Lost buoyancy
Flooded compartment does not contribute to the total buoyancy. It can be added by modeling compartment and choosing "Non-buoyant" volume.
Damage analysis in MAXSURF Stability is done by the lost buoyancy method rather than added weight method.
Added weight
Mass of the fluid in flooded compartment is added to the total weight of the ship. When the weight is added, hydrostatics of the ship change, and the final stage is calculated over a set of iterations. This method can be used by modeling flooded compartment as a tank, filling it to waterline level, and spilling it. (add a spilling key point with 'Fluid Simulation' on)
The formula for Righting Arm is:
Lost buoyancy and added mass methods give the same Righting Moment results, however for added mass method, displacement is increased. As a result, Righting Arm curve for added mass method will be lower than for lost buoyancy method.