Question 1

What is "Dock Water Allowance" (DWA), how is it calculated, and why is it relevant to a Chief Officer loading in a port with brackish or fresh dock water?

Accepted Answer

DWA is the amount by which a vessel may be loaded beyond (deeper than) her applicable load line marks when the density of the water in which she is floating is less than 1.025 (standard seawater), calculated by the formula DWA = (FWA / 1000) x (1025 - Dock Water Density in kg/m3), giving an allowance in mm that is added to the relevant load line freeboard/draft to determine the permitted loaded draft in that dock water A vessel's load lines are assigned based on standard seawater density (1.025 t/m3). When loading in water of lower density (e.g. brackish estuary or fresh river water, common in many ports), the vessel will sit deeper for the same weight loaded, and conversely will rise as she moves into denser seawater on departure. To allow for this, the Fresh Water Allowance (FWA — the difference in mm between the summer salt water and summer fresh water draft/freeboard for that vessel, often found on the load line certificate or calculated from FWA = W/(4*TPC) in mm) is used pro-rata against the actual dock water density to calculate the Dock Water Allowance: DWA = (FWA/1000) x (1025 - dock water density). This DWA is then added to the permitted seawater draft, allowing the vessel to load to a deeper draft in less dense dock water while still being at (or above) the permitted seawater draft/freeboard once she reaches sea water of standard density. The Chief Officer must obtain an accurate dock water density (using a hydrometer) to apply this correctly and avoid overloading.

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.