I think that the U-Boats need to be stronger against the Transports and Able to sink a ship on a random value, or just incur standard damage.
The U-Boat sank the Lusitania after all, with 1 shot, right? The Transports are not that different, and The Battle ships and Cruisers are still boats...That have radar, granted, but that should only still reduce the value that the U-Boat sinks it in 1 shot.
T.Sr.
The Purist