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Boats for Beginners

The floors of a ship are called decks, the walls are called bulkheads, and the stairs are called ladders. There are no halls or corridors in a ship, only passageways. There are no ceilings in a room, only the overhead in the compartment. Openings in the outside of the ship are ports, not windows. Entrances from one compartment to another are called doors. Openings from one deck to another are called hatches. The handles on the watertight hatch or door are called dogs.

When you close a door or watertight hatch, you secure it. If you close down the dogs on the door or hatch, you dog it down. You never scrub the floor or wash the walls, rather you swab the deck and scrub the bulkheads. When you get up to go to work, turn to. You never go downstairs, you lay below, and if you are going up from one deck to another, you lay topside. If you are going up the mast or into the rigging you are going aloft.

A ship, to be classed as a fighting ship, must be capable of inflicting damage and of sustaining or avoiding damage. She must possess sufficient speed and, maneuverability to execute her mission and the capacity to proceed independently to a scene of action. The type of a warship is determined by the degree to which each of these qualities has been stressed in her design.

Capacity to "dish it out" is a primary attribute of fighting ships. A fighting ship must also be able to "take it." A ship may be designed to absorb punishment, to mitigate its effect, or to avoid it. Her hull will be subdivided into separate spaces, or provided with bulges or blisters, to confine the effects of flooding and explosion. This is called compartmentation. Damage control systems consisting of provisions for counterflooding, fire fighting, etc., are developed in varying degree in all types. Speed and maneuverability in themselves constitute factors of protection in smaller types in which armament and protection have been sacrificed for these qualities, while submarines depend for protection largely on their ability to make themselves invisible by submerging.

Without the capacity to reach a scene of operations, execute a mission, and return to a base, even the fastest and most powerful fighting ship would be of little value. To bring an enemy to action is the battleship's primary function, and these ships must carry crews and provisions necessary to take them into battle with enough shells and fuel and food aboard to permit them to fight and return.

Since cruisers are often required to perform independent missions at great distances, sea keeping capacity is a vital consideration in their design. Carriers must also be designed to accommodate provision for extended operations and fuel for their aircraft. Destroyers, as they often operate with battle fleets or in convoy, must also carry provisions for such work, subject to limitations of size, while submarines are required to remain in enemy waters for extended periods. It will, therefore, be seen that sea keeping is a very important factor in the design of all more important types of fighting ships.

The bulky hull of the battleship reflected capacity to accommodate provision for sea repair and for storage of immense quantities of fuel, water, food, ships stores, and lubricating oil, while the proportions of destroyer hulls indicated limitations in sea-keeping capacity inherent in their type.

Fighting Trim refers to a warship sailing condition that could be set by some warships of the late 19th and early 20th centuries. These ships had the ability to alter their freeboard. i.e. the portion of the ship above the waterline. Water-holding tanks designed specifically for the purpose were intentionally flooded, to lower the silhouette and the target area of the ship.

In high seas, most ships must sacrifice either speed or seakeeping ability, and neither can be achieved without size. To survive in high sea states and maintain speed, conventional displacement ships must be large. The relationship between a ship's maximum speed and its hull length is called "hull speed." For example, to reach a speed of 30 knots, a vessel must be at least 550 feet long. This limit on maximum speed applies to virtually all ship types, commercial and military. Consequently, small, conventional displacement ships are unable to do high-speed missions.

In addition to speed, a ship's size also limits its ability to perform in a seaway. There is a close relationship of size to capability in a seaway for several generations of ship hull forms. For example, to be fully operational in a seaway of 15-foot-high waves, a vessel must be 500 feet long. This sea-state limitation further emphasizes the unsuitability of small, conventional displacement ships for high-speed missions, especially in high seas.

Some advanced hull forms use dynamic lift to achieve high speeds without adhering to conventional size restrictions. However, these craft, which include planing hulls, hydrofoils and hovercraft, are highly susceptible to the effects of high sea states.

Though they may achieve high speeds in calm, inshore waters, the higher sea states found offshore require these ships to slow down for the safety of the vessel and its cargo as well as for the comfort of its passengers. A catamaran must slowdown in high seas to avoid passenger and crew seasickness, severe structural slamming and a wet deck. In high seas, performance of hulls that depend on dynamic lift also suffers: planing hulls and hydrofoils are subject to loss of lift, and air cushion vehicles can experience venting.

Seakeeping thus limits these advanced, high-speed vessels from providing an overall effective platform for many open-water applications--including ferrying, search and rescue operations, and military missions.

The quest to improve seakeeping led to development of the SWATH hullform. Utilizing submerged submarine hulls, wave-piercing struts and an elevated platform, the SWATH hullform has a low waterplane area that is less affected by waves than its predecessors. The result is increased stability in high seas. SWATH hulls, however, are still restricted to lower speeds. This lack of speed limits the effectiveness of SWATH, and to date, ship designers and operators are faced with the dilemma of choosing either speed or stability.

Efforts to improve the seakeeping ability of the faster hullforms met with little success. Lockheed Martin, therefore, decided to attempt to increase the speed of the stable SWATH design. The SLICE hull form is the result.

Resistance of a ship comprises two principal components: (1) viscous resistance stemming from the friction of the water against the hull, and (2) wavemaking resistance leading to the formation of the waves following the ship, known as the Kelvin wake. At high speeds, wave-generating resistance composes 50 to 60 percent of total resistance. This produces the distinct Kelvin-wake pattern behind the vessel.

The transverse wave component of the Kelvin-wake, observable as the large periodic waves within the V-pattern, is the larger part of the total wave drag. Until the 1970s, designers could do nothing to counteract the effect of wave drag on resistance and total performance. The hull parameter governing wave resistance is known as the Froude number. The Froude number relates the speed of a vessel to its length by the formula, where V is the vessel's speed, L the vessel's length and the acceleration due to gravity.

At low Froude numbers (low speed-to-length ratios), the wave resistance is low and the viscous resistance dominates. As speed (Froude number, or F) increases, wave resistance becomes a higher percentage of total resistance--until at the critical or "hump speed," wave resistance exceeds viscous resistance. This large increase occurs when F = 0.4, and is maximum at F = 0.5. Conventional ships always operate at Froude numbers below this primary hump speed To achieve high speed, naval architects design their ships to operate below the F = 0.4 threshold by incorporating long lengths. Only Navy ships with high installed-propulsion power can operate at a Froude number above 0.4.

The first attempt by naval architects to reduce wave resistance was the bulbous bow, which is widely used on cruise ships, ocean tankers and cargo vessels. This design cancels a segment of the wave created, thereby reducing the energy in the Kelvin wake. The bulbous bow lowers the height and increases the period of the transverse wave created by the ship. While the bulbous bow reduces the energy of the Kelvin wake by about 10 percent, this reduction occurs only at the design speed of the ship. Therefore, this design improves the efficiency of transit at cruise speed but provides little improvement at other speeds.

The SWATH ship Sea Shadow is an extension of this cancellation idea. This ship's lower hulls have a bulbous, "Coke-bottle" shape, with thin struts that connect the lower hulls to the superstructure. This shape counteracts wave resistance, reducing the energy of the Kelvin wake by about 20 percent. The Sea Shadow's sculptured lower hull doubles the limited cancellation effect of the bulbous bow. But again, this effect is realized only at the optimum design speed.

Theory shows that at high Froude numbers, the transverse portion of the Kelvin wave is virtually eliminated, reducing the wave resistance to low Froude number values. This leads to the idea that to significantly reduce wave resistance, a ship should operate at large Froude numbers, thereby surpassing the limiting hump. The idea is analogous to a supersonic jet overcoming wind resistance by surpassing the sound barrier.

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Page last modified: 07-07-2011 12:40:29 ZULU