Glossary of Nautical Terms

(I try to give the nautical meaning; many words have obvious alternative, everyday meanings.)
APPARENT WIND
The wind felt aboard a moving vessel. It is the combination of the true wind and the boat wind. A boat in forward motion always pulls the apparent wind forward of the true wind.
BACK
The wind is said to back when its direction changes in a counter-clockwise direction. A wind that was blowing from the south and shifts to the east is often said to be "backing to the east." Also see veer and haul.
BOAT WIND
The wind produced by the boat itself as it moves forward. A boat powering forward at 5 knots in a dead calm creates a 5 knot boatwind.
COMPASS COURSE
The course to be steered as indicated by the ship's compass. This course takes into account the deviation from the Magnetic Course that must be considered for a particular compass and its environment aboard a particular ship, both of which influence its accuracy.
DEAD RECKONING
From "deduced reckoning": a technique of navigation in which a plot is kept, starting at a known location (a fix) and, by careful observation of course and speed, the current position is deduced.
DEADHEAD
An old log which has lost most of its bouyancy, and now floats nearly vertical, with one end down, and the upper end just at or below water level. Deadheads are common in old harbors where logs were floated in for loading on boats.
DEPARTURE
the last fixing of position obtained from land or coastal references prior to navigation on open water
DEVIATION
The correction applied to Magnetic Courses in order to convert them to Compass Courses. It takes into account the error of the compass, caused by both internal and external influences on the compass. The deviation of a compass varies with the heading of the ship..
DR PLOT
Deduced Reckoning plot: a charting of course and speed by which one keeps track of ship's position. Positions are usually calculated (deduced) once an hour, or more often as needed. Whenever possible, the DR postion is confirmed by a fix.
DR POSITION
The position of the ship according the navigator's track of course and speed since the last fix.
FETCH
(1) The distance of open water over which a wind has blown before it reaches your location.
(2) The ability to sail to windward with a resulting course sufficient to carry you to a destination, e.g., "We can fetch the mark on this tack."
FIX
A known position, obtained by sighting of objects of known position and applying their bearings from your position, or by close proximity to the known location itself, or by application of other ingenious methods developed over the centuries by sailors and mathematicians.
GAM
A heard or school of whales; a social meeting, visit, or the like, as between vessels at sea; [Nautical] (of the officers and crews of two whaling vessels) to visit or converse with one another for social purposes.
HAUL
Applied to the wind direction: to change in a clockwise fashion. Also see veer and back.
HOBBS TIME
Elapsed time as indicated by a counter which is activiated by an oil pressure sensor on an associated engine. Named after the first manufacturer of such a device.
KNOT
One nautical mile per hour. A nautical mile is equal to one minute of lattitude, or about 6076 feet. Hence, a knot is about 1.15 statute miles. Often explained to landlubbers as "a mile and an eighth."
LOW WATER DATUM
A plane of reference for measurement of water depth. For Lake Huron, for example, charted depths refer to the level of the lake when the water guage at Goderich reads 576.8 feet above sea level. During the summer months, it is not unusual for the lake to rise three feet above this level, and, during periods of extreme high water, the lake has risen as much as five feet above the reference level!
MAFOR
A concise weather forecast coded in strings of five numerals and transmitted by radiotelephone. Details available.
MAGNETIC COURSE
The course to be steered as indicated by the reference to Magnetic North instead of True North. This course takes into account the variation from the True Course that must be considered for a location on earth. A perfect compass would point in reference to True North, but compasses must often have a deviation applied to them to correct for errors.
PAINTER
The line at the bow of a small boat, used to tow it or to moor it.
RHUMB LINE
Technically, a course line that crosses all meridians at the same angle, but casually used to mean the straight line course between two points. On mercator projection charts, rhumb line courses are straight lines.
RPS
A Reversible Pilot Seat; first seen on early Boston Whaler boats.
SCOPOLMINE
In the U.S., a prescription drug that suppresses motion sickness. Usually taken via a trans-dermal (through the skin) patch worn on the neck. It's effective at suppresing nausea, but has side effects in some people of drowsiness and hallucinations.
SET
The affect of winds and current on a boat's progress, often in conflict with the boat's intended course.
SPEED OVER GROUND (SOG)
The speed of the vessel relative to the bottom, as opposed to the vessel's speed through the water. The speed over ground includes the negative or positive effects of the currents in the water.
SPLAKE
A freshwater predator gamefish. The hybrid offspring of a Lake Trout and a Brook (or Speckled) Trout, it was introduced to Lake Huron by Michigan and Ontario fisheries in an attempt to bolster sport fishing. It is presently not reproducing well, and it is maintained by stocking. The named "SPLAKE" was formed by choosing the "Speckled" Trout name in favor of the "Brook" Trout; otherwise the fish would be called a "Brake"...
TRUE COURSE
The course to be steered as measured from True North. This course is calculated or measured from charts, then converted to Magnetic Course or Compass course by the application of variation and deviation respectively.
TRUE WIND
The true speed and direction of the wind, not modified by the effects of the boat's movement.The actual wind, i.e., not the apparent wind felt on the boat.
VARIATION
The correction to be applied to adjust reference bearings and courses from True North to Magnetic North. It varies as a function of one's location on earth.
VEER
The wind is said to veer when its direction changes in a clockwise direction. For example, a wind blowing from the south changes to the west. This is described as "the wind veered to the west." Also see back and haul.
WESTING
Progress in a westerly direction. In days prior to the determination of longitude, a vessel navigated by sailing to a particular latitude, then sailing due east or west as needed until reaching the desired destination. A ship was said to be "running her westing down".
VMG
Velocity Made Good, i.e., speed made good in the desired direction. Distance gained towards destination versus time, as opposed to simple speed through the water.

Portions Copyright © 1995, 1996, 1997 by James W. Hebert. All rights reserved.
Last modified: February 23, 1997
URL:http://continuousWave.com/glossary.html
JWH