Does the GLM become obsolete when the vessel is modified? Not unless the hull envelope or tank arrangement is modified. For example, putting a bulb on the bow would require that the computer model of the hull be updated also, and this would have to be done by your naval architect. But a change in the superstructure or equipment (if it only affects the weight and CG, not the buoyancy or tank volumes), can be accounted for simply by adding or changing weight items in your GLM. In other words, as a GLM user you can update weight numbers but not the hull and tank models.