beam. The width of a vessel at its widest point; also the side of a ship.

Beaufort wind scale. A visual description of the sea state used to estimate wind speeds and wave heights.

broach. To cause a vessel to veer and turn its beam (side) to the wind and waves.

capsize. To roll a vessel on its beam and to cause it to overturn.

coaming. A raised wall or border around the edge of a ship, its hatches, or other openings to prevent water from entering.

dead reckoning. The determination of a vessel’s position based on course and distance run.

deadweight tonnage. The carrying capacity of a vessel. Originally expressed in long tons (2,240 pounds), today the standard is metric tons.

displacement tonnage. The actual weight of a vessel. Originally expressed in long tons (2,240 pounds), today the standard is metric tons. It is numerically equal to the weight of the water displaced by the vessel (i.e., the water beneath the hull).

draft (or draught). The distance from the surface of the water to the lowest part of the vessel beneath the surface.

drift. The speed of a current in knots.

fetch. The distance of open ocean over which the wind has been blowing.

FPSO. Floating production, storage, and offloading system. A special tanker designed to be moored in an offshore oil field.

freeboard. The distance from the waterline to the lowest part of the deck of a vessel.

GM. An abbreviation for the distance from the center of gravity of a ship to its metacenter; a measure of a vessel’s stability.

gross tonnage. The total enclosed space or internal capacity of a vessel calculated in tons of 100 cubic feet (2.83 cubic meters) each. This is derived from old merchant ship traditions where 1 ton of merchandise on average occupied 100 cubic feet. Under the new regulations of the International Maritime Organization, it is now expressed in cubic meters.

heaving to. A maneuver to protect a boat and crew in rough seas. For a powered vessel, it usually means to head into oncoming seas, at a slight angle to the waves, with just enough speed to maintain steerage way through the water. For a sailboat, after reducing sail, the jib is backed (so the wind is against the sail, rather than pushing it), and the main sail or storm trysail is set to barely maintain forward motion. The rudder is then brought to the leeward side of the vessel and locked there. When the sails are adjusted properly the boat is essentially stationary, making very slow forward progress while also moving sideways.

Mayday. The international radiotelephone distress signal. It is derived from the French m’ aider (“help me”). Before the advent of voice radio communications, in Morse code the distress signal was SOS, signifying “Save Our Souls.” It is transmitted or signaled as … - -- …, or dot dot dot, dash dash dash, dot dot dot.

metacentric height. Literally, the height of the metacenter of a floating vessel above its center of gravity. Its significance is that it is a measure of ship stability.

net (registered) tonnage. A measurement of the earning power of a vessel carrying cargo, equal to the gross tonnage minus spaces on

the vessel that cannot carry cargo, such as the engine room. This is often used as the basis for assessing fees on the vessel.

OBO. Literally, ore-bulk-oil, meaning a combination carrier, a bulk carrier that can carry various types of cargo. The Derbyshire is an example.

pitchpole. When a vessel flips end over end, usually because of sailing too fast down the face of a high, steep wave, the bow becomes buried in the trough and the breaking wave carries the stern over.

Plimsoll marks. Marks painted on the side of a vessel that indicate how deeply it can be submerged under various sea conditions.

rig. On a sailboat, the spars (masts), sails, and standing rigging.

rode. A line connecting an anchor to a vessel; it can be rope or chain.

Ro-Ro carrier. Ro-Ro stands for “roll on–roll off” and signifies a vessel that carries cargo that is driven on and off the vessel.

run-up. Referring to a tsunami, the maximum elevation reached by the wave on land.

scantlings. A set of standard dimensions for ship design.

set. The direction toward which a current is flowing.

significant wave height. The average height of the highest one-third of a group of waves.

soliton. A solitary wave. Solitons can propagate on the surface or below.

SOS. See Mayday.

TEU: Twenty-foot equivalent unit. Refers to the original standard size of containers used on container ships. Today, 40-foot containers are common; one of these would equal 2 TEUs.