you're only off by at lesst four orders of magnitude, assuming the ship was completely filled with iron. you can tell because the city is still thereI used 150k tons (but I see now it was more like 100k tons) moving at 8 knots. Oh and I completely ignored the exponent on the joules-to-megatons conversion... So it's more like 1/150000 Nagasakis. That's more reasonable to expect a bridge to resist I suppose.