In a recent game--Emperor level as France on a randomly selected Standard Continents map--was ferrying a single Transport carrying an Army Leader across a strait to conquered enemy (Iroquois) territory. Four Destroyers and Two Cruisers provided escort. While stacked together in mid-strait an invisible enemy Sub engaged the Transport, sinking it, while ignoring the escorts. So if Destroyers can't detect Subs what's the best way to protect Transports from Subs? Thanks.