Contents:	The SWATH Ship Concept
The Swath Concept A Brief History SWATH FAQ's Ride Quality Tri-Hull Development Our Model Testing Maritech Project -Lightweight Structure Other |Home|About|Contact|	WATH is an acronym meaning Small-Waterplane-Area Twin-Hull. The waterplane is the horizontal plane cross-section of a ship’s hull at the water surface. A SWATH ship’s key advantages are: (1) ability to deliver big-ship platform steadiness and ride quality in a smaller vessel and (2) ability to sustain a high proportion of its normal cruising speed in rough head seas. SWATH ships typically have two submarine-like lower hulls completely submerged below the water surface. Above water, a SWATH resembles a catamaran. Its haunch areas are connected to each submerged hull by one or two relatively thin vertical members, or struts. The longitudinal cross-section of each strut is somewhat streamlined to decrease wavemaking resistance. A typical SWATH International ship cross-section is shown below. Ship motions are caused by the waves on the ocean surface which produce forces on the hull that decrease rapidly as the hull is moved further below the surface, as with a submarine. Wave exciting forces can also be made smaller if the amount of waterplane area at the design waterline is decreased. With careful selection of strut size it is even possible for the net vertical wave force on the columns and hulls to be essentially zero over a narrow range of wave frequencies. However, the objective for a balanced SWATH ship design is NOT to minimize ship motions at the expense of speed-power or payload capabilities. Instead, the relative proportions for the strut waterplane area and submerged hulls are selected to reduce motions and accelerations well below accepted criteria for seasickness or onset of degraded performance of personnel or equipment. For this reason, and in contrast to a mobile oil drilling platform, the amount of waterplane area selected for a SWATH ship is small, but not extremely small. Although there is no commonly agreed on line of demarcation, all SWATH ships will have less than 50% as much waterplane area as a monohull of equal displacement. During the design process, if the total amount of strut waterplane area is decreased, the transverse spacing between the hulls must be increased to regain adequate transverse stability to resist heeling moments due to wind or movement of all passengers to one side of the ship. Because wider hull spacing results in greater structural weight for a given vessel length, choosing very small waterplane area will usually result in a larger, more expensive ship. Smaller struts also mean less space for propulsion machinery, make access to the lower hulls more difficult, and are usually worse from a structural standpoint. While thinner struts decrease wavemaking resistance, they also increase the hull’s wetted area, which results in increased frictional resistance. These other design considerations are why much of the research and development effort on SWATH ships has focused on understanding and being able to predict how much waterplane area a SWATH vessel can have and still offer significantly superior seakeeping. Adequate clearance to the underside of the connecting structure is also essential to permit the SWATH ship to ride over surface waves that are typically present in coastal waters. This type of operating mode results in the smallest vertical motions and is called platforming. For rough seas, with wave heights exceeding the amount of cross-structure clearance, SWATH ships are designed to have a sufficiently short heave period to provide inherent contouring behavior at low speeds. Ensuring this behavior is largely dependent on the designer having selected enough waterplane area.
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