Does anyone know what was the weight of the Ark before and after the animals were loaded on to it?
The draft of the Ark (the depth of the vessel submerged under water) is given in scripture as being 22.5 feet (15 cubits, as recorded in Genesis 7:19-20).
From this figure, together with the dimensions of the structure, can be calculated the displacement, which is the volume of water that the vessel displaces.
The weight of this volume of displaced water can then be calculated which will be the weight of the vessel (as is known by By Archimedes' Principle).
Archimedes' Principle - the weight of a floating object is the weight of the fluid it displaces.
All the details and calculations are shown by Creation Concepts. They estimate a weight of :
47million lb or about 24 thousand tons
Note: This only gives the weight of the ark when afloat with all its contents.