D   Delta

Keep clear of me,

I am manoeuvring with difficulty


A kind of adjustable underwater foil,  for resisting leeway,  which can be raised or lowered more or less vertically in a case.   When beating to windward the daggerboard is lowered to resist leeway:  when running before the wind it is raised to reduce frictional drag.

Not to be confused with a centreboard,  which pivots inside its case,  or with a leeboard,  which pivots in two dimensions outside the boat,  although all three perform the same function.

A very small dinghy with its daggerboard raised

On these sailing dinghies, you can clearly see the difference between a centreboard and a daggerboard. Illustration Claudia Myatt.


In his 1906 treatise Folkard describes 'sliding keels',  invented by Captain (later Admiral) Schank,  as something we would now (early 21st Century) recognise as a dagger board:  he reminds us that they had been " . . in use for centuries . . by the natives of Pernambuco in their catamarans." and that they were successfully tested by the Admiralty in 1791.




A locator buoy, with a float,  a weight to keep it upright and a 2 metre staff with a flag.   They are kept attached to the taffrail of the boat attached to one of the horseshoe buoys.   They are thrown overboard if a crewmember falls in the water; the flag marks the position of the MOB.

They may also be used to show the location of a worksite,  crab & lobster pots and the ends of a fishing net.


From Latin,  the singular form of 'data':  a single item of data,  or information.

A reference baseline from which measurements are made.

An arbitrarily defined level.

Chart datum is the level of Lowest Astronomical Tide at a Standard Port.  Height of Tide is measured in metres above chart datum (CD).   Charted depth is measured in metres below chart datum.

A geodetic datum is a standard level from which measurements are taken for geographic surveys.

Ordnance Datum Newlyn (ODN) is the datum level for the Ordnance Survey of Great Britain,  defined as Mean Sea Level at Newlyn,  in Cornwall,  between 1915 and 1921.

The Tunnel Datum for London Underground is Ordnance Datum Newlyn -100m.

The tunnel datum for the Channel Tunnel is ODN-200m

Datum line:  see 'Direction'.

Davy Jones' Locker

The bottom of the sea.

Sailors who die,  and are buried,  at sea go down to Davy Jones' Locker.

Those who die ashore go to Fiddler's Green.


A crane made of a swinging boom together with blocks and tackle,  which may be swung over the stern of a boat to lower and recover the dinghy.  Sometimes davits are used for loading cargo, or lifting an anchor on deck. They are often mounted in pairs.

One of the two davits on the starboard quarter of the Thames Barge Hydrogen,  out of Maldon,  tending the ship's dinghy.

One of a pair of davits

The twenty-four hour period between noon and the next noon.

When related to a date,  the twenty-four hour period between midnight and the next midnight.

The hours of daylight between sunrise and sunset.

Day beacon

A fixed structure,  without lights,  for guiding ships.   It may have an identifying coloured board.   It may form a transit with another feature,  such as a chimney.

Dead in the water

Under way (IRPCS Rule 3 (i)) but not moving through the water.   There is an implication of loss of control and vulnerability (to an enemy).


A wooden block with holes but no sheaves which is used for adjusting the tension of the standing rigging.

Once known as "deadmen's eyes".   (Chatterton)



A metal or wooden covering over a portlight to prevent waves from breaking the glass,  or to reduce stray light which might confuse the helmsman or other vessels.


De(a)d Reckoning

A calculation (according to the RYA) of position using only the boat speed,  the heading and the distance run,  but ignoring the tide.   It’s not clear whether it should be spelled ‘dead’,  the meaning of which (in this context) is not clear,  or ‘ded’,  which might be a contraction of ‘deduced’.
Falconer (in 1815) thought it was "a judgement of the ship's position using the distance run and the course steered allowing for drift,  leeway &c".   The RYA would call this an Estimated Position.
Smyth (in 1867) used almost the same words as Falconer.   They both used the spelling "dead-reckoning".

The calculation of a De(a)d Reckoning (DR) position (according to the RYA)  is shown under Estimated Position.   The RYA's Estimated Position is the same as Falconer and Smyth's Dead-reckoning.


Blocks of timber linking the keel to the ship's bottom at the extreme ends where the run of the bilge trends upward to the waterline.



Planks of timber from the fir tree.




The floor beneath ones feet on the outside or inside of the boat.   On a big ship the decks are equivalent to the storeys,  or levels,  of a house.   One stands and walks on the deck,  leans on the ceiling and looks up at the deckhead or headlining.




The overhead of a compartment.   The underside of the deck above.


Deck shoes

Deck shoes have 3 important features.

They must be smart and fashionable,  with leather laces which stretch and break.

The soles must not slip on the deck.

The soles must not mark the teak or glass-fibre deck.



n   To sailors,  the original meaning of ‘deep’ was ‘hidden’,  ‘mysterious’:   in Medieval times sailors had no concept of deep,  or depth,  as we now understand the words.   'The Deep' was the dark,  mysterious,  dangerous place below the surface of the sea.

To measure (or plumb) the deep (hidden,  mysterious) water a shaped piece of lead (Latin;  Plumbum,  Pb) was attached to a line and lowered to the sea bottom.   The line had marks (of leather,  calico,  etc) at 2,  3,  5,  7,  10,  13,  15,  17,  and 20 fathoms;  at 4,  6,  8,  9,  11,  12,  14,  16,  18 and 19 fathoms were ‘deeps’,  with no marks.
Plumbing the deep had more relevance to fixing a position than to knowing the depth of water.
See Marks and Deeps.

n   A submarine trench or gulf,  deeper than the surrounding sea bottom.

adj   Deep water as opposed to shallow water.   Deep is relative and subjective;    if you can't swim,  3m is deep water.   The ends of swimming pools are marked as 'deep end' and 'shallow end';  most people cannot stand on the bottom,  at the deep end,  with their head above water.



An angular measurement equal to one three hundred and sixtieth of a complete circle.

There are 60 (angular) minutes in a degree,  and sixty (angular) seconds in a minute.

Directions at sea are now (early C21) measured in degrees clockwise from True North,  and may be read directly from a gyroscopic or fluxgate compass adjusted to True North,  or calculated from a magnetic compass.

Thus,  from our position,  North is 000°T,  East is 090°T,  South is 180°T and West is 270°T.

Degrees of Latitude are measured from a point at the centre of the earth:  the equator is at 000°North and South;  the North Pole is at 090°N;  the South Pole is at 090°S;  Greenwich (near London) is at 51.476852°N.

Degrees of Longitude are also measured from a point at the centre of the earth.   Greenwich is at 000°East and West;  Moscow is at 37.6° E;  San Francisco is at 122.4194° W;  The International Date Line would be at 180° East and West if small Pacific island states didn't keep changing their minds about which day is which.

One radian is 360° divided by 2 pi, or about 57.3°

Points are an earlier (and more or less obsolete) measure of angles;  a point was 11.25°:  one-thirtysecond of a full circle.   An early (Age of Sail) compass showed thirty two points with names rather than degrees.   Sailors were required to be able to 'box the compass';  ie,  to name the points,  in sequence,  in a clockwise direction.




A region of low atmospheric pressure (normal atmospheric pressure is 1013 millibars),  usually associated with warm and cold fronts.    If points of equal pressure are linked by curved lines (called isobars) each isobar encloses an area of lower pressure.   The synoptic chart on the right shows the distribution of isobars,  4 millibars apart,  around a depression centred to the South East of Greenland,  on a Polar chart.   Winds blow around the depression clockwise in the Southern Hemisphere,  anti-clockwise in the Northern Hemisphere.   Winds do not blow parallel to the isobars,  but are angled about 15° inward toward the lower pressure.   Isobars are packed more closely where the winds are stronger.

See Frontal system,  Isobar,   Cyclone,

A North Atlantic Depression



The distance from the surface of the sea to the seabed.   The numbers on a chart are charted depth,  which is the distance from chart datum to the seabed:  the difference is tidal height.


Depth of water = charted depth + height of tide



Derelict is cargo that is on the sea bottom, and which no one has any hope of reclaiming.   See Lagan.




The error of a compass mounted close to metal or a magnetic source.   The angle between the North shown by such a compass and magnetic North.

The Deviation of the steering compass is unique to the boat on which it is mounted at the time it is measured.   Any change to any electromagnetic structure on the boat (such as the engineradar,  radio  or even the metal rim of spectacles placed close to the compass),  or the position of the compass,  will change the Deviation of the compass.

Deviation varies with the heading of the boat.   For this reason the person who ‘swings the compass’ (or,  more accurately,  'swings the boat for the compass')  (see Annex 13 of SOLAS Chapter  V) prepares a deviation table with three columns and a graph.   The first column shows the ship's head in °C,  the second shows the deviation and the third shows the ship's head in °M.

Compass deviation table


n   A sort of priming made by dampening and bruising gunpowder.

n   "Devil to pay,  and no pitch hot"   There’s a job to be done and no-one to do it.

The seams between wooden planks were caulked (payed) with oakum and hot pitch.

The seam between the waterways and the uppermost wale was called the devil:  it was especially difficult to caulk.

Following some minor infringement of discipline aboard ship there would be the devil to pay.   The unfortunate caulker involved would hang over the side of the ship between the devil and the deep blue sea.



Temperature at which moist air becomes saturated.


Traditional sailing vessels of the Indian Ocean and the Red Sea.   They had usually one mast with a long oblique yard and either a lateen or a settee sail.




n   All charts have tidal diamonds scattered over the chart.   Each represents a point at which the tidal stream has been measured.   Also on the chart is a table of tidal diamonds which lists the one-hour periods before and after the High Water hour of the reference port,  together with the set and the drift of the tide,  at that diamond,  during each one-hour period.   The set is expressed in degrees True.   There are two values for drift:  the first is the drift,  in knots,  at the Spring tide;  the second is the drift,  in knots,  at the Neap tide.

See Tidal Stream Atlas.

n   A gemstone of compressed carbon.   "A girl's best friend" because it is valuable and saleable.


An adjective describing a liquid fuel or a type of internal combustion engine.


Petro-diesel (derived from mineral oil) is a mixture of about 75% paraffins with between 8 and 21 carbon atoms per molecule,  and about 25% aromatic hydrocarbons.   It also contains additives.

Biodiesel is obtained from vegetable oil or animal fats (biolipids) which have been transesterified with methanol.

Diesel fuel has a high flash point,  and is difficult to ignite with,  say,  a match.

In a diesel engine,  diesel oil is atomised to very small droplets,  and mixed with air at a very high pressure (and temperature).   Under these conditions it ignites and burns rapidly,  further increasing the temperature and pressure of the gas and driving a piston to operate the engine.



In an almanac Standard ports have a complete table of tides,  with times and heights of high and low water.   Secondary ports do not have tide tables;  instead,  they have a table of differences from a nearby standard port.   These tables of differences are a little tricky to use.

The UKHO website Easytide obviates these calculations by providing tide tables and curves for both standard and secondary ports.




A small,  usually un-deckedboat,  usually designed to be rowed,  but often sailed with mastboard and rudder.

The Dinghy Cruising Association is very liberal in its definition of a cruising dinghy. 


n   The two ends of the needle of a compass point toward the North and the South magnetic poles.   The poles are usually below the horizon so that one end of a freely suspended needle would point downward.   At the equator the dip of a magnetic needle would be minimal:  it would be horizontal.   At the (magnetic) poles the dip would be extreme and the needle might almost be vertical.

The dip of special "dipping needles" (up to 2m long!) combined with tables of dip were used,  at one time,  to estimate latitude.

v   To dip the ensign is to lower it halfway down the staff and then haul it up again.   Dipping the ensign is an act of courtesy and recognition of another vessel.

Dipping distance

The distance,  in nautical miles,  at which a light just appears above,  or disappears below,  the horizon.   The distance depends upon the height of the light and the height of the observers eye.   There is a table in the almanac.   The chart gives the height of the light (on a lighthouse) above Mean High Water Springs.

Dipping lug

A lugsail the tack of which is tacked down to a bow;  both the tack and the yard are passed abaft the mast and reset on the other side when changing tacks.

Dipping the eye

When passing the eye of a warp over a bollard it should be passed under warps already on the bollard and then up through their eyes before being dropped over the bollard.   This ensures that any of the warps can be removed without disturbing the others.


An angle relative to a datum line.

For direction on the sea (or the surface of the earth) the datum line is between us and the true North pole.

The direction of a bearing;  in practice,  the datum line is from us to the magnetic North pole (eg NorthEast,  or 045°M) as determined by a bearing compass.   This must be converted to an angle with True North before plotting on the chart.

The angle from North of a heading or of a course.



Directional light

Refers to the light on a fixed structure,  such as a lighthouse,  which can be seen from only one direction.   Directional lights are often placed near a bend in a channel:  if a particular light (perhaps white) can be seen then the observer is in the middle of the channel:  if a different light (perhaps red or green) can be seen then the observer is out of the main channel.   Directional lights must be interpreted in the context of the appropriate chart.


v   To leave a ship.




When an object is immersed or floated in water it displaces,  or moves aside,  the water.

The displacement of a floating object (like a boat) is the weight of water displaced by the object.   The weight of water displaced is the same as the weight of the floating object.

Displacement is one of the ways in which a boat might be measured*.

Most boats displace more or less the same weight of water when moving as when stationary:  they are displacement boats.   Their maximum speed through the water is limited by the approximate equation

v ≈ 1.34√L

where v is the maximum hull speed

and L is the waterline length


By contrast,  planing boats lift almost out of the water at speed and skip across the surface,  displacing  very little water.

Hydrofoil boats are lifted out of the water by their hydrofoils.

Air cushion vessels (hovercraft) are lifted entirely out of the water by fans driven by the engines.

Wing-in-ground-effect vessels (which the drafters of the IRPCS erroneously call “Wing-in-Ground (WIG) craft”) are lifted above the water by short wings flying on a cushion of air (the ground effect) close to the water.

*See also Thames MeasurementPortsmouth Yardstick.



A vessel is in distress when she can no longer be managed by her crew.



v   To move beneath the water.

Submarine (vessels) are able to manoeuvre on the surface and under the water.

People swim under water (dive) for recreational purposes or to work.

v   To move from a height to and below the surface of the water in a controlled way.

A person (or bird) who dives is a diver.


Instrument for measuring distances on a chart.

See Caliper



n   The basin of water between two piers,  possibly within a lock, for fitting out or repairing ships,  or for loading and unloading cargo and passengers.   See Dry dock.

v   To bring a boat into a dock and make fast.


A facillity for building and repairing ships.


A piece of fitted material lashed or mounted around the cockpit of a yacht to provide some protection from spray.

The folding canopy mounted over the companionway is a sprayhood,  not a dodger.



n   An iron hook with a sharp point and a handle or loop.   Sacks or planks are pierced with the hook and dragged or lifted while manually loading and unloading a ship.   Dogs (and their handlers) were replaced by fork-lift trucks;  the need for them has been replaced by containers.

n   A bench dog is a wooden or metal pin which fits securely into a hole in a bench to restrict the movement of items being worked upon.

v   To fasten down a hatch cover securely.



At one time,  a Dutch fishing vessel used on the Dogger Bank in the North Sea.   It had a mainmast and mizzen mast,  like a ketch.



A makeshift windvane.   Redundant audiotape may be tied to one or both shrouds to show the direction and strength of the apparent wind.




Navy watches are four hours long;  but,  if they were all four hours long the same people would be on watch at the same times every day.   To vary this the First Dog Watch,  from 1600 to 1800 (ship’s time) and the Last Dog Watch,  from 1800 to 2000 (ship’s time) are each two hours long.

see also Bell.



Ocean regions near the Equator, characterized by calms or light winds; the calm winds characteristic of these areas.




n   Nearly 40 species of marine mammal,  of the Order; Cetacea,  SubOrder; Odontoceti.


n   A structure of several posts,  lashed together and embedded in the seabed;  used at one time for warping ships around the harbour.

n   A species of fish (mahimahi,  dolphin-fish or dorado) found worldwide.

These dolphins in Amsterdam harbour are no longer used to warp ships,  but to moor and to protect the abutments of bridges.


Dorade is a sailing yacht designed in 1929 by Olin Stephens of Sparkman & Stephens and built 1929–1930 by the Minneford Yacht Yard in City Island, New York.   The first vessel to use dorade ventilation boxes,  which were so named after the yacht.

n   A deck-mounted ventilation box which allows air below but not water.

Usually a rectangular box fixed to the deck, and fitted with vertical baffles. The baffles alternate to be free at the floor of the box, or free at the roof, forming a series of chambers.

Scupper holes perforate the wall of the box at the floor of each chamber.

Dorade boxes operate on the principle that air can pass freely through the chambers, yet rain or sea water will be trapped in successive chambers and will drain through the holes in the sides of the box.   See Cooke,  p483

n   A species of edible fish;   Sparus aurata,  gilt-headed bream.


A simple,  lightweight boat with a flat bottom traditionally used in North America for fishing.



v   To lower or furl a sail quickly,  perhaps in a sudden squall or when approaching a quayside.


A solid cylinder of wood used to fasten two timbers together.

Treenails (trunnels,  trennels,  tr'n'ls) were solid cylinders of wood often several inches* long and an inch or more in diameter used to fasten planks to frames and the futtocks of frames together.   The holes into which they were driven were almost exactly the same diameter as the treenail.   The ends of treenails were slotted and fitted with wedges;  the inner wedge expanded the treenail as it reached the bottom of the hole;  the outer wedge was driven in later to expand the outer end of the treenail.

Common dowels are machine made from an inferior wood,  such as birch,  and are widely used in simple carpentry.   They are usually glued in place rather than wedged.   Boatwrights prefer to make their own dowels (?treenails) from the same wood as the timbers to be joined.

*   1 inch ≈ 25mm




A haulyard (ha’lyard (or even halliard)) is used to haul a sail up a luffwire or mast;  a downhaul is used to haul it down again.

A line attached between the tack of a sail and the deck,  used to tighten the luff,  may sometimes be called a downhaul,  but  is more properly a tackline or tack purchase.   In the words of Commodore Luce;  "The yard of a lug-sail hooks to an iron traveler on the mast; the hauling end of the halliards should have an eye in its end, to be placed over the hook of the traveler before hoisting, and used as a down-haul."   It's possible to see how this might have been misunderstood.

In his description of an Houarios,  Falconer describes the use of a 'downhaul rope' attached to the heel of a gunter topmast.


In the direction the wind is blowing.

Vessel A is said to be downwind of vessel B when the wind reaches vessel B before it reaches vessel A.

If you throw your hat into the air it will blow away downwind.



n   An anchor drags when it fails to grip the sea bottom and is pulled through the sand or mud as the boat is carried downwind or downtide.

n   a force acting against the relative motion of an object,  such as a boat or a sail, moving in a surrounding fluid.

Both friction and turbulence contribute to drag.


Draught,  Draft

The depth of a boat below the surface of the water.   The vertical distance from the surface of the water to the bottom of the keel at its lowest point.

The depth of water needed to float a boat.



n   The speed of the tidal stream,  in knots, which may be found from a table of tidal diamonds or from a tidal atlas.

The table of diamonds and the tidal atlas are grouped in 1-hour periods either side of the High Water hour of the reference port.   The figures imply that the drift is constant during each tidal hour and that it changes abruptly at the end (and beginning) of each hour;  in fact,  there is a progressive change from one hour to the next.

The course which a vessel makes over the ground is the resultant of its heading,  speed and leeway,  and the set & drift of the tide.

n  The difference between the size of a bolt and the hole into which it is to be driven.   A bolt which is 1mm in diameter larger than its hole has a drift of 1mm.

n   A metal rod which is driven tightly into a bored hole to join timbers together.

See dowel,  treenail.

vb   To drift is to allow a vessel to move with the wind and tide.   A vessel may be intentionally adrift,  perhaps when the occupants are fishing,  or it may be adrift because it is not under command;  in either case it is still under way according to the definition in the IRPCS.

A sailing boat hove to is not adrift;  it is under the control of its sails and rudder.   It is under way.

A person with no clear objectives might be said to drift,  perhaps from place to place or from job to job.

adj   A drift net is one which,  buoyed up by glass or cork floats,  'drifts' at the surface of the sea.   In practice,  they are usually anchored at one or both ends,  or attached to the ship which has cast them.


A fishing boat using a long drift net supported by floats.


n   The water,  overboard.   An item lost overboard has fallen "into the drink".

n   A potable liquid,  often alcoholic.



The large fore-and-aft sail,  usually gaff-rigged,  on the mizzen mast of a square-rigged vessel.   See Spanker



A sea anchor deployed from the stern of a boat,  and used to reduce the speed of the boat when running before a very strong wind.

See anchor,  Sea anchor.



Deliberately to drag an anchor (or other object) along the sea bottom to slow a boat or to maintain steering.


A dock,  closed by a gate (caisson) or lock,   which can be flooded or dried.   A vessel is floated into the drydock and shored.   The gate is then closed and the water pumped,  or drained,  out.   It allows the bottom of the ship to be cleaned or repaired.

A drydock is the modern equivalent of careening.


Digital Selective Calling (radio).


The DUKW (Duck) was a six-wheel-drive amphibious modification of a 2½ ton truck used by the U.S. military during World War 2 and the Korean War.

It was designed by a partnership under military auspices by Sparkman & Stephens and (GMC),  and was used for the transport of goods and troops over land and water. It excelled at approaching and crossing beaches in amphibious warfare attacks,  but was intended only to last long enough to meet the demands of combat. Surviving DUKWs have since found popularity as tourist craft in marine environments.

D: 1942;  U: Utility (amphibious);  K: all-wheel drive;  W: two powered rear axles.

Dumb sheave

A sheave is a pulley wheel,  around which the rope or wire moves,  inside a block.

A dumb sheave has no pulley wheel;  the rope moves in a groove in a block (usually of wood).


n   Loose material,  often wood or miscellaneous small items packed under the cargo to keep it away from the ship's bottom and between the items of cargo to prevent it's moving.

n   Personal baggage.


Something which must be done.

It is the lookout's duty to look for and report other vessels.

It is the boatswain's duty to maintain the masts,  spars and rigging.



John Starkie

April 2021

If you disagree,

or can't find a word

please let me know.