If you're hunting warships you should try to hit them in the amo bunkers or in the engine rooms. That way you can sink them with one torpedo. As far as DDs are concerned, make sure you set the running depth of the torpedo to 1m and the settings to fast. Also try to fire when the destroyer is between 400-1000m away from you. If it is further away he will listen to the torpedo and avoid it. If you fire from small distances electric or not the ship has no time to avoid the torpedo. Check out these links, they will help you aim at the correct areas
http://www.34thflotilla.com/resources/schiffskarte.pdf
http://www.34thflotilla.com/resources/shipchart.pdf