Think of it this way; at 805 lbs, the wing is designed to bend up to a certain amount before it is damaged (or departs). If you add water ballast to go to 900 (ASW-19) or 1000 (ASW-19B) lbs, the weight of that water resists this bending, therefore, you can weigh more before damage occurs. Some gliders, with inner and outer tanks (LS-8 18 comes to mind) the POH/FM specifies which tanks should be filled in the case of partial water loads - presumably for the same reason. I have heard the argument that the incompressible nature of water keeps the tank (and wing) from deforming, adding strength, but looking at open class gliders, the wings do seem to bend!

The difference between max weight with and without water ballast has nothing to do with the max weight of non-lifting parts. That defines the load on the wing attachment to resist the bending. The difference between max weight with and without water just changes the bending of the wing.

BGA data sheet has max weight of non-lifting parts, for your info: https://members.gliding.co.uk/wp-con...2114_asw19.pdf .

If you have a pilot max weight of 196 lbs you may be wise to do a new weight and balance. There may be some lead in there somewhere...