Torpedo versus Bomb
The torpedo would of course have been caught by torpedo nets but would not have necessarily hit too high against the armoured belt. It could be set to run at an appropriate depth and even explode underneath the target.
The 18 inch torpedo Mark XII, an 18-inch-diameter (460 mm) model weighedg 1,548 pounds (702 kg) with an explosive charge of 388 pounds (176 kg) of trinitrotoluene (TNT).
The Upkeep on the other hand was and its overall weight was 1,280 pounds (581 kg), of which 600 pounds (272 kg) was Torpex.
The power to weight was clearly better. The bomb speed in the bounce compared with the speed of the torpedo in water would also be considerably greater which meant it could also have had a greater degree of success against a target underway.
The torpedo could also have directional problems whereas the highball, if launched in the right direction would probably have been more accurate.