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Seafaring A – Z Alphabet – “T” is for…


Seafaring “T” is for…

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Tankers – All ships used to be “general cargo vessels”, they lifted whatever cargo was going and whether it was suitable of not, there was very little in the way of specialisation. Ships plied their trade wherever carried whatever was available.

Over time vessels became more sophisticated and evolved, so instead of a mixed hold full of all kinds of different commodities or goods, owners began to feel that specialisation was worthwhile as trade and the level of demand for vessels to perform particular tasks was worthwhile. The certainty of trade flows began to give the confidence to specialise, and so the move towards recognisable ship types began.

This was especially true with liquid cargoes, and specialised “tankers” have become one of the mainstays of shipping. Tankers play an important role in international trade with a share of over 33% of the world tonnage. They come in varied sizes ranging from handysize tankers to ultra large crude carriers (ULCC) with a deadweight tonnage ranging between 320,000 to 550,000.

Tankers carry cargoes such as crude oil, finished petroleum products, liquefied natural gas (LNG), chemicals, edible oils, wine, juice, and molasses. These were cargoes that for millennia, were carried in amphora, or pots, then casks and barrels. Then someone had the bright idea to carry the liquid without the weight and hassle of barrels or other vessels.

It was lighter, cheaper, faster and easier – and it became the norm. With the move to tanks filled with liquid, the need for specialised designs was born. A liquid cargo in a cask was in essence handled no different from bales of wool or cotton. Suddenly bulk liquids changed everything.

The 2700-ton “Gluckauf”, built in Newcastle upon Tyne in 1886, became the world’s first “true” oil tanker, with separate tanks for the oil built into her hull. The engines were placed right aft and she had a navigational bridge almost amidships. In essence she had all the traits of a modern tanker.

From 1886 onwards the tanker industry saw significant technological advancements. Vessels increased dramatically in size and carrying capacity and progressed from coal-fired engines via steam-turbines to diesel engines.

From the turn of the century onwards demand for oil rocketed. Initially tankers carried already refined products, as the trend was for refineries to be placed close to oil fields. Politics, wars and costs meant that refineries were soon built in importing nations, and so the need to ship crude oil took off.

In 1959 the 114,356 dwt “Universe Apollo” became the first tanker to pass the 100,000-ton figure: within a decade ships five times that size were being planned. The biggest oil tankers were built in the 1970s after the 1973 oil crisis. Thus a new generation of tanker was born, the VLCCs (Very Large Crude Carriers) and the ULCCs (Ultra Large Crude Carriers).



Training – Seafaring was, and perhaps still is, essentially an occupation where performance depended heavily on experience. Whether a seafarer was heading to be deck crew or to take command, the training and gaining of experience was the same.

Youngsters were accepted at about 15 years of age to be led, guided, and socialised, into the work culture of the sea. With each new experience or task they gained some new insight and climbed the metaphorical career and rank ladder.

When vessels began to travel further, the art, science and practice of ocean
navigation demanded a higher level of educational attainment. So there was a split from those who were more “hand on”, practical sailors – to those who needed to be able to manage complex calculations and learn nautical astronomy and navigation.

Until the nineteenth century vocational training in the practical work of ships, loosely grouped under the term “seamanship,” remained experiential but from the middle of that century, state-aided navigation schools, endowed nautical schools and private establishments began to provide some training in aspects of seamanship.

While some aspects could be learned at sea, to really hone the skills, techniques and science of navigation, knowledge had to be obtained from “mathematical practitioners” ashore.

These “professors”, offered their services in increasing numbers from the sixteenth century onwards, and eventually a more formal structure of nautical education through fee-paying schooling emerged.

These nautical schools were attended following a general education before going to sea (pre-sea) or between voyages (in-service) as a step in the ladder to command. This approach to maritime vocational education and training was continued in the twentieth century when nautical education was largely absorbed into the state tertiary education system.

It was also in the mid-nineteenth century that the idea of providing an industrial training in seamanship for youngsters in need of care, through shore establishments and static training ships, was developed. These were charitable establishments supported by voluntary subscription though the majority relied on government grants. A few were orphanages, most were industrial training ships and three were reformatories. Their product, a minority of those taking up a sea career, mostly went to sea as ratings. It took the 1914-18 war for the need for all seafarers to pass through a pre-sea training in seamanship to be recognised, and the 1939- 45 war for that aim to be achieved.

Finally, the past 150 years has seen the introduction of the new technologies of power propulsion and radio communication, of new manning groups aboard ship, engineers and radio operators, and of related educational provision ashore. The marine engineering and radio schools followed a similar development to the navigation schools: mostly initially private establishments, then drawn into the state tertiary system in the twentieth century.

The earliest structured form of maritime training, familiar to generations of mates and
masters well into the twentieth century, was that of the “seafarer apprenticeship” – and this took in time at college with a mix of seatime. Seafarers work towards gaining their Certificate of Competency, and the requirements are laid down within the International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW), 1978.

Away from the need to study at a shorebased college, the shipping industry has long embraced the concept of distance learning and computer based training. The world leader in this is Videotel, and the company has been developing training material since 1973 when the Intergovernmental Maritime Consultative Organization (IMCO), the forerunner to the International Maritime Organization (IMO), wanted to improve the quality of training materials available to seafarers and called on the company’s services. As an indication of how prevalent this form of training is, Videotel’s training solutions are in use on more than 12,000 vessels worldwide.



Tugs – A tug or “tugboat” is a vessel that is used to assist the movements of other larger vessels, by either pushing or towing them.

Tugs give a degree of control and added manoeuvrability to vessels which may perhaps have difficulty in a crowded harbour, river or a tight channel. Or they are used to move by themselves, such as barges, disabled ships, log rafts, or oil platforms.

Powerful for their size and strongly built, some tugs are even ocean-going and can be found serving as icebreakers or salvage boats, sometimes even used as “guard” vessels for offshore construction, pipe or cable laying.

Tugboat engines typically produce 500 to 2,500 kW (~ 680 to 3,400 hp), but larger boats (used in deep waters) can have power ratings up to 20,000 kW (~ 27,200 hp) and usually have an extreme “power: tonnage-ratio”. They are built to be powerful and agile too.

A tugboat’s power is typically stated by its engine’s horsepower and its overall bollard pull. The largest commercial harbour tugboats in the 2000s-2010s, used for towing container ships or similar, had around 60-65 tons of bollard pull, which is described as 15 tons above “normal” tugboats.

Early tugboats had steam engines, but today most have diesel engines. They also usually have a firefighting role and capability, and many have fire monitors, allowing them to assist in the event of fire, especially in harbours.

Tugboats are highly manoeuvrable, and various propulsion systems have been developed to increase this capability and increase safety. This makes them ideal for their role as providing thrust for other vessels.

They are not limited to harbour work. In the USA and mainland Europe, rivers such as the Mississippi, Danube and Rhine often see the employment of “river tugs” – these act as towboats or pushboats – able to move large barges or lighters. Their hulls often feature a flat front or bow to line up with the rectangular stern of the barge, often with large pushing knees.

While out at sea, large deep sea tugs provide vitally important salvage capabilities. They are often used to rescue vessels or to remove them from hazardous conditions, and dangers.

When in a port, there are strict relationships between the tug and the vessel being towed. These are the United Kingdom Standard Towage Conditions (the UKSTC) – and are used in many other countries. They date back to 1933 when UK tug owners first adopted a set of “national” conditions in a standard form contract. The UKSTC were drawn up to protect tug owners and exclude any possible liability which the tug may have towards the tow.


Ship carrying cargo

Tonnage – Tonnage is a measure of the cargo-carrying capacity of a ship. The term derives from tonnage was the tax on tuns (casks) of wine that held 954 litres (252 gallons) of wine and weighed 1016 kilograms (2,240 pounds).

The confusion between weight-based terms (deadweight and displacement) stems has long been problematic, and in the 19th Century the “Moorsom System” was introduced, which calculated internal volume, not weight. This system evolved into the current set of internationally accepted rules and regulations.

The size and weight of vessels are key to many calculations, and the applications of Tonnage Measurement are many and varied and are used in the assessment of costs and also statutory requirement.

Harbour and Pilotage Dues are usually based on either Gross or Net Tonnage. While Light and Canal Dues, though there are other sundry costs such as Agency, Towage, Dry Docking, P&I, Registration and Statutory Surveys, as well as the criterion for the application of IMO Conventions, e.g. SOLAS, MARPOL, STCW.

Under the IMO International Convention on Tonnage Measurement of Ships (1969), “Gross Tonnage” applies to vessels, not to cargo. While “Net Tonnage” is a vessel’s gross tonnage minus deductions of space occupied by accommodations for crew, by machinery, for navigation, by the engine room and fuel.

There are other measurements too –

• Displacement Ton – this is the unit for the total weight of a ship and her contents – this is the weight of water displaced, under any particular condition of loading

• Lightweight Ton – is the unit for the fixed weight of the empty as-built ship. The ‘Lightweight Tonnage’ is the weight commonly used as the basis for determining the scrap value of ships.

• Deadweight Ton – is the unit for the variable weight of the total contents of a ship under any particular condition of loading. It is the difference between the Displacement Tons and the Lightweight Tons of the ship. The ship’s ‘Deadweight Tonnage’ is typically quoted as being the maximum deadweight applicable under the International Load Line Regulations when floating at her Summer Load Line draught.

• Deadweight Cargo Capacity Tonnage – is the Deadweight Tonnage less bunkers, water and constant weights.

The major canals have their own calculations too:

• Panama Canal – “Panama Canal Tonnage” is comparable to the tonnage as determined by the IMO International Convention on Tonnage Measurement of Ships (1969). The tonnages stated on the Panama Canal Tonnage Certificate are therefore identical to those quoted on the International Tonnage Certificate (1969).

• Suez Canal – “Suez Canal Tonnage” is different from all other tonnage remaining based on the old Moorsom System of tonnage measurement, i.e. with gross and net tons being equivalent to 100 cubic feet or 2.83 cubic metres. The tonnages stated on the Suez Canal Special Tonnage Certificate are different from those quoted on the International Tonnage Certificate (1969).

Territorial Waters – In 1958, the United Nations proposed a Convention on the High Seas, often referred to as the “Geneva Convention on the High Seas” which examined the law of the sea, and the technical, biological, economic and political aspects of issues relating to control and management of the seas.

Work began, but it took until 1982 to bring everything together in one convention – this was the United Nations Convention on the Law of the Sea, also known as UNCLOS.

Amongst many other issues, it set about defining what the distinct areas of the seas are in relation to States – so laid out the concept of International waters, or “High Seas”. Which are in contrast with internal waters, territorial waters and exclusive economic zones.

These territorial waters or a territorial sea are coastal waters extending at most 12 nautical miles from the baseline (usually the mean low-water mark) of a coastal state. The territorial sea is regarded as the sovereign territory of the state, although foreign ships (both military and civilian) are allowed innocent passage through it; this sovereignty also extends to the airspace over and seabed below.

Historically, the concept of territorial waters saw two different concepts developed—one held that the area of jurisdiction should be limited to cannon-shot range, the other that the area should be a much greater belt of uniform width adjacent to the coast. In the late 18th century these concepts saw a compromise that proposed a fixed limit of 3 nautical miles.

In 1793 the United States adopted three miles for neutrality purposes, but although many other maritime states during the 19th century came to recognise the same limit, it never won universal acceptance – and now 12 miles is considered the limit of territorial waters.

Working from the baseline, moving back towards the land mass there are internal waters. While moving outwards, there is the State’s territorial sea extends up to 12 nautical miles (22.2 km; 13.8 mi) from its baseline.

Closely related to the issue of territorial control, there is a buffer between territorial waters and the high seas, this is the “contiguous zone”, an area of water extending from the outer edge of the territorial sea to up to 24 nautical miles from the baseline.

In the contiguous zone the State can exert limited control for the purpose of preventing or punishing “infringement of its customs, fiscal, immigration or sanitary laws and regulations within its territory or territorial sea”. In essence it means that any perpetrators can be pursuit, with the right of hot pursuit. Beyond that 24 miles comes the “Exclusive economic zone” – the EEZ extends from the outer limit of the territorial sea to a maximum of 200 nautical miles and actually includes the contiguous zone.

A coastal nation has control of all economic resources within its exclusive economic zone, including fishing, mining, oil exploration, and any pollution of those resources. However, it cannot stop passage or loitering above, on, or under the surface of the sea that is in compliance with UN Convention. The EEZ is subject to a specific legal regime – and while the State can control the exploring and exploiting, conserving and managing of natural resources, other States can use the waters with the same freedoms of international waters.

True north

True north

True North – True north (geodetic north) is the direction of a meridian of longitude which converges on the North Pole. True geodetic north differs from magnetic north and the horizontal angular difference is called “magnetic variation” or “declination”.

Compass roses on marine charts show two graduated scales. One is referenced to True North, while the other is referenced to Magnetic North. Gyro compasses used aboard vessels yield “True” headings, while magnetic compasses will yield magnetic headings.

If a magnetic compass is in use, mariners must be able to convert back and forth between true and magnetic headings. They accomplish this by either adding or subtracting the magnetic variation—which is the angle between magnetic and geographic meridians.

Magnetic variation is given in degrees and minutes, with an easterly or westerly component. If converting from magnetic headings to true headings, a west variation is subtracted from the magnetic to obtain true. An easterly variation would be added.
Variation. This is the difference, in degrees, between true and magnetic. Variation can be east or west. On the above compass rose we can calculate the variation visually by drawing a straight line, starting from the centre then going through the inner rose and continuing across the outer rose. The difference between the two is variation.

Converting between true, magnetic, and compass is necessary when navigating, and bearings and headings need to be converted from true to magnetic or magnetic to compass. For instance, the set of a current is always given in degrees true.

The easiest way to convert between true, magnetic, and compass is mathematically by adding or subtracting the appropriate amount. The rule to follow is when going down the list of true to magnetic or magnetic to compass we add westerly variation or deviation and subtract any easterly variation or deviation.

While when going up, or in the opposite direction, as in calculating compass to magnetic or magnetic to true, subtract westerly and add easterly.

An easy way to memorise the formula is to come up with a mnemonic that includes the letters T, V, M, D, C, A, W. A popular one which cadets seem to remember easily is: “True Virgins Make Dull Companions, Add Whiskey”.



Tropics – The tropics are a region of the Earth surrounding the equator. In the North, the Tropic of Cancer is the circle marking the latitude 23.5 degrees north, where the sun is directly overhead at noon on June 21, the beginning of summer in the northern hemisphere.

To the south, the Tropic of Capricorn is the circle marking the latitude 23.5 degrees south where the sun is directly overhead at noon on December 21, the beginning of winter in the northern hemisphere.

When the lines were named 2000 years ago, the Sun was in the constellation of Capricorn during the winter solstice and Cancer during the summer solstice (hence the names). Now due to the precession of the equinoxes the Sun is no longer in these constellations during these times, but the names remain.

The word “tropic” itself comes from the Greek “trope”, meaning turn (change of direction, or circumstances), inclination, referring to the fact that the sun appears to “turn back” at the solstices.

The tropics or “tropical zone” lies roughly in the middle of the globe and include the Equator and parts of North America, South America, Africa, Asia, and Australia. The tropics account for 36 percent of the Earth’s landmass and are home to about a third of the world’s people.

The tropics are warm all year, averaging 25 to 28 degrees Celsius (77 to 82 degrees Fahrenheit) owing to exposure to the sun. This also means the tropical seasons are broken up into just two: the wet and the dry season.

The heat experienced in the tropics drives weather patterns, and there can be severe problems for seafarers, in the form of tropical cyclones. These are rapidly rotating storm system characterised by a low-pressure centre, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain.

Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane or, typhoon, tropical storm, cyclonic storm, tropical depression. They are often simply called cyclones, which refers to their cyclonic nature, with wind blowing counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis Effect.

Tropical refers to the geographical origin of these systems, which form almost exclusively over tropical seas. Tropical cyclones typically form over large bodies of relatively warm water and derive their energy through the evaporation of water from the ocean surface, which ultimately re-condenses into clouds and rain when moist air rises and cools to saturation.