lone transport rules #17
Description
unescorted transports may pass subs.
The requirement of an escort is for offloading purposes only:
a transport is not allowed to offload land units for an amphibious assault in a sea zone containing 1 or more ignored enemy submarines unless at least 1 warship belonging to the attacking power is also present in the sea zone at the end of the Combat Move phase.
from http://www.axisandallies.org/forums/index.php?topic=39150.0