Swath Yacht Technology stands up to the number test but will this new innovative superyacht technology take off? This article explains some of the swath boat technology at work.
All conventional displacement yacht hulls are subject in varying degrees to the upward and downward motion of waves. At the heart of the SWATH yacht design is the observation that waves only exist on the surface of the water and that below the surface the water is calmer. Therefore, waves do not affect a boat’ designed with its displacement permanently below the surface. To take advantage of the calmer waters below, yacht builders — Abeking & Rasmussen among them — have developed the unusual looking SWATH yacht.
A&R is the fist shipyard to have built a SWATH yacht. Their yacht design places the superyacht SILVER CLOUD’s entire superstructure above the water on top of a platform, which thin struts connect to a pair of submarine torpedo-shaped hulls. The struts are slender to minimize their exposure to waves. The swath boat design is similar in some ways to that of a car chassis, with two struts located on each side, a bit like wheels on a car. On the 134’6” Silver Cloud the “torpedoes” are 11’6” (3.50m) in diameter and run about 3’7” (1.1m) below the surface. A 6’10”-air gap between the water surface and the superstructure prevents cresting waves from making contact with the underside of the bridge deck.
After extensive testing, A&R engineers determined that the most effective design allocated 80 percent of displacement to the torpedoes (40 percent each) and divided the remaining 20 percent of displacement between the four struts. The struts then contribute only in small part to the vessel’s buoyancy; engineers installed ballast tanks inside the underwater torpedo-like hulls to help manage overall weight distribution. Also housed within the torpedoes are the fuel tanks and two 1,100 hp C32 Caterpillar engines, which provide the yacht with a top speed in excess of 14 knots. The vessel has a range of 3,900 nm.
Silver Cloud features a Costa propulsion bulb, connected to the vessel’s rudders in the axis of the propeller shaft, to help economize on power and fuel. A&R Engineers calculated that Silver Cloud’s Costa propulsion bulb adds about 0.5 knots to the cruising speed and increases the luxury yacht’s range by about 400 nm. Active fins fitted forward and aft on each hull act a bit as suspension does on a car. Using an advanced electronic control system, the fins help “fine-tune” the superyacht’s response to waves. According to A&R, fin stabilizers fitted to conventional ships and yachts are effective in suppressing rolling but do little to prevent pitching, slamming and heaving, which the SWATH does very well. The build contract went as far as specifying the vessel’s maximum acceptable vertical acceleration.
During sea trials in a head sea with waves topping 6’, Silver Cloud recorded a vertical acceleration of 0.035g rms and roll angles as low as 1.2 degrees rms (“g” equals the force of gravity and “rms” represents the statistical average.) According to A&R, these numbers are almost never achieved in ships with a hull shorter than 328’. In comparison, the vertical acceleration in cargo ships is limited to a maximum of 0.15 g rms and the NATO naval vessels require a maximum of 0.2g rms. By the time A&R delivered its first seagoing SWATH yacht in 2008, it had already built 10 commercial SWATH ships, including seven patrol boats. The German shipyard celebrated its 100th anniversary in 2007 and has built more than 6,400 vessels since its first, an unnamed 15’ workboat. One of its most recent yachts is an elegant conventional twin-screw yacht that has taken its place this year on the list of the world’s 100 largest yachts.
For more information, visit http://www.abeking.com