I did a back of the envelope calculation. 20 KT blast at 500 feet from a 1000 ft by 1000 ft steel (iron) plate. Assuming (worst case) that ALL the energy that intercepts the plate is converted to heat.
You could either vaporize a layer of plating 0.02 millimeters thick or you would melt a layer 0.5 millimeters thick. An inch thick layer of steel is 25 millimeters thick.
So, at 500 feet you are going to probably vaporize a few thousandths of a millimeter, melt a few tenths of millimeter, and get the rest of hull nice and warm but not melting, if the large majority of the energy is deposited in the hull (another worst case calculation).
For these calcs, if your ship is more like 300 long by 100 wide it doesnt matter (its smaller but it intercepts less energy). Its the 500ft distance that matters.
Anybody wanna check this calc? I could easily have dropped a zero or two here and be way off.