Considered as the environment for boats and ships of all kinds and sizes, the term sea is used to denote all waters large enough for the operation of these craft, from creeks and ponds to lakes and oceans. The wind and the ships moving across the sea create a pattern of undulations ranging from minute ripples to waves of gigantic size. The currents moving through it must also be taken into account in all ship operations and in some ship-design problems. The variations in density, resulting from the amount of salts in solution, determine the variable-ballast tank capacity of submarines and the ability of a submarine to “sit” on a layer of dense water while largely supported by a less dense layer above.
Considering the overall surface configuration, termed the seaway, the classical concept of a train of regular waves is highly unrealistic, but it has some practical uses. The normal seaway is highly irregular, with waves of different heights and lengths traveling in many directions. For analytic purposes, it may be considered as made up of a multitude of very low waves, having a wide range of lengths and periods and traveling in various directions, superposed to produce the actual seaway. When this is done, a useful approach is to use statistical methods to define the seaway by its spectrum, which indicates the amplitudes of its many (theoretically infinite) wave components.
The sea is also home to teeming masses of marine life, many of which are detrimental to ships. Marine borers attack wood exposed on underwater portions of the hull. Barnacles cling to the underwater hull, roughening its surface and increasing the ship’s resistance to travel through the water. Sea water is highly corrosive to most materials, and severe electrochemical effects cause rapid disintegration of submerged metals that are unprotected.
Ship motions in waves
Treated as a rigid body, a ship partakes of six oscillatory motions in a seaway. Three are translatory motions of the whole ship in one direction: (1) surge is the oscillation of the ship fore and aft; (2) sway is the motion from side to side; and (3) heave is the up-and-down motion. The other three oscillations are rotary: (4) roll is the angular rotation from side to side about a fore-and-aft axis; (5) pitch is the bow-up, bow-down motion about an athwartships axis; and (6) yaw is the swing of the ship about a vertical axis. Yawing is not necessarily oscillatory for every service condition. All six of these motions can and do take place simultaneously in a confused sea, so the situation is most complex.
The forces and moments caused by waves are balanced by three types of forces and moments opposing them: (1) those inertia reactions developed by the acceleration of the ship and cargo and the adjacent water; (2) those that result in damping the oscillatory ship motion or reducing its extent by the generation of surface gravity waves, eddies, vortexes, and turbulence; the energy required for setting up these disturbances is carried away and lost; (3) those of hydrostatic nature that act to restore the ship to a position of equilibrium as, for example, when the ship rolls to an angle greater than that called for by the exciting moment.
The behaviour of a ship in waves is too complex for the motions in all six degrees of freedom to be completely described mathematically. However, the longitudinal motions of pitching and heaving can be treated as a coupled system (neglecting surging), under the assumption that lateral motions do not exist at the same time or are reduced by stabilization to minimal values. Similarly, rolling can be treated along with heaving and swaying on the assumption that pitch and heave do not occur or have negligible effect. Equations of motion can then be set up that equate the wave exciting forces and moments to the three types of forces associated with the motions that were described above.
The theory of rolling was developed in the 19th century by Froude. The theory of coupled pitching and heaving is more recent, stimulated by the work of Boris Korvin-Kroukovsky in the 1950s, who applied a so-called “strip” method in which the ship was divided longitudinally into strips or segments. The total force and moment acting on the ship and the resulting motions were assumed to be the result of the integration of all the forces in the individual strips without appreciable interference. Model tests in many laboratories have confirmed the basic soundness of this approach, although refinements are continually being made. Computer programs for solving the equations and calculating the pitch-heave motions of any ship are commonly used in the design stage.
The pioneering work of Manley St. Denis and Willard J. Pierson, Transactions of the Society of Naval Architects and Marine Engineers (1953), showed how the motions of a ship in an irregular seaway can be statistically described by assuming that the irregular motions are the sum of the ship’s response to all the regular component waves of the seaway described by its spectrum. This powerful tool has permitted the extension of calculated motions (or those measured in a model tank) to the prediction of realistic irregular sea responses and hence to the comparative evaluation of alternative ship designs under realistic conditions.
Work by various investigators along the above lines has shown that longitudinal weight distribution and overall ship proportions have a much greater effect than details of hull form on pitching and heaving, and on the associated shipping of water, slamming, and high accelerations. In general, a short pitching period in relation to ship length is found to be advantageous in raising the limit of speed in rough head seas. This suggests concentration of heavy weights amidships, if possible, and favours long, slender hulls over short, squat ones.
Effect of shape and proportions
The most important single factor in cutting down the increased resistance, as well as motions, of ships running in waves appears to be a small fatness ratio; in other words, a small underwater volume compared with the ship length. This slenderness is difficult to work into ships intended to carry cargo but relatively easy for passenger ships. For reduction in the magnitude of ship motions in waves, it is important that the damping forces and moments be as large as practicable. Moderate flare in the above-water sections at bow and stern, large beam compared with draft, and fineness of the underwater sections all help to achieve the result. A deep-sea sailing yacht embodies these characteristics to a high degree.
To keep the ship reasonably dry while undergoing the rolling, pitching, and heaving motions that remain, large freeboard is essential, especially at the bow. To prevent slamming under the bow when it lifts out of water and then drops heavily upon the surface, the forefoot underwater must also be deep.
A good degree of damping is most necessary to avoid deep rolling. If this cannot be achieved by a transverse form suited to the service, such as that of a sailing yacht with a deep fin keel, it is accomplished by adding long fins on each side in the form of roll-resisting or bilge keels. When placed along the lines of flow, these keels add little to the ship resistance in calm water.
Active roll-resisting fins serve to quench the greater part of the roll on a fast ship with a reasonable expenditure of weight, space, and cost. These fins, much shorter than bilge keels but extending several times as far outboard when in use, are rotated mechanically about transverse axes to produce angles of attack and girthwise forces which continually oppose the rolling motion. Since the moments of the roll-resisting forces increase as the square of the ship speed, the active fins are ineffective at low speeds.
Passive roll-resisting tanks of flume or U shape have been extensively installed in ships. In these, the tank dimensions and quantity of water or other liquid are arranged so that the liquid moves from one side of the ship to the other to counteract the rolling motion. Active tanks make use of controllable (and reversible) axial-flow propellers placed in ducts connecting the port and starboard tanks to control the flow.
Considering the vertical accelerations involved, pitching and heaving, or a combination of the two, are particularly objectionable for passenger comfort and safeguarding of cargo. They often necessitate a reduction in speed or a change of course. Some form of passive pitch-resisting fin may be evolved which will accomplish its primary purpose without introducing detrimental features.
Hydrostatic and hydrodynamic loads in service
The naval architect must know the loads imposed upon a ship in all the conditions of its expected service in waves so that a hull structure may be designed to withstand them. Aside from the static distribution of load along the length when the ship is floating at rest in calm water, there are many possible buoyancy distributions in waves for the same loading condition of the ship. Further, the wave action and the ship motion in waves generate dynamic forces which, under certain conditions, may be extremely important. When the bow and stern are on wave crests, with a wave trough between, the ship hull sags or bends downward in the middle. As the middle body reaches a wave crest, with the ends over wave troughs, the ship bends the other way, or “hogs,” and the ends droop because of the greater buoyancy amidships. Waves also produce torsional moments and the hull twists in the seaway, as when the ship is traveling obliquely through waves. Both bending and twisting actions involve shear in the structural members, as when a region that was square in shape under no load takes the shape of a rhombus when deformed. When the ship rolls, racking strains are induced in the hull because the above-water portion wants to continue to roll as the underwater portion starts to roll back the other way. Ship motions also induce inertia forces similar to those felt in elevators when starting or stopping.
It often happens that a part of the hull and an adjacent wave surface, each parallel to and approaching the other, meet with a heavy shocklike impact known as slamming. This can occur if the bow of a ship emerges from the water on a violent up-pitch and drops down upon a rising wave surface. It can also occur if a large wave strikes an overhanging part of the ship, such as the flaring hull under the forward end of the flight deck on an aircraft carrier. The tremendous blow against one end of the hull causes the whole structure to vibrate in an action known as whipping. The strains thus caused may be as great as those encountered in sagging and hogging over large waves.
Other natural loads are those caused by wind and ice. Typhoon and hurricane winds may blow with velocities of 100 knots (185 km per hour) or more. In subfreezing weather the sea spray freezes on the exposed portions of the ship, thereby adding a substantial weight. Icebreakers must be able to withstand the shock of ramming thick solid layers of sea ice and to survive the squeezing action of pack ice.
Many of these loads may be reduced by judicious operation of the ship; for example, by slowing down or heaving to in a storm. Ship structures are designed to withstand most of them, but the exercise of good seamanship significantly lessens their intensity.
