affair. If I understand the Rules correctly, u-boats don't harm the Allies economies at all unless and until they sink enough merchant ships to put the Allies below the number they need for their convoys. But the Allies, particularly the UK, start with a healthy surplus of merchant ships and gain more from each Minor that the Axis conquer. Using just the 3 u-boats that the Germans start with or are in construction, is it possible for the Axis to put the Allies below this threshold before their u-boat losses become non-cost efficient? If not, than is it the case that the Germans should either go All In and build several more u-boats, or not build any at all?