Simply put, Design Hydrostatics is a subset of the complete Hydrostatics & Stability calculation. The idea is to make it possible to compute hydrostatics with a single button click.
Without Design Hydrostatics, the usual steps to compute hydrostatics are:
1. Select the surface(s) and/or mesh(es) to be included in the calculations
2. Start the Orca3D Hydrostatics & Stability command (Orca3D->Stability->Compute Hydrostatics & Stability)
3. Define the flotation condition (weight/cg or flotation plane, etc.)
4. Set the options (Mirror about Centerplane, etc.)
5. Click Calculate
While this gives you a lot of flexibility and options, many times while you're shaping the hull you just want the hydrostatics at a particular flotation plane or weight/cg, and you don't want to have go through each of these steps every time. The Design Hydrostatics allows you to define a special condition (the "Design Condition") once, and then while you're working with the hull you only need to click the Compute Design Hydrostatics icon to get hydrostatics computed in that condition.
To define the Design Condition, go to Orca3D->Stability->Define Design Condition. Select the surface(s) and/or mesh(es) to be included, define the flotation plane, and check any options (such as Mirror about Centerplane). The Design Condition is now defined, and to get hydrostatics quickly just click on the Compute Design Hydrostatics button.
Keep in mind that if you delete one of the surfaces or meshes that were included in your Design Condition, you will need to redefine the Design Condition. Also, the surfaces or meshes that you select for a standard Hydrostatics & Stability calculation will now become the surfaces and meshes for the Design Condition, unless you redefine it.