18.5mm is just under 3/4 inch. With a straight-on hit, penetration by any of these 'pop-guns' should not have been a problem, but the upper curve that was actually above the surface presented extreme angles and a small target to the incoming round.
EDIT: I see that this is pretty much a repeat of Ken's comment.
2d EDIT: Was there any penetrating hits just below the waterline, where a flatter target angle could have offset water drag? I just remembered posts that discussed that the Japanese had developed shells that could be effective through water.
That is what 3" AP rockets were for.
Soon after some encouraging results from the initial deployment, trials of the weapon were conducted against targets representing U-boats. It was discovered that if the rockets were fired at a shallow angle, near misses resulted in the rockets curving upwards in seawater and piercing the targets below the waterline. Soon Coastal Command and the Royal Navy's Fleet Air Arm aircraft were using the rockets extensively.
The first U-Boat destroyed with the assistance of a rocket attack was U-752 (Kapitän-Leutnant Schroeter), on 23 May 1943, by a Swordfish of 819 NAS. The rockets used on this occasion had solid, cast-iron heads and were known as Rocket Spears. One of these punched right through the submarine's pressure hull and rendered it incapable of diving; the U–boat was scuttled by its crew. On 28 May 1943, a 608 Squadron Hudson destroyed a U-boat in the Mediterranean, the first destroyed solely by rocket. These rockets were, among other factors, credited with making it too dangerous for the Germans to continue operating their Flak U-Boats, which were initially designed with heavy anti-aircraft weaponry to hold off air attacks.
From then until the end of the Second World War in Europe, Coastal Command and the Fleet Air Arm used the rockets as one of their primary weapons (alongside torpedoes, which, to a certain extent they replaced) against shipping and surfaced U-Boats.
Edited by DougRichards, 22 May 2017 - 1631 PM.