The way I read the rules is that it will take multiple hits to sink a modern battleship (one with +5 defense). Note that not all of the battleships listed in the ship data tables have the +5 defense.
For each torpedo hit, you'll roll the attack die and add the torpedo value (5, 4, or 3 depending on the torpedo type). The defender rolls and adds the defense value.
So the first hit can only cause light damage. If a subsequent hit causes light damage, the battleship will move to Heavy damage. Ships with heavy damage have a -1 defense modifier, which makes it so you can get Heavy damage on a subsequent hit and cause the damage to go to Crippled. Crippled ships have a -3 defense modifier, so there is a decent chance you can sink the battleship.
Also, there are critical hits that can further reduce the defense values or cause other damage.
In general, it is hard to sink battleships. Many modern battleships had torpedo bulges that reduced the effect of torpedo hits.
Of course, you could always get lucky, do light damage with a critical hit, roll a 6 to roll again on the Heavy Critical Hit table, roll a 1 for a magazine hit and then a 6 for a magazine explosion that sinks the battleship.
