D (Delta)

The single-letter signal means, ‘Keep clear of me; I am manoeuvring with difficulty’. Few yachtsmen are likely to make it, if only because they would hardly find time to do it if they found themselves in such straits, but every private owner should know what it implies if made by a bigger craft. The Morse is ‘-. .’. Phonetically it is Delta. In times of fog or bad visibility a long and two shorts on a ship’s whistle means that she is not under command, or restricted in her ability to manoeuvre, or constrained by her draft, or under sail, or fishing, or towing or pushing another vessel. (Collision Regs, Rule 35c).




A Centreboard, usually in a dinghy, which retracts vertically. Some bigger sailing boats have dagger-boards, but their disadvantage is that they are not free to swing aft if they strike the bottom. (Although the vertically retracting board has a specific name, Dagger-board, the pivoted board lacks one. ‘Centreboard’ applies to both types).


A small marker buoy (not usually for navigation) having a vertical stick or staff through its centre, with perhaps a flag or light on top. Dan-buoys are used to mark lobster pots, anchors and so forth, or as racing marks. They are also used in conjunction with horse-shoe rings as life-saving aids in man overboard situations.


A patent anchor. The design originated in the USA, and the name has now become generic for a number of similar anchors,just as ‘CQR’ is often wrongly used for Plough anchors which are not of that specific design. A Danforth has excellent holding power, and so have some of its copies. An important point is that the shank should be strong enough and stiff enough to match the loads which its high holding power can impose. In other words, the poor imitations bend ….


The preferred pronunciation rhymes with ‘have it’, but the long ‘a’ is quite OK if you crave it. A davit is a mini-crane, usually a single steel tube formed into walking-stick shape. Davits may be of other shapes, and may be single to lift an anchor aboard, or in pairs to lift a dinghy or ship’s boat.


Typically a small sailing boat, a few feet more or less than twenty, without a cabin, and used for pleasure sailing as opposed to racing. Like so many other sailing terms, this one is ill-defined (and not only by me). It is also used of small power craft, provided that they are not used for racing nor for overnight sleeping. But a fast runabout is not called a day-boat nor is a sea-angler’s skiff.


An unlit beacon, normally rather large and erected on the land, to provide a reference point for pilotage by daylight. Day-marks often take the form of towers of brick, stone or steel, and may be several hundred feet high.


An RYA course teaching basic navigation and helmsmanship

Dead reckoning

Commonly abbreviated to DR. This is the type of navigational computation which deduces a position by plotting a heading and distance run (through the water) since leaving a former position. It makes no allowance for the effects of tidal stream, or current, or for leeway. When those influences are taken into account the result is an Estimated Position, or EP. (The ‘dead’ is supposed to derive from ‘deduced’, by the way.)


A block without a sheave. Normally has three smoothed holes bored through the wood, with well-faired entry and exit, to take the lanyards used for bowsing down the shrouds of a sailing vessel. Alternatively, and more likely on a modern boat, the block has a single hole and is used, perhaps at the clew of ajib, to give a single whip purchase.


A solid cover which can be shipped over a Scuttle for protection. (Or you could say ‘fitted over a porthole’, but what’s the good of having this excellent dictionary and not using the proper terms !)


The rise of the bottom of a boat from the keel outward to the turn of the bilge. A flat-bottomed boat has no deadrise. The sharper the angle of the V in a V-bottomed boat the greater her deadrise. In planing boats a flat bottom gives best speed for a fixed power, but the ride is harsh. Increasing deadrise makes for a softer ride, but speed suffers slightly. The term deep-vee is applied to hulls with twenty-five degrees or more of deadrise.


A heavy timber fillet which fills the angle between the keel and sternpost (or transom) of a wooden boat. What you would call a big bracket if you were fixing a massive shelf in the kitchen. There may also be a deadwood between keel and stempost, but similar and smaller brackets in other parts of the ship are called Knees.


The Decca Navigator System used low frequency radio transmissions from several different base stations to allow a receiver to accurately triangulate its position. First deployed in World War II, Decca was shut down in the spring of 2000 when GPS made it obsolete.


Need I?

Deck light

A thick piece of glass let into the deck to allow light below. Some¬times of ‘bullseye’ or ‘bottle-end’ shape, and sometimes a prism.

Deck log

A rough note-book in which the crew in the cockpit can keep notes for neat compilation of the proper Log at some later time.

Deck organiser

A series of sheaves used to route control lines along the deck or coachroof.

Deck saloon

A raised coachroof with large windows that allow people inside to see out while seated.

Deck spreaders

Spreaders extending outboard, sideways at deck level.


A boat may be fully decked, though the fact is not so likely to be men¬tioned as when she is ‘half-decked’, which implies decking over the bows (prob¬ably as far aft as the mast), a narrow strip of decking along each side, and a small after-deck.


The underside of the deck, as viewed from the cabin. Even if there is an inner lining or ceiling under the deck it will be called the deckhead. The word Ceiling has a special use which I could easily tell you now, but rather than be checked out of the lexicographer’s union, I’ll use that wicked q. v.


Please see Deadrise.


Of a flag, it means that some known pattern of flag is embellished with some other symbol. For example, the Cruising Association’s ensign is the British blue ensign ‘defaced by a white anchor on a red ball in the fly’. (See Fly.)


The known position from which a course is laid for Dead reckoning. For example the last reliable fix from shore bearings before the land disappears from view and you are obliged to rely on dead reckoning.


Meteorologically speaking an area of relatively low atmospheric pressure and harbinger of bad weather. Depression may result in the skipper who hears it is coming.


The vertical dimension of a hull between Sheer and Keel. Very often called the Moulded depth. (See Moulded.)

Depth gauge

A scale of feet or metres painted vertically on a harbour wall, or pile, or the like, which shows the actual depth of water at that place. Example, the present depth at the entrance to a lock. A Tide Gauge is similar in appearance, but conveys a different message.


Same thing as an echo-sounder.

Designed waterline

See waterline


The compass error resulting form the influence of magnetic materials aboard the ship. In small craft deviation can often be avoided by siting the compass where it is several feet clear of the engine or other objects made of iron or steel. Otherwise one calls in a compass adjuster who will place little magnets in positions where they compensate for the effects of inbuilt metals. Or you can make a Deviation card which records the errors for various headings. (See Heeling error and Variation.)


Direction finding (see radio direction finding)


Differential GPS. A system to improve the accuracy of GPS by using signals from a network of ground stations.

Diamond stays

A pair of lateral mast stays, leading from the masthead, over Spreaders and back to the mast above deck level. (Thus outlining a diamond shape against the sky.)


A foghorn operated by compressed air. Characteristically it emits a deep booming note which ends with a short grunt. (Compare with the Tyfon.)


A small open boat, used under oars, sail, or outboard. A small sailing boat used for racing and having a centreboard and not a fixed keel. The term is automatically applied to almost any yacht’s tender unless she is shaped like a launch and of fourteen feet or more in length. Thus it includes inflatables. But note that a small fast runabout driven by a powerful motor is not a dinghy.

Dip-pole gybe

A method of gybing a spinnaker pole, where one end of the pole stays permanently attached to the mast. The outboard end is released from the guy, and lowered to pass inside the forestay, whereupon the new guy is clipped in and the pole rehoisted on the new windward side of the boat.

Dipping light

A light seen at such a distance that it appears and disappears over the horizon as the ship lifts to the swell. The line of sight to the horizon is naturally below the horizontal (unless you are swimming) and this angle is the Dip. When using a sextant to measure the altitude of a heavenly body above the horizon, a correction must be made for dip.

Dipping Lug

A four-sided sail with a Yard at the head which sets on the leeward side of the mast, and has to be changed from side to side as the boat tacks. To get the yard and the sail round the mast they have to be dipped or dropped, partially or completely. See sketch under Lugsail.

Direction Finding

See radio direction finding

Dirty Wind

Off the lee quarter of a sailing boat the wind has been disturbed by vortices shed from her sails, and its direction is shifted somewhat to weather. Another boat will be hampered in this region of dirty wind, and may find it impos¬sible to overtake through the lee quarter, even though she might be the faster if enjoying the same ‘clean’ wind.


(1) The weight of water which is displaced by a floating boat, and likewise the weight of the boat herself. A boat weighing, for example, one ton immerses until she makes a hole in the sea which formerly held a ton of water. The pressure to be exerted by the surrounding water is then the same as it was formerly. In design, and therefore in sales pamphlets, the displacement should be the weight of the complete boat in sea-going state, with fuel, stores and crew. (See Thames tonnage and Register tonnage.)

(2) In navigation, ‘displacement’ may refer to the distance by which a position is east or west of a Meridian of longitude.

Displacement Hull

A hull which always floats in the water and never planes across it. Displacement craft are relatively slow but are sea kindly and economical of fuel. (See Drag.)

Displacement/length ratio

This is the standard formula used by naval architects (and percipient private owners) to assess whether a particular boat is light or heavy for her length. The underlying reason for the formula is that a boat which is twice as long as another, can be expected to weigh about eight times as much if she has the same displacemenUlength ratio. The two boats are then comparable in terms of ‘heaviness’ or ‘lightness’. To find the D/L ratio you take her weight in tons and divide it by the cube of one-hundredth of her waterline length in feet. It looks like this



The tons which are used internationally by yacht designers for this formula are the old-fashioned British kind of 2,240 Ib each, a fact which should be noted by North American readers. As a guide in case you are minded to work it out for your own boat, the sort of answer you can expect ranges from 400 Heavy, through 300 for Medium, and 200 for Light. Cl allude, as always, to seaworthy cruising boats) The other widely used formula for comparing boats is the sail Area/Displacement ratio.

Distress Signals

There are fourteen internationally agreed signals which may be made when a vessel or her crew are in danger and help is needed. A less urgent signal is to show the letter V by flag or Morse code, meaning that assistance is required, though there is no immediate danger.

Docking Line

Just a rope which is used to make a boat fast to a harbour wall, a pile, or even another boat. Six docking lines are normally required to moor a boat alongside: two Breast Ropes; two Springs; and two Warps.


A canvas screen erected to keep wind and spray at bay. More esteem will accrue to your image if you refer to it as a Weather cloth.

Dog watches

In the Royal Navy the shifts of duty known as Watches are each of four hours, but the period from 1600 to 2000 is split into the first and second dog watches, each of two hours. This makes a total of seven watches in the twenty¬four hours, instead of six, and this uneven number ensures that a fixed pattern of watches does not result in the same men doing the same watch day after day. There’s little of value to you or me in this point, except that the phrase is very common, and equally commonly misunderstood.


A raised shelter over the entrance to the cabin, so called because it has very roughly the dimensions of a dog-kennel. More properly called a Companion. (See Wheelhouse.)


A fixed pile of timber, concrete or metal which is used for mooring a ship, and especially for warping in or out of a dock. Also used of a navigational beacon if standing in water and rather massively built.


A flat-bottomed double-ended pulling boat whose sections show straight flared sides. The thwarts are removable so that the boats stack one inside the other on a ship’s deck, so to be carried to the fishing grounds where they are launched for (e.g. long-line) fishing. A mast may be stepped and rigged with a smalllugsail. The name dory has also been applied commercially to a triple-vee or Cathedral-hulled boat which bears no relation at all to a dory – other than the fact that both are boats.

Double the angle on the bow, to

A navigational technique for finding one’s distance off a mark, such as a headland. Laying a course to pass off the headland, a bearing is taken of the mark – let us suppose that it lies ftfteen degrees off the bow. Maintaining the same course the distance by log is measured until the mark shows double the angle on the bow (thirty degrees off). The direct distance be¬tween ship and mark is now equal to the distance she covered between taking the two readings. To get the best accuracy it is necessary to take into account the effect of any tidal stream, since it is the true distance the ship has moved over the bottom that is required. (See Four-point bearing.)

Douglas Protractor

A navigational protractor made on a square sheet of transparent plastic instead of the conventional circular form. The square edges make it easy to align the protractor with a meridian, or with Magnetic North. (See also Course-setting protractor.)

Down Helm

Bear away


You guessed it – a rope rigged for the purpose of hauling something down, commonly a sail or the tack of a sail. (See Inhaul.)

Dowse, to

To capture a sail and hold it quiet, by lashing, tricing, bagging, or just sitting on it. Also, as ashore, to extinguish a lamp.


Dead Reckoning


Please see Bonnet


(1) The downward slope of the bottom of a keel which is deeper aft than forward.

(2) The old word used to be ‘resistance’, but drag has been dragged into naval architecture from aerodynamics. The water drag of a hull can be considered as composed of four different types of drag. First is Form drag which in essence is due to the difference in pressure on the back and front faces of a moving object (your hand when swimming, for example). Second is Skin drag, which is akin to friction but results from the clinging of a thin layer of water (the Boundary layer) to the surface of the hull. This layer moves with the boat, and there is thus a shearing force between it and the stationary fluid farther away from the skin. Third is the Induced drag, which results from the difference of pressure (lift) on the weather and lee sides of the keel. Fourth, and of great significance in surface craft, is the Wave-making drag. Wave-drag effectively limits the speed of any displacement craft, and for the following reason. When moving through the water, a hull creates bow and stern waves, with a hollow between. Any wave in water has a natural speed, proportionate to the square root of the length between wave crests, and when the wave is hull-generated the length between crests is directly related to the length of hull. Hence a boat’s speed is related to the square root of her hull length. It works out that a practical speed in knots is found by taking the square root of the water line length in feet and multiplying by a factor between 1.2 and 1.4. Thus a twenty-five foot waterline boat can be expected to move at between six and seven knots. The corollary is that the factor can be found if you divide the speed in knots by the square root of the water line length NI-YD I or in words ‘V over root L’.

Drag, to

An anchor drags when it slides across the bottom and will not stay put. Try more Scope.

An anchor drags when it slides across the bottom and will not stay put. Try more Scope.


The depth of water a vessel draws, in other words the depth of water she needs to float.

Draw, to

A boat draws three feet when the lowest part of her hull, keel, rudder, etc., is three feet below the water surface. A sail draws when it is filled with wind and it pulls on the sheet.

Dress, to

A mast is dressed by attaching the standing and running rigging. Rig¬ging is dressed by coating it with linseed oil, lanolin or the like. A ship is dressed (overall) by rigging all her signal flags (bunting) in a line from stem to masthead and down to the stern. Sails used to be dressed with catechu or cutch, red ochre, linseed oil, potassium bichromate, and beeswax in various magic proportions to preserve the cotton against mildew and rot … but those days are past.


(1) The Nautical Institute of London confines this term to ‘the distance covered in a given time by the movement of a current (USA) or tidal stream (UK)’. The same Institute uses the word “set” for the direction of that movement. Most of us who navigate our own boats use drift to represent the combination of both those meanings – that’s to say the change of position due to stream or current. (Please also see Dead Reckoning.)

(2) A noun, meaning the distance between two blocks in a tackle or the amount of take-up available in a rope. As the blocks of a tackle approach drift gradually lessens and it is zero when they are Chock-a-block.

(3) (of stays) The linear displacement of the lower end of a stay, which sets it at an angle. For example, a lower shroud might have a drift of two feet, relative to the Cap shroud.


An especially lightweight headsail for winds of up to about Force one. Would be in cloth of 2 oz. (See Ghoster.)


Any device designed to drag astern so as to create a slowing or rearward pull. Used occasionally by motor vessels in heavy seas, and more often to cross a harbour bar. The rearward drag prevents the stern from being slewed round by a following sea. A more prosaic use of a drogue is to keep a dinghy from bumping the backside of her parent boat in the night. For this job the drogue usually takes the form of a bucket, which is more effective when streamed from the stem of the dinghy than from her stern.

Drying Mooring

A mooring where a boat is sometimes afloat and sometimes aground. When the tide leaves her, and she grounds, the boat is said to be ‘dried out’. Used in that sense the words simply mean that the water has left her. The same words may be used in their ordinary sense of the planks of a boat which has been ashore and under cover for a long time, so that the wood is thoroughly dry.


Digital Selective Calling. A compulsory system for new marine VHF systems by which short text-based messages can be broadcast in digital format. One of its major advantages is the ability to send a Mayday message via a single button press.

Duck Up

A real old seaman’s term this. Most of us would think of ducking down, but when an old salt lifts the corner of a sail, or raises a boom to clear his head, he ‘ducks it up’.

Duplex Working

Radio Telephone

Dutchman’s Log

A small chip of wood (or other buoyant material) which is cast over a bow, and whose passage down the hull is timed. If the length of the hull is known it is then possible to calculate the vessel’s speed.


Designed Water Line


See Aramid.


Danger Zone. For example, a practice firing range.