with destroyers on stock, the "D" is their active sonar (the pinging part) which doesn't really care how fast you're going. The part that looks like a giant pac-man is their passive sonar (this is the guy that can hear you and you need to watch your speed) and covers a much larger area than the active (also be aware that later in the war Japanese merchants start to equip the passive sonar and if they hear you will begin zig-zagging before you ever fire a shot.) It's wide open in the back because the sound of their own engines would drown out a submarine's electric motors struggling to keep 8 knots, which is why you'll see them suddenly making these big wide sweeping turns when they're on patrol or escort duty. Then you have the full circles that stretch out even further. If there's one, it's how far they can typically see (though this varies with weather and their skill). Two circles means the ship is equipped with radar and if you're on the surface chances are it already knows you're there, so good luck!
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