Don't forget, just because a destroyer turns up, its crew have not necessarily detected you. As mentioned above, they may simply be on a patrol course that has coincdentally intersected your route.
Especially at night or in dense fog, it's entirely possible to cruise by not 500m away, and they won't see you. Don't forget that submarines are mostly submerged, and have a smaller profile to spot than a warship, which is mostly out of the water and far bigger than you.
Often, unless they have radar, you'll spot them before they spot you. Therefore, if you dive and go silent, the destroyer will usually drive on past without even attacking. If so, you've successfully evaded the attack.
Note also that sound is often a better detection means than sight. if you're underwater, your hydrophone listens out to 34km from your position (although the operator seems deaf if a sound is beyond 20km). Take a dive every few hours to check your hydrophone to ensure there's nothing sailing towards you. But, be aware that within 2-3km, any destroyers will also hear the sound of your propellors in the water if you're moving faster than about 3kts while submerged.
Today's lesson: Frequently when it's dark or foggy, it can be better to use the hydrophone to check for nearby ships than using the lookouts. However, it also then makes you more easily detectable for the enemy.
Welcome to SHIII. Give 'em hell!
