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Seafaring A-Z Alphabet – D is for…


Over the past year we have looked at the seafarer welfare issues and challenges affecting those at sea today. We’re building an A-Z of the most important words and phrases in seafaring and those which are having the greatest impact – send us your own suggestions for entry into the Crewtoo A-Z.

Let’s get this rolling with:

Davy Jones

Davy Jones

Davy Jones – The infamous Davy Jones’ locker is an idiom often used by seafarers referring to the bottom of the sea. It is used as a euphemism for drowning or shipwrecks in which seafarers and unfortunate vessels are consigned to the deep, and sent to Davy Jones’ Locker.

The earliest known reference occurs in the Four Years Voyages of Capt. George Roberts, by the author Daniel Defoe. Published in 1726 in London, and which captures the fearful nature of this most terrifying of fates was formed and first captured the imagination.

The origins of the name of Davy Jones, as the sailors’ devil, are unconfirmed – with some early references calling Davy Jones the “ghost of Jonah”. Other explanations of this nautical superstition have been put forth, including an incompetent sailor or a renowned pub owner who kidnapped sailors and placed them in a locker before forcing them to sea.

While the name has long been in fairly common usage it re-emerged into the public consciousness when Davy Jones was tied up with the legend of the Flying Dutchman in the Pirates of the Caribbean film series. Davy Jones’s locker was portrayed as a sort of limbo, in which the lost souls of seafarers drowned at sea were ferried to the afterlife.

Dead reckoning

Dead reckoning

Dead Reckoning – In essence “dead reckoning” (DR) is like taking an educated guess at the vessels position. It is based on knowing where you started, and assuming you know the direction and the speed, then after a certain amount of time you should be able to work out where your current position is.

The real definition is that it is the determination of position without the aid of celestial navigation from the record of the courses sailed, the distance made, the known starting point, and the known or estimated drift.

The dead reckoned position can sometimes be supplemented with an “estimated position” – which works on similar lines, but which is corrected for effects of current, wind, and other factors.

The longer the time period the more uncertainty with the dead reckoning position creeps in. Navigators have to be very careful when using this approach, as the potential for errors and uncertainty is high.

It is important to plot the vessel’s DR position:

  • At least every hour on the hour.
  • After every change of course or speed.
  • After every fix or running fix.
  • After plotting a single line of position

While navigation is ever more technical and sophisticated a properly maintained DR plot can be important, as it can also allow the navigator to examine a future position in relation to a planned track. It allows the OOW to anticipate charted hazards and plan appropriate action to avoid them.


Dead weight


Deadweight – Deadweight tonnage (DWT) is the unit of measure of how much mass a ship is carrying or can safely carry so as to settle at the correct loadline marks or “plimsoll line”.

It is the capacity of the vessel, and so doesn’t reference or include the weight of the ship just what it can carry. DWT is therefore the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew. Whatever is carried affects the deadweight of the ship.

Deadweight tonnage is a good indication for ship owners and charterers of how much revenue the vessel is capable of generating – as it indicates how much cargo can be carried. There is another important measurement which sits alongside this figure, and that is the “lightweight” tonnage. Lightweight tonnage is the weight of the ship, including machinery and equipment.

The deadweight should never be confused with displacement. Displacement is the weight of water that a ship displaces, and so includes the vessel itself. Displacement is the sum of the deadweight and lightweight.




Distress – Whenever a ship is in distress there are both obligations and procedures to be adhered to. The Safety of Life at Sea (SOLAS) Convention, which was brought about after the loss of the Titanic, contains details of who should do what and how.

Masters of vessels that receive or are notified of a ship in distress are obliged to respond to the information. Ships can be requisitioned by the master of a ship in distress or the search and rescue authorities. Once rescued persons are to be treated humanely and delivered to a place of safety.

When a vessel is in trouble it will issue a distress signal or distress call. This is an internationally recognised means for obtaining help and indicating that something is wrong.

Distress signals are commonly made by using radio signals, displaying a visually detected item or illumination, or making an audible sound, from a distance. These are defined in the International Regulations for Preventing Collisions at Sea (COLREGS) and in the International Code of Signals.

They are commonly referred to as “Mayday signals”, and must only be used where there is grave and imminent danger to life. Other levels of signals exists for lesser problems or emergencies.

Distress can be indicated by any of the following officially sanctioned methods:

  • Smoke signal
  • Transmitting a spoken voice Mayday message by radio
  • Transmitting a digital distress signal
  • Transmitting a digital distress signal by activating (or pressing) the distress button (or key) on an Inmarsat-C satellite internet device
  • Sending the Morse code group SOS by light flashes or sounds
  • Burning a red flare
  • Lighting a non-pyrotechnic visual distress signal device
  • Emitting orange smoke from a canister
  • Showing flames on the vessel (as from a burning tar barrel, oil barrel, etc.)
  • Raising and lowering slowly and repeatedly both arms outstretched to each side
  • Making a continuous sound with any fog-signalling apparatus
  • Firing a gun or other explosive signal at intervals of about a minute
  • Flying the international maritime signal flags “N” November and “C” Charlie
  • Displaying a visual signal consisting of a square flag having above or below it a ball or anything resembling a ball (round or circular in appearance)
  • Launching distress rockets
  • Using automated radio signals such as a Search and Rescue Transponder (SART) or an Emergency Position-Indicating Radio Beacon (EPIRB).




Dock – With so many docks all over the world it perhaps seems odd to think that there was a point when ships operated without them. But there was a time when ships would sit on the mud, or would just anchor off and cargo was shifted by small boats.

As with so many things, it was the Ancient Egyptians who seemed to get there first. The earliest known docks were those discovered in Wadi al-Jarf, an ancient Egyptian harbor dating from 2500 BC located on the Red Sea coast.

While only slightly latter, a dock from Lothal in India dates from 2400 BC and was even cunningly located away from the main current to avoid deposition of silt. This was the earliest known dock equipped to both berth and service ships, and the dock walls were actually constructed from kiln-burnt bricks.

It seems our ancestors were a shrewd, wise and canny lot – they understood tides, river flows and the need to have somewhere purpose built to offload cargo.

For the centuries hence, it seems the old ways were the best – and there were no real major developments. Dock walls were on the river or sea front and the vessels tied straight up.

It was the rapid industrialisation and development of cities which meant that things had to change – and as vessel sizes increased, it was also important they could be kept safe, stable and secure while loading or discharging.

The next leap forward came with the concept of a “wet dock” – and cutting away from the river or sea, into which vessels could enter through a lock and cargo operations could be conducted.

The advent of the wet dock or impounded dock was development in which the water is impounded either by dock gates or by a lock, thus allowing ships to remain afloat at low tide in places with high tidal ranges. The level of water in the dock is maintained despite the raising and lowering of the tide. This makes transfer of cargo easier and more predictable.

The world’s first enclosed wet dock with lock gates to maintain a constant water level irrespective of tidal conditions was the Howland Great Dock on the River Thames, built in 1703. The dock was merely a haven surrounded by trees, with no unloading facilities. So the accolade of being the world’s first commercial enclosed wet dock, with quays and unloading warehouses, goes to the Old Dock at Liverpool, built in 1715 , which held up to 100 ships. The dock reduced ship waiting, giving quick turnarounds, greatly improving the throughput of cargo – and helping to make Liverpool one of the world’s most important port cities.

Gradually, docks began to specialise as bulk cargoes and oil demanded their own facilities for handling the cargo but it was the advent of containers which shaped the modern dock landscape.

With so many boxes being lifted, loaded and discharged, the dock areas needed far greater space, or “hard” to store containers. They needed bigger, faster and more efficient cranes. They also needed to accommodate bigger vessels.

In a 300 year turnaround, the largest container vessels are now so big that dock innovations focus on how they can be kept on the river or waterfront. The idea of locking the box behemoths of today in and out it too much of a challenge, and so once again innovators have had to find ways of facilitating cargo work.

Yet again Liverpool has been at the forefront of change – newly built, and just about to receive its first vessels, Liverpool Two container berth is actually constructed on the traditional waterside, dockfront model. The major difference is in the use of dredging to create a “docking pocket” into which the huge vessel can sit when the tide goes out.


Double hull

Double hull

Double Hull – Traditionally, vessels were built with a single hull – but with demands for safer and cleaner ships there was a shift to creating a “double bottom” effect. This is where the ship hull design and construction incorporates two complete layers of watertight hull surface.

The outer layer forms what can be considered the normal hull of the ship, however, the double bottom provides a second inner hull which is somewhat higher in the ship. This is a far from new concept, back in 1850s the SS Great Britain had a double bottom.

Progress being what it is, it was soon apparent that rather than just a doubling of the bottom, that an entire double hull could and should be created.

This forms a redundant barrier to seawater in case the outer hull is damaged and leaks. Naturally the concept is that two hulls are better than one. Indeed in some cases where vessels have sustained hull damage when running aground then the double hull has prevented either sinking and/or pollution.

A double hull provides two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship, and a second inner hull which is some distance inboard, typically by a few feet, which forms a redundant barrier to seawater in case the outer hull is damaged and leaks.

In low-energy collisions, double hulls can prevent flooding beyond the penetrated compartment. In high-energy collisions, however, the distance to the inner hull is not sufficient and the inner compartment is penetrated as well.

Would the Titanic have been saved if she had a double hull? Well, perhaps – it would certainly been less likely to flood to the extent it did. In more modern, times perhaps the Costa Concordia gives a better indication – sadly it seems that no amount of engineering and design can really stand up to incompetence or hubris. If ships skirt with rocks or icebergs, there is usually like to be only one winner – regardless of how many hulls the vessel has.

Perhaps we will soon see triple hull ships as the norm? Some could consider LNG tankers as triple hulls of sorts, as the gas is enclosed in giant tanks within the existing double hulls. At the moment, triple hulls are the preserve of navy vessels, but if there were a major oil spill from a double hull tanker, then pressure would build as it so often does.

In a scene from the popular cartoon Futurama, the idea of multiple hulls is mocked when an announcement is made of the breaching of a 6,000 hull tanker. “Oh, the fools! If only they’d built it with 6,001 hulls! When will they learn?” come the plaintive cries…when indeed…?


Dynamic Positioning

Dynamic Positioning

Dynamic Positioning – Just over 50 years ago work began on a project to control ships using thrusters and engines – a way of making a ship follow a track or stay in position. This was the birth of dynamic positioning or DP.

DP use is as varied as the vessels it is installed upon, and the technology has found its way into all aspects of the marine industry. Mining, wind farms, cable laying, drilling, oil rig supply and support, cruise ship and superyacht port visits, and many more uses beside are all made faster, safer and more effective with the use of DP.

The genesis of DP was the need to be able to drill down into deeper water for oil. The traditional jack up rigs of the 1950s in the Gulf of Mexico were reaching the limit of their usefulness – so another answer was needed.

There are also rumours that the US military had been perfecting a means of keeping vessels stationary as they sought to recover a lost Russian submarine deep in the Pacific.

Using technology pioneered by the reclusive billionaire Howard Hughes, the vessel “Glomar Explorer” (T-AG-193), was a deep-sea drillship platform initially built for the United States Central Intelligence Agency Special Activities Division secret operation “Project Azorian” to recover the sunken Soviet submarine K-129.

DP soon began to show its worth, allowing the advent of deepwater offshore drilling, and a new era of exploration.  When Shell launched the Eureka, the vessel was the most astonishing technological leap forward.

Eureka was equipped with thrusters fore and aft, capable of rotating through 360 degrees and was the first true DP vessel with many of the characteristics which the most modern vessels have today.

The DP system is controlled and monitored by a DP Operator – in most instances a certified OOW who is able to set up the system, and make sure that the vessel is doing as it should be, while also anticipating any problems which may arise.

A key aspect of DP is the inputs and position referencing it uses – satellite navigation has grown hand in hand with the boom of DP, and not surprisingly the growth of GPS and the technological advances have paved the way for growth and for ships to do ever more, with ever more certainty and assurance.

At its most basic level DP is about countering forces within six degrees of freedom that a ship experiences. A ship is free to change position as forward/backward (surge), up/down (heave), left/right (sway), and these movements are about three perpendicular axes, often termed pitch, yaw, and roll.

By constantly checking on the vessel’s movement, the DP system can measure each movement and counter it – while trying to achieve the goals set by the DP Operator.

Today, ships are able to perform incredible feats of construction and engineering at sea because of the DP systems they are fitted with. They sense the inputs from their surroundings, and are able to build models of movement which allow them to counteract them. Enabling DP Operators to use the system to perform intricate and sensitive tasks which would be almost impossible without DP.