"If you can’t explain it simply you don’t understand it well enough" Albert Einstein.
The problem with displacement is not that it’s a difficult concept.
Quite simply it’s the amount of fluid displaced by a body immersed in that fluid. The problem is that it in relation to boats the term is used by different writers in different context and the context is not always clearly defined.
The context may be the plans and specifications for a boat in the early stages of planning. It may be the boat actually floating on the water, or it may be information used to carry out structural or stability calculations. These numbers do not necessarily match the weight of the floating vessel in service at any particular time. They might vary and for good reasons.
Displacement is displacement, the amount to water displaced is exactly equal to the weight of the actual boat at any given time, but the term "displacement" is employed while the vessel is in the planning stage on the drawing board, or when the numbers are on a specification sheet being used by a naval architect, engineer or notifying body, or in a sales brochure.
When a designer starts a project one of the first things to determine is how much volume needs to go under the waterline for the vessel to float on its’ lines. This is largely a matter of
judgement based on experience. It may or may not be revised later in the design process as a result of a weight study or evidence that the final weight might be different to initial expectations,
possibly due to the owner’s fit out requirements or changes to the specification.
In design terminology this volume allotted to the submerged part of the hull or hulls is referred to as the vessel’s Designed Displacement, sometimes expressed as Displacement DWL (Displacement Designed Waterline).
How close the displacement and the actual figures are depend on the designer’s judgement and possibly other factors such as whether the vessels was built, fitted and loaded in a manner intended by the designer.
The displacement figure used by the designer is used to calculate the righting moment which in turn is used in calculations for the structure, stability and compliance with various classifications. Underestimating the displacement can lead to the vessel being out of trim or dragging it’s transom which is very detrimental to performance in multihulls.
It can also lead to lower than intended wing clearance in a bridgedeck catamaran. On the other hand if the displacement is overestimated there are no significant detrimental effects on the sailing qualities or the integrity of the structure, except that the vessel might be over engineered if a higher than necessary figure was used.
Just like people boats tend to gain weight over time and accordingly the displacement figure needs to err on the conservative side. A vessel with a displacement figure that looks high relative to another boat of similar configuration in the sales brochure is not necessarily a heavier boat.