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Pounds and tons

Long weekend permits two more chapters to be completed. These are shorter than the others, recognising the slimming-down exercise to come.

One of the things that Peter and I discussed was the stupid Imperial measurements they used to use. He reminisced about how godawful non-decimal currency used to be. Here's a piece of my own thesis that had me saying "what the?":

The finished weight of an LCVP was around 6 tons and allowing for 10% additional weight for crating the dismantled boat and engine, each disassembled boat would require just 10 shipping tons (11 m3). Given that the net tonnage of a Liberty ship was 4,380 tons (12,000 m3), it followed that as many as 1,000 LCVPs could be carried disassembled in a single ship. Of course, so many valuable craft would not be risked on just one ship but you get the idea.

It's correct of course but it had me going for a moment there. Since when is 4,000 = 1,000 x 10?. What's going on here is that we are using four different kinds of "tons" in the one paragraph. The weight of an LCVP was around 13,000 lbs, about 6 long tons or 6 metric tons (or tonnes). Shipping tons were 400 cubic feet - that's right, a measurement of volume not weight. But when we talk about the tonnage of a ship, we use tons of 100 cubic feet. Net tonnage refers to the useful cargo space, ie less the machinery and all. So it all works out.