I used to play at 80+% realism, but got tired of trying to find the convoys... anyways I've had DG shootouts with armed trawlers and the odd flower corvette. I go ahead flank and attack head-on, only turning full rudder when I need to. If you aim well you can take out the ships gun with a few shots, even with HE ammo.
I once took on a black swan frigate, it had two guns working and I took out both with AP ammo using my first two shots! I also once tried attacking a V&W DD by DG, after an acoustic homing torpedo took out the rear ammo bunker. But it sunk me.
I've tried sneaking into ports, backing up right beside a large merchant ship (too close for their guns) surfacing and then DG-ing them. It's tricky, as soon as you get the "ship destroyed" message you have to flank out of there, otherwise the collapsing ship may crush you... I've learned that the hard way.
I wouldn't try that with a warship though... :hmm: