fill the hulls with water and measure the weight or gallons that empty out .this is easily converted to a volume measurement don't forget to multiply the results by 2 for the entire boat volume. Is your thought to get displacement this is even easier it is the boat weight.
I think your question should have been how many people will the 21 hold and still be fun to sail
I have carried 6 adults and a few kids and the boat still sailed great.
Much better than a 14 with 2 or a 16 with 4 or even a 18 with 4 on board
As for lines a CADD computer and measurments in cross section fo every 1' of lenght would probably work.
The first comment was not to be taken seriously that much weight would probably damage the boat