Yup Colin BUT if you're building something from scratch and the reference books give varying terms which some could read as "weight" ........?If you look at early books, such as Rees's Naval Arcitecture from 1819, they worked out from the King's Rules and then a Mr Parkyn of Chatham came up with a better version so, for example Victory was calculated at 2162 tons by KR, 1839 by PR and 1840 actual. Adamant with 50 guns was 1044 tons by KR, 870 by PR and 886 actual. Tonnage then was specified to be at the light watermark.A further check was made, where possible, by reverse engineering the hull model.
Going later, one of the standard references, Janes, for 1898 doesn't give any displacement figures atall. Conway 1860-1905 give displacement for most, displacement (load) for some and displacement (normal?) or displacement(standard?) for others. Janes WW1 either give the same 3 options or just the number of tons with no explanation.
What actually caused me to ask the question was the current build which is a simple steam trawler from 1905/6.A general reference book gave 276GT, Register tonnage, as part of a mortgage, was 306, Admiralty weights give 360t light and 547t load.
OK, general guidelines say you can always add weight and in this case I've run out of lead flashing BUT if you're building, for example, a model of a WW1 Z class destroyer at 1:100. The tolerances can be so little as to make the boat fall over, or worse, as soon as you hit the wet stuff BUT if you change from using the Europlans at 1:100 to 1:96 to match your opposition you get a 13% uplift in available weight.
Thank you for your input sofar. I hope you don't think it a futile exercise but I'm sure you know that we wouldn't want to be overweight