The ships constant is the difference between a vessels design lightship and it's actual displacement when empty. The ships constant is determined by a draught survey. The ships constant will vary over the life of the vessel.
Generally, a ships lightship value will increase over time; particularly for vessels with large cyclical passenger loads, ships prone to corrosion, and ships which aren't regularly dry-docked. One estimate of lightship increase is a rate 0.5% per annum. Additional inclining tests should be performed after any substantive additions, relocation, or removal of weights, as well as periodically over the life of a vessel.