Question 1

A vessel's "summer displacement" (the displacement at her summer load line draft) is made up of two components. What are they?

Accepted Answer

Summer displacement is made up of the vessel's "lightship" weight (the weight of the vessel as built — hull, machinery, outfit, and equipment, but with no cargo, fuel, water, stores, ballast, or crew) and her "deadweight" (the total weight of everything the vessel carries beyond the lightship weight when loaded down to her summer load line — cargo, fuel, fresh water, stores, ballast, crew and effects, etc.); the sum of lightship weight and deadweight equals the summer displacement Displacement at any draft represents the total weight of the vessel and everything aboard her at that draft (equal, by Archimedes' principle, to the weight of water displaced by the underwater hull volume). This total can be broken down into two fundamental components: the lightship weight, which is the weight of the vessel herself as constructed — including the hull structure, machinery, fittings, and permanently installed equipment, but excluding any variable loads — and the deadweight, which represents everything else the vessel can carry: cargo, fuel and lubricating oil, fresh water, stores and spares, ballast water, and the crew with their effects. The summer displacement — the displacement at the vessel's summer load line (the deepest draft she is normally permitted to load to in temperate zone summer conditions) — is therefore simply the sum of the lightship weight plus the summer deadweight. A chief officer must understand this fundamental relationship as the basis for all loading and stability calculations: knowing the lightship weight (and the "constant," which accounts for drift from the original figure) and subtracting it from the displacement at the intended draft gives the deadweight available for cargo, fuel, water, stores, etc.

Question 2

The yacht is loading additional stores and fuel before a long passage, raising the displacement significantly. What effect does this have on stability that you, as OOW, should be aware of?

Accepted Answer

Increased displacement generally raises the centre of gravity if weight is added high up, and free surface effects in partially-filled tanks can reduce the effective metacentric height (GM) Adding weight changes the vessel's centre of gravity depending on where it is stowed, and partially-filled tanks create free surface effects that reduce GM. OOWs should understand these basic stability principles even on smaller vessels without a dedicated loading computer.

Question 3

During a survey, watertight door seals in a transverse bulkhead below the waterline are found to be perished and not closing properly. As Master, what is the significance of this for the vessel's damage stability?

Accepted Answer

Significant — a failed watertight closure compromises the subdivision the vessel's damage stability calculations rely on, potentially allowing progressive flooding between compartments in the event of damage Damage stability calculations assume watertight subdivision functions as designed. A bulkhead door that cannot seal properly defeats that subdivision, allowing flooding to spread beyond the single compartment the design assumed — this must be rectified before further operation.