Small Waterplane Area Twin-Hull [SWATH]
Small Waterplane Area Twin-Hull [SWATH] ships have been used as ferries for cars and passengers, cruise vessels, oceanographic research, patrolling, and other off-shore crew service functions. The SWATH acronym has also been applied in trade terminology to tri-hull vessels. Some 50 commercial SWATH vessels have been placed in service worldwide, with displacements typically below 1,000 tons. However, there is a SWATH cruise ship having a displacement of about 11,500 tons.
Ships which carry a substantial quantity of cargo are typically constructed with a monohull having large displacement characteristics. This type of hull however is susceptible to the forces of waves encountered at sea, as well as those encountered while docked. These wave forces often cause the vessels to pitch, roll and heave to a large degree. The motion of the vessel often causes passenger and crew discomfort and increases the risk of dislodging and shifting of cargo. Such motion can damage the vessels as they move relative to off-shore rigs, docks and other ships to which they are moored. This lack of stability also hinders the loading and off-loading of cargo.
Small water plane area twin hull vessels, which are referred to by the acronym SWATH, and which also have been called semi-submerged vessels, have heretofore been developed for high stability and safety at sea. This type of vessel has two submerged, parallel, torpedo-shaped hulls and vertical struts which extend upwardly a substantial distance above the water line to a connecting cross structure that spans the struts. The superstructure and deckhouse of the vessel is supported upon this cross structure. Because the submerged hulls provide the entire buoyancy of the vessel, such have not been capable of transporting large tonnages of cargo. If, however, a ship could be developed with the cargo capacity of a typical monohull vessel but also with the safety and stability of a SWATH vessel, such would be ideally suited for use in the transporting of cargo.
Conceptually, SWATH vessels date back at least to the 1940s. A SWATH vessel generally includes two submerged, parallel, torpedo-like pontoons, each of which is provided with one or more vertical struts which project upwardly a substantial distance above the water line and which at their upper ends support the above-water superstructure of the vessel. As used herein, the superstructure of the vessel includes all parts of the vessel above the wet-deck, and it includes the portion of the vessel between the wet deck and the weather-deck. The combined buoyancy of the two (or more) pontoons and the immersed portion of the vertical struts is sufficient to support the superstructure of the vessel a predetermined distance (e.g. 5-8 feet) above the water level while the boat is at rest, as well as when it is under way in relatively calm waters. It is well known to provide SWATH vessels with underwater horizontal stabilizers or canards to assist in maintaining stability of the boat in terms of pitch and roll while moving at medium to higher speeds through calm or rough waters.
The above-water superstructure of a SWATH may be designed and outfitted to function substantially the same as a conventional mono-hull boat or ship. That is it may be fitted as either primarily a cargo vessel or a passenger-carrying vessel. During more recent years, a number of SWATH vessels, ranging in length from approximately 60 feet to over 200 feet, have been built and tested or operated as ocean or seagoing vessels with reasonable success.
All of the reasonably well designed SWATH vessels that have been built and operated in the 1980s provided much enhanced riding stability over any known conventional hull or catamaran construction. A properly designed SWATH inherently provides a much more "level" ride as far as minimizing the amount of pitch and roll inherent in more conventional designs.
The SWATH vessel generally includes two laterally separated, parallel, streamlined, underwater pontoons, each connected to a superstructure located above the water surface by one or more slender, surface piercing struts. The primary purpose of this hull form is to provide smaller motions in a seaway than can be obtained by other hull forms, over a wide range of speeds. The vessel is supported by the buoyancy of the submerged pontoons and, to a lesser extent, the submerged portion of the strut, both at rest and under way. The design water line is located on the struts part way between the tops of the pontoons and the bottom of the upper platform.
Adequate hydrostatic stability is obtained by selection of the amount of separation between the struts on either side of the vessel and the length and thickness of the struts at the water line. Usually, a propeller is located at the aft end of each pontoon behind a streamlined tail cone, and a rudder, attached to the overhanging strut, is located behind each propeller. In order to maintain stability at high speed, at least two approximately horizontal hydrofoil fins are added as far aft on the pontoons as is practical. Sometimes a single hydrofoil spanning the space between the pontoons is used. These fins, called stabilizer fins, are often actively controlled, along with additional forward hydrofoil fins, called canard fins, to reduce motions caused by ocean waves.
Some problems however have been encountered in practical application of SWATH vessels. Thus, practical limits are imposed on the depth of the pontoons below the surface by the water depths in the harbors. Often, these draft limits cause the pontoons of the SWATH vessel to be sufficiently near to the water surface that when running at moderate to high speeds at level trim, reduced pressure occurs on the upper side of the pontoon tail cone and a depression is formed in the water surface above the propeller. The reduced pressure on the tail cone increases drag on the vessel and the water surface depression can cause the propeller to ventilate (ingest air), resulting in vibration and loss of propeller efficiency.
In order to improve performance under these conditions, the vessel is usually trimmed down by the stern, utilizing the stabilized fins, usually two or three degrees to alter the flow pattern at the tail cone and move the propeller further below the water surface. Trimming the stern down decreases the clearance between the water surface and the superstructure, increasing the possibility of "slamming" when heading into even moderate seas.
One severe shortcoming that has been encountered in SWATH vessels is that when the vessel encounters even moderately rough head seas (e.g., as little as 10-ft. seas), the bow of the SWATH structure tends to plough into or abruptly land atop the head seas, resulting in the bottom or "wet-deck" of the superstructure forcefully "slamming" against the oncoming waves and/or creating the danger of swamping and sinking the boat. In any event, the impact with the head seas violently and abruptly alters the movement of the vessel (e.g., the vessel immediately brakes or stalls), and this can cause serious injury to passengers or cargo on board.
Furthermore, the resulting slant of the deck caused by trimming down of the vessel can be undesirable for some SWATH vessel applications. Depression of the water surface over the propeller can also be produced by downward deflection of the trailing edge of the aft stabilizer fin, usually located not far ahead of the propeller. To avoid propeller ventilation from this source, deflection of the stabilizer fin is usually restricted, causing a corresponding loss in its control effectiveness.
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