on a personal feel, I think the damage of the Heavy Flak should be 30/70% Direct/AoE Otherwise, there is no point to try to wait for arming time if the direct hit does most the damage.
Due to the fact that the shell has a warhead in it, (it is supposed to be a flak cannon, it just doesn't have the timer to airburst like flak shots do,) it should have a bigger reward for hitting an armed shot, and not just hitting a boat. Otherwise, You might as well just go back to the Gentleman Days and grab your Merc + Flak + Flak Lesmok, All Engineer Galleons again...