Contents:|	Small-Waterplane-Area Tri-Hull Ship Designs
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| In this Article: Why is SI looking at Tri-Hulls? What about Powering? Sample Tri-Hull SWATH General Arrangements.	Swath International is unique in actively working to extend SWATH ship design principles and experience to small-waterplane-area tri-hull vessels. Concept level designs have been developed for 2 applications: a mini-cruise ship a fast, open ocean car ferry with long range hile the acronym SWATH has been applied exclusively to twin-hull small-waterplane-area configurations, it is equally apt for tri-hull configurations. In fact, at least two of the early SWATH concept patents, those held by Blair (1930) and Lang (1971), included tri-hull small waterplane area configurations. There has been considerably less development work on tri-hull geometries and no tri-hull SWATH vessel has been built. However, beginning in the 1980s more research and development has been carried out than is commonly known. It has been shown that a tri-hull small-waterplane-area design can provide substantially improved speed-power performance compared with a twin-hull SWATH vessel of equal displacement. It has also become apparent that three hulls are advantageous for some design applications because they enhance the exterior appearance and vessel layout. Why does a tri-hull SWATH have superior powering performance? The usual powering superiority of tri-hull configurations at moderately high speeds is primarily the result of decreased wavemaking resistance, but it is also important that the hull configuration not incur a large frictional resistance penalty. By way of background, the total resistance of a twin-hull SWATH ship at moderately high speeds is roughly 50% frictional and 50% wavemaking + form drag. SWATH ships are considerably shorter than a monohull or conventional catamaran of equal displacement tonnage. This is partly to minimize the weight penalty caused by the more deeply submerged hulls of a SWATH ship and the fact that smaller waterplane area makes necessary wider hull spacing than a conventional catamaran. Shorter length also minimizes the frictional resistance penalty from the submerged hulls' considerably higher wetted area per foot of length. Lastly, twin-hull SWATH configurations often have relatively high wavemaking resistance because their short length and greater structural weight make them less slender than would be desirable. Geometrically similar ship forms of different sizes are called GEOSIMS. For comparison purposes it is useful to think of the 2 hulls of a SWATH ship as geosims of a single-hull small waterplane area ship having twice as much displaced volume, or buoyancy. Each dimension of a geosim, such as hull length, is related to the corresponding dimension of the reference hull by the linear scale ratio, which is defined as the cube root of the ratio of their displacements. So, the length of each SWATH ship hull is (0.500)^0.333 = 0.794 x the length of a monohull small-waterplane-area ship of twice the displacement. An important factor affecting speed-power performance is the ratio of hull wetted area to displaced volume. For geosims this ratio follows the "square-cube law". That is, wetted area is proportional to (scale ratio)2 while displaced volume is proportional to the (scale ratio)3. The wetted area of each hull of a SWATH is 0.794 x 0.794 = 0.63 x that of the monohull, but the displaced volume is 0.50 x that of the monohull. It follows that the wetted area per ton of buoyant volume for the SWATH hull is (0.63/0.50) = 1.26 x that for a geometrically similar monohull. This means that SWATH ships have 26 percent higher total frictional resistance than a monohull small-waterplane-area ship of twice the displacement. By extension, the square-cube law suggests it is disadvantageous to adopt a trimaran if the total buoyant volume is merely redistributed into 3 smaller but geometrically similar hulls. The wetted area per ton for such hulls would be 1.44 x that for the monohull with three times larger displacement. Total frictional resistance would be 44 percent higher than is the case with the monohull, and about 14.5% greater than for a comparable twin-hull SWATH ship. For this reason, SI's tri-hull small-waterplane-area ship designs don't have 3 geometrically similar hulls. Instead, the designs have a small-waterplane-area centerhull with 80 to 90 percent of the total displaced volume and 2 relatively short, slender vertical struts providing the rest of the buoyancy. This type of tri-hull configuration, which could be described as a "SWATH-like center hull with outriggers" was first publically advocated by William O'Neill in 1986. In general, the total wetted area (and, therefore, frictional resistance) of these configurations is no more than 5 percent larger than for a comparable twin-hull SWATH ship, but the reduction in wavemaking resistance is much greater (see comparison chart), so there is a worthwhile net decrease in total resistance.
Sample Tri-Hull SWATH General Arrangements	| Home | About SIL | Products | Designs | Links | FAQs | Contact | What's New | Events | Swath Concept | Last Updated: June 24th, 1999