1.1 Ship Dimensions - Depth
Vertical Merchant Ship Dimensions - depth
A ship’s dimensions can be measured in length, depth and width. Every commercial vessel has a list of exact measurements and dimensions based on the building plans and load capacity.
Vertical dimensions
All ships have vertical dimensions or depths, variable or non-variable, with measurements taken from specific reference points.
Depth: the vertical distance from the bottom of the keel to the main deck (deck line). The depth of the vessel is non-variable.
Moulded Depth: The vertical distance between the insides of the moulds (including the double bottom). It indicates the inside height of the vessel. It is used to determine the vessel’s cargo carrying capacity in relation to her stability.
Draught or draft: the distance from the waterline to the underside of the hull. The draft is a variable dimension.
A distinction can be made between the loaded draft and the light draft, as well as salt water draft and fresh water draft. Due to the salinity of seawater, the draft in sea water is less than draft in fresh water, because sea water has a higher specific gravity affecting the buoyancy of the vessel.
The draft of a ship can therefore change, based on the load, the season and the geographical position of the vessel.
Air Draft: the distance from the waterline to the highest point of the vessel. When proceeding through a channel that is spanned by a bridge, the air draft should of course be less than the vertical clearance of the bridge. When a vessel’s air draft is greater than the vertical clearance will allow, we speak of a top-hampered vessel.
Freeboard: distance from the waterline to the lowest outside deck (or deck line).
Additional measurements
Waterline: the actual water-level line on the hull when the ship is afloat.
Underkeel clearance (UKC): The distance between keel and seabed.