The Hull

The hull is the main body of the ship below the main outside deck. The hull consists of an outside covering (or skin) and an inside framework to which the skin is secured. The skin and framework are usually made of steel and secured by welding. However, there may still be some areas where rivets are used. The steel skin may also be called shell plating.

Marine vehicles are categorized as either displacement craft or dynamic-lift craft. Displacement craft derive their lift from buoyancy (displacement). Dynamic-lift craft derive their lift dynamically, such as by hydrofoils or planing surfaces. The drag of displacement craft is primarily frictional and wavemaking. The drag of dynamic-lift craft is primarily frictional and induced (drag induced by lift). In high-speed craft of either type, frictional drag is normally more than half the total drag. It is important to reduce frictional drag, although all types of drag are reduced to achieve a large lift-to-drag (L/D) ratio. The following are dynamic-lift: hydrofoil ships, air cushion vehicles (ACV), seaplanes, wing-in-ground effect (WIG) craft, planing hydrofoil ships, surface effect ships (SES) and ram wing craft. Displacement craft include: slender monohull ships, catamaran ships, SWATH (Small Waterplane Area Twin Hull) and displacement hydrofoil ships.

To achieve high speeds, i.e. speeds greater than 30 knots, various types of hull have been developed, some making use solely of hydrostatic support like monohulls but by increasing the number of hulls, as with catamarans and trimarans in particular; hydrodynamic support has also been used by means of underwater "wings" which serve to lift the load-carrying hull out of the water above a certain speed; other principles have been developed, in particular those using aerostatic support, such as air cushion vehicles.

Vessels constructed with two, three, or more hulls are referred to as catamarans, trimarans, or generally as multihull vessels. Multihull vessels have the advantage of more lateral stability than a monohull vessel, but with a wetted surface area that is normally higher than that of a monohull vessel of similar size.

The term "tumblehome" (alternatively spelled "tumble home" or "tumble-home") refers to a property of a water vessel's hull wherein its topside, or some portion thereof, is inclined inward. Generally according to conventional tumblehome-type monohull designs, the two sides of the vessel are sloped, above the maximum beam (widest width or breadth), toward the middle or center of the vessel.

Monohull tumblehome hull designs typically are beset with certain undesirable characteristics. They have a poor righting arm, since the tumblehome feature decreases the water plane area as the ship heels (tilts to one side; lists). They have poor damage stability, because the above-water volume is limited by the tumblehome feature. They have restricted useable superstructure volume and width at the deck level. They are characterized by unusual sea-keeping responsiveness along with ongoing concern regarding the dynamic stability in a seafaring way; this is probably attributable to the broad transom (which has large above-water volume associated therewith) in combination with the wave-piercing bow (which has limited above-water volume associated therewith).

The organisms that generally foul vessel hulls are the typical species found in natural marine intertidal and subtidal fouling communities. Typical invertebrate phyla associated with marine fouling communities are arthropoda (barnacles, amphipods, and crabs), mollusca (mussels, clams, and sea slugs), porifera (sponges), bryozoa (moss animals), coelenterata (hydroids and anemones), protozoa, annelida (marine worms), and chordata (sea squirts and fish), as well as macroalgae (seaweed). Fouling organisms tend to concentrate in sheltered areas of the hull, such as sea chest intakes and rudder posts, and develop in areas where anti-fouling coatings have been compromised (Ranier 1995; Coutts 1999; Godwin personal observation). Anti-fouling coatings wear off along the bilge keel and weld seams, and are inadequately applied in some cases, which makes the surfaces susceptible to settlement by fouling organisms.

If the coatings applied to the hulls of modern commercial vessels are maintained, they act as a deterrent to the settlement of marine organisms on vessel surfaces below the water line. Studies have shown that there are areas on the hull where the coatings are compromised, thus allowing settlement of marine fouling organisms. Fouling organisms have also been noted to exist in sheltered areas around rudder posts, and within sea chest intakes. Sea chest intakes tend to harbor a diverse community that is sheltered from the turbulence created by movement through the water. Even properly maintained vessels can transport fouling organisms when these factors exist. Slow moving vessels that have long residence times in port are more likely to develop fouling organisms, than those that have short residence times, and are transiting more often. Towed vessels, such as, overseas cargo barges, floating dry docks, vessels from decommission yards, or any floating platform, are examples of this type of dynamic.

The main centerline structural part of the hull is the keel, which runs from the stem at the bow to the sternpost at the stern. The keel is the backbone of the ship. To the keel are fastened the frames, which run athwartship. These are the ribs of the ship and gives shape and strength to the hull. Deck beams and bulkheads support the decks and gives added strength to resist the pressure of the water on the sides of the hull.

Construction of a Hull


The skin, or shell plating, provides water-tightness. The plates, the principal strength members of a ship, have various thickness. The heaviest plates are put on amidships. The others are put on so that they taper toward both ends of the ship (from the keel toward the bilge and from the bilge toward the upper row of plates). Using plates of various thickness reduces the weight of the metal used and gives the vessel additional strength at its broadest part. The plates, put on in rows from bow to stern, are called strakes. They are lettered consecutively, beginning at the keel and going upward.


The bottom row of strakes on either side of the keel, are called garboard strakes. The strakes at the turn of the hull, running in the bilge, are bilge strakes. The strakes running between the garboard and bilge strakes are called bottom strakes and the topmost strakes of the hull are sheer strakes. The upper edge of the sheer strake is the gunwale.


The interior of the ship is divided by the bulkheads and decks into watertight compartments. A vessel could be made virtually unsinkable if it were divided into enough small compartments. However, too many compartments would interfere with the arrangement of mechanical equipment and the operation of the ship. Engine rooms must be large enough to accommodate bulky machinery. Cargo spaces must be large enough to hold large equipment and containers.

Bulkheads and Decks


The engine room is a separate compartment containing the propulsion machinery of the vessel. Depending on the size and type of propulsion machinery, other vessel machinery may be located there (such as generators, pumping systems, evaporators, and condensers for making fresh water). The propulsion unit for Army vessels is a diesel engine. The "shaft" or rod that transmits power from the engine to the propeller leads from the aft end of the engine to the propeller.


The waterline is the water-level line on the hull when afloat. The vertical distance from the waterline to the edge of the lowest outside deck is called the freeboard. The vertical distance from the waterline to the bottom of the keel is called the draft. The waterline, draft, and freeboard will change with the weight of the cargo and provisions carried by the ship. The draft of the ship is measured in feet and inches. Numbered scales are painted on the side of the ship at the bow and stern.

External Parts of the Hull

The relationship between the drafts at the bow and stern is the trim. When a ship is properly balanced fore and aft, she is in trim. When a ship is drawing more water forward than aft, she is down by the head. If the stern is too far down in the water, she is down by the stern. If the vessel is out of balance laterally or athwartship (leaning to one side) she has a list. She may be listing to starboard or listing to port. Both trim and list can be adjusted by shifting the weight of the cargo or transferring the ship's fuel and water from one tank to another in various parts of the hull.

The part of the bow structure above the waterline is the prow. The general area in the forward part of the ship is the forecastle. Along the edges of the weather deck from bow to stern are removable stanchions and light wire ropes, called life lines. Extensions of the shell plating above the deck are called bulwarks. The small drains on the deck are scuppers. The uppermost deck running from the bow to the stern is called the weather deck. The main deck area over the stern is called the fantail or poop deck. The flat part of the bottom of the ship is called the bilge. The curved section where the bottom meets the side is called the turn of the bilge.

Below the waterline are the propellers or screws which drive the ship through the water. The propellers are attached to and are turned by the propeller shafts. A ship with only one propeller is called a single-screw ship. Ships with two propellers are called twin-screw ships. On some ships (especially landing craft) there may be metal frames built around the propellers (called propeller guards) to protect them from damage. The rudder is used to steer the ship.


The decks aboard ship are the same as the floors in a house. The main deck is the first continuous watertight deck that runs from the bow to the stern. In many instances, the weather deck and the main deck may be one and the same. Any partial deck above the main deck is named according to its location on the ship. At the bow it is called a forecastle deck, amidships it is an upper deck, and at the stern it is called the poop deck. The term weather deck includes all parts of the forecastle, main, upper, and poop decks exposed to the weather. Any structure built above the weather deck is called superstructure.

Weather Decks


The front end of the ship is the bow. When you move toward the bow, you are going forward, when the vessel is moving forward, it is going ahead. When facing toward the bow, the front-right side is the starboard bow and the front-left side is the port bow.

The central or middle area of a ship is amidships. The right center side is the starboard beam and the left center side is the port beam.

The rear of a vessel is the stern. When you move in that direction you are going aft, when the ship moves in that direction it is going astern. When looking forward, the right-rear section is called the starboard quarter and the left-rear section is called the port quarter.

The entire right side of a vessel from bow to stern is the starboard side and the left side is the port side. A line, or anything else, running parallel to the longitudinal axis or centerline of the vessel is said to be fore and aft and its counterpart, running from side to side, is athwartships.

From the centerline of the ship toward either port or starboard side is outboard and from either side toward the centerline is inboard. However, there is a variation in the use of outboard and inboard when a ship is on berth (moored to a pier). The side against the pier is referred to as being inboard; the side away from the pier as outboard.

Locations and Directions Aboard Ship

How a Ship is Compartmented and Numbered

Knowing how a ship is compartmented is crucial for navigating its vast interior. Each compartment of the ship is stamped with a series of alphanumeric numbers, known as "bull's-eyes," which give information on where you are, and what that compartment's function is. The information is given in the following order: deck number, frame number, relation to the centerline of the ship, and compartment usage. Each of these parts is separated by a hyphen.

Decks above the main deck are numbered 01, 02, 03, etc. and are referred to as levels. Below the main deck, there are the first, second, third decks, etc. (remember, on a carrier the hangar deck, the one below the flight deck, is the main deck.). Frame numbers tell you where you are in relation to the bow of the ship; the numbers increase as you go aft. The third number in the bull's-eye reflects compartmentation numbers in relation to the ship's centerline. EVEN numbers are to PORT, and ODD numbers are to STARBOARD. The numbers increase as you travel outboard. The last letter stamped on the compartmentation number indicates what the compartment is used for. Below are some typical codes:

Carrier Compartment Usage Codes


Supply and storage


Living quarters






Ship control


Trunks and passages









Example Bull’s-eye: 3-75-4-M


Indicates the third deck.


Indicates the compartments forward boundary is on or immediately aft of ship’s frame 75.


Indicates the fourth compartment outboard of the centerline to port (even numbers to port, odd to starboard).


Indicates the compartment is used for ammunition (see above).

Figure 1.1. Carrier Deck Schematic

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