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