2.1 Types of Vessels

Generally speaking a vessel can be classified according to the purpose she serves.

The most common purposes are:

1) transportation of cargo or/and passengers;

2) assistance and Service (by so called “special purpose vessels”);

3) the catching of fish (fishermen);

4) peace keeping (warships).

Furthermore we can make the distinction between sea-going vessels and vessels for Coastal trade and/or inland waterways.

 

1) Transportation

Liners and Tramps

Vessels that have been designed to transport cargo or/and passengers are called merchant ships. They may be classified as liners or tramps.

A liner carries cargoes between two fixed destinations.

Her sailing schedule has been prearranged - she has a fixed homeport, port of destination and port(s) of call, and fixed ETA’s and ETD’s (Estimated Times of Arrival and Estimated Times of Departure). A liner-vessel is allowed to carry up to 12 passengers.

Freighters that carry cargoes according to schedules that are not fixed are called tramps.

Homeports, ports of destination, ports of call, ETA’s and ETD’s differ with every voyage.

Merchant ships may carry general cargoes, bulk cargoes, refrigerated cargoes, heavy cargoes, timber, and many more.

General Cargo Ships

General cargo is cargo that has been packed in crates, boxes or bags, or cargo coming in pieces (unpacked cargo items).

Cargo is loaded and discharged by the vessel’s own derricks or by shore based cranes.

The conventional general cargo ship has several tweendecks, so that the cargoes for the various destinations can be reached and discharged without having to remove cargo for other destinations first.

Under the influence of cargo-palletization the “open freighter” was fitted with two or even three hatches side by side.

Both types of vessels, the general cargo ship and the open freighter, are becoming rather obsolete, since general cargo is more and more transported by vessels that have been designed to carry general cargo in containers.

Watch the video clips below.

https://youtu.be/dSdcuCSmAsY

Bulk Carriers

Bulk cargo is unpacked cargo of one commodity.

Dry bulk cargo, such as grain, ore, fertilizers, etc. is carried in specially designed vessels with holds that have been divided into compartments by longitudinal and transverse separations, so that the ship's stability will not be affected by a full cargo.

The holds of these ships are often constructed in such a way that they are self-trimming; this means that the surface of the cargo is constantly made equal by special pumps in rolling circumstances or when the vessel has a list.

Dry bulk cargo is loaded and discharged by cranes with grabs or by pumps.

Liquid cargoes such as crude oil, petroleum, edible oils, etc. are carried in tankers, for example in Very Large Crude Carriers (VLCC's), Chemical tankers, such as Liquefied Petroleum Gas tankers (LPG carriers) or Liquefied Natural Gas tankers (LNG carriers).

Product tankers are small tankers that carry different sorts of oils.

For safety reasons tankers must be fitted with double bottoms. These spaces also provide storage for fuel, lubricating oil and waters.

Tankers are divided into compartments by longitudinal and transverse bulkheads.

Cofferdams are empty spaces between the tanks and in the double bottom. They serve as separations to prevent liquids from leaking from one tank into the other.

Cofferdams often serve as pump-rooms. Pumps for loading and discharging the cargo may be installed in these compartments. Tankers are often loaded and discharged in the offing by means of flexible pipes. This system of wet bulk handling reduces the number of lay days.

Container ships

Cargo that has been containerized is carried by Container ships.

Containers are most often measured in Twenty Feet Equivalent Units (TEU’s) and are stowed in a cellular arrangement in Rows, Bays, and Tiers.

The rows run abeam, or athwartship; the bays run fore to aft, and the tiers are horizontal layers. The three- figure code on each Container refers to this stowage system. Thus, each Container can easily be found.

Container ships are sometimes equipped with their own gantry cranes that load and discharge the containers. Container ships may carry general cargoes, liquid cargoes or refrigerated cargoes.

The advantages of carrying cargo in containers are: short lay time because of efficient and rapid cargo handling; few stevedores are required; less pilferage because the cargo has been stored in locked containers.

Roll-on / Roll-off ships (Ro / Ro ships)

On a Ro / Ro ship cargo is rolled on and rolled off by lorries or trailers.

The great advantage of this system is that no cargo handling equipment is required.

The loaded vehicles are driven aboard via ramps through special stern and bow doors and are properly secured for the passage. Upon arrival in the port of discharge, the vehicles are released and driven ashore to their destinations.

Coasters

A coaster carries cargo along the coast or on sea- voyages. Trans-Atlantic voyages are quite common.

A coaster is of limited length and tonnage.

Her engine room is situated aft. Often there are no tween decks, and the cargo spaces have no obstacles, so that a variety of cargo can be handled.

Hatches are very broad and cover most of the main deck surface. Because of her limited length she will hardly experience any problems related to longitudinal stresses. However, due to the broad hatchways, transverse strengthening are necessary to avoid difficulties caused by transverse stresses.

Refrigerated-cargo vessels (Reefers)

Refrigerated-cargo vessels are ships that carry perishable cargoes, such as meat or fruit. These cargoes require cooling and must be stored in spaces that have precise temperature- and humidity Controls during the voyage.

Reefers, as these ships are also called, are equipped with refrigerating plants.

Lash-vessels

“Lash”stands for “Lighter Aboard Ship”. A Lash-vessel has a main deck that is flat and without any obstacles. A lighter is a Container that floats in the water. The containers may be hoisted on board by the vessel’s own heavy derricks that stack them on board.

Another way of loading the containers on board is by submerging the vessel first (for this she must be equipped with a powerful pumping-system), then have tugs or push boats tow or push the lighters over the Lash-vessel, after which the vessel will emerge again and will “pick up” the lighters. This type of vessel is also referred to as a “Seabee”.

Heavy-load vessel

Heavy-load vessels have been designed to lift and carry extremely heavy cargo on the main deck.

Their most prominent features are very heavy derricks (“booms”), masts and lifting-blocks.

Their cargoes, such as drilling platforms, engines, yachts, trains, derelicts and wrecks, are loaded onto the main deck, which is flat and free from any obstacles.

A special way of loading and carrying heavy cargo is performed by submerging the ship and have the cargo- module float over it. She must be equipped with a powerful pumping-system.

After pumping the ship empty the vessel will emerge again and will pick up the cargo.

Timber Carriers

Timber is a raw material from which wood-products are manufactured.

Vessels that carry timber can easily be recognized by their tall derricks.

A timber carrier has been designed in such a way that she can carry a tall deck cargo.

Her Plimsoll Mark is provided with a special timber Load-line that indicates the maximum draft to which she is allowed to be loaded under certain circumstances and in different seasons.

Multi-Purpose Vessels

Cargo ships that carry both general cargo, bulk cargo and containerized cargo are called multi-purpose (or multi-loads) vessels. These ships are equipped with a variety of cargo handling gears to load and discharge the different types of cargoes.

An OBO-ship has been designed to carry oil/bulk/ore. She has been subdivided in such a way that oil can be carried in the largest compartments and ore can be carried in the smaller compartments.

Passenger Ships

Passenger ships, such as cross-Channel ferries, have been designed to carry passengers and their vehicles on a prearranged route.

Their main features are more or less the same as the features of the Ro/Ro vessels.

Cruise ships have been especially designed to carry holidaymakers.

2) Assistance and Service

Vessels that render assistance and Service have been designed to perform specific tasks, for example assisting other vessels, or providing special Services to navigation.

A tug is a vessel that assists other vessels with entering or leaving the port, tows an oil rig to its position or assists with a salvage operation.

There are sea-going tugs and harbour tugs.

Their engines must be capable to develop enormous powers.

The largest and most powerful tugs are often fitted with Controllable Pitch Propellers (C.P.P.) that have adjustable blades.

Their manoeuvrability will be enhanced even more by bow thrusters and stern thrusters.

One of the main features is that the aft deck of a towing interfere with the towing-line.

A salvage vessel is a vessel that rescues other ships and their cargoes from loss at sea.

She must be equipped with heavy derricks to lift wrecks from the seabed.

A buoyage vessel places and maintains buoys. Her aft deck is flat and provides room to carry or haul in the buoys with her hoisting installation.

A survey vessel performs marine research. She is equipped with oceanographic instruments to carry out all kinds of measurements and assessments.

A supply boat supplies oil rigs with Stores, spare parts and supplies for domestic use.

Her aft deck must be flat.

Additional duties may include the towing of rigs and extinguishing fires, for which they must be equipped with high-capacity fire-extinguishing pumps.

A SAR-vessel performs Search and Rescue when a ship is in distress. She must be capable to develop high speeds and must be equipped with the most modem communication equipment to maintain contact with Rescue Co-ordination Centres (RCC).

A Firefloat is a fire fighting vessel. She must have a powerful fire-extinguishing system on board.

A pilot tender (or pilot launch) is a small boat that may be launched from the pilot boat.

The pilot will embark the ship that has requested pilotage from the pilot tender. She is often fitted with a sheltered aft deck to prevent the pilot from getting wet.

A cable layer lays cables on the bottom of the sea. She is fitted with a huge horizontal wheel that reels off the cable.

This type of ship is often equipped with a Dynamic

Positioning System to keep her in the exact position when the submarine cable is reeled off.

A lightship serves as a beacon for navigation and is anchored in the vicinity of crowded channels or seaways. She is usually not self-propelled, which means that she has to be towed to her position.

Icebreakers are designed to ride up the ice and crush a way through for other ships to follow.

This requires a powerful engine and a considerable strengthening of her stem.

A dredger deepens out harbours and ports, fairways, approaches and entrances, inland waterways, anchorages, roadsteads, etc. Spoil is discharged into an integrated hopper or into a hopper that is moored alongside. To keep her in position she is often spudded.

There are bucket dredgers, grab dredgers (or backhoe

dredgers) and suction dredgers with drag heads. A cutterhead is used to disintegrate rocky bottoms.

3) Fisherman

Large and modem fishing vessels (fishermen) are capable of catching and processing enormous quantities of fish. They may be underway for weeks before they return to their homeports.

Large refrigerating plants on board provide deep- freezing facilities. Before the fish are deep-frozen they are often stored in RSW-tanks (Refrigerated Sea Water Tanks). Powerful winches are used for hoisting the nets on board.

4) Man-of-war

Peace-keeping vessels (warships / men-of-war) are measured by weight, contrary to merchant ships, which are measured by volume. Warships must be capable to develop speeds of 25 knots and more, and must therefore be equipped with a high-power propulsion plant.