Representing weights

You can represent weights of as-yet-undetailed items in Tools/Weight Schedule. Create points with weight attributes to stand in for the missing items; for example, a point named 'ballast' with a unit weight of 4800-kg to represent a ballast casting. Place it where you expect the center of gravity of the ballast to be. It will be included in the weight schedule and combined with other weights. Later, when you've created geometry for the ballast and assigned it unit weights, you can delete the point, or change its unit weight to 0.
If the model has symmetry, be sure to make this point Symmetry Exempt, or you keel could be twice as heavy and might sink the boat when you open Hydro.