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. The following illustration shows the 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.