"Sturdy" Liberty Ships

[Qairyl]

In a mission earlier...ran into a pair of ships, a C3 and a Liberty. Immediately went in to attack, fired two torps each at both ships. The Liberty was hit twice, the C3 once. Since the Liberty was closer to me, I went after it. I waited for a bit to see if it'd go down, and finally saw "Enemy Unit Destroyed" at the bottom of the screen; but it wasn't the Liberty. The C3 had gone down after just one hit. The Liberty was still floating there, dead in the water (one of the torps had taken out the props). Water was too rough to use my deck gun, so I had to keep firing torps at it. Shot a third, direct hit amidships, barely anything but a black mark on its hull. After waiting to see if it'll sink for another five minutes, I fire again, hit a bit further towards the bow than the previous torp. Wait again.....it's a bit lower in the water, but not sinking any lower. So finally, exasperated, I turn around so it's in my periscope at 180 and fire a rear tube at him. Hits at almost the tip of its bow, and it finally flips over and sinks. My question is, is there an easier way to sink these apparently extremely sturdy Liberty ships other than five torpedoes along one side?

-Qairyl