Marine Log - September 2007 - (Page 20)

ICE CLASS TANKERS HIGH SPEED CRAFT Metacentric height (GM) does increase as hull separation increases but so does roll inertia, virtual roll inertia and roll damping. This leads to a reduction in motions at increased hull separation. The use of GM alone is only applicable to calculate roll motions for monohull type vessels and has been found to be not applicable to multi-hulls. It has been found that the Incat 112m Wave Piercing Catamaran (WPC) will have significant improvements in sea-keeping over the Incat 96m and 98m vessels and this can be related to the increased hull separation (overall beam), increased length and vessel mass. Additionally the 112 m WPC’s increase in tun- at 40 knots nel clearance and arch shape provides significant improvements. The 96 and 98m WPC’s carry an impressive 500 tonnes at 40 knots with 28 MW power. The 112m WPC can carry 1,000 tonnes deadweight at 40 knots—long the Holy Grail of high speed craft design— with 36 MW power. Davidson and Roberts say that industry discussion wrongly suggests that catamarans are not suitable for good sea-keeping. Generally most of this discussion suggests that catamaran motions are similar to monohull and monohull type vessels in head seas, but poor in beam seas. To increase the speed of a vessel, the length to breadth (L/B) ratio must be increased to reduce the wave making resistance. For a monohull this means a very long vessel or the stability of the vessel will be reduced too far. Various designs have been designed to overcome this limitation by adding outriggers to a monohull to provide the necessary stability, while reducing L/B to an appropriate figure. The L/B of each hull of the catamaran is restricted only by the size of the water-jet. On a catamaran, it is easier to achieve a high L/B because each hull can accommodate two water-jets whereas a stabilized monohull would have to fit three or four. An unstabilized monohull would not be able to achieve the same L/B ratio unless it was very long. The disadvantage of length in this respect is that the vessel lightship weight would become unreasonable and the vessel may not be practical for some ports. In general, a high speed catamaran has shorter length than a monohull or stabilized monohull of similar speed and carrying capacity. For commercial operations in today’s economic climate, the efficiency of the vessel is very important. It can be said that at equivalent payload and power, the vessel that can go faster should be the most efficient, as to gain speed, the hulls must be efficient. In their conclusions, Davidson and Roberts say that seakeeping is heavily dependent on the size of the vessel, length, beam and weight. A wider, longer vessel will generally have improved sea-keeping as compared to a shorter, narrower vessel. The improvement in sea-keeping given the right sea-state could be significant. In the right conditions, motions could be reduced to near insignificance if the vessel operates in wave periods outside of its own natural periods. Ride control is very effective in reducing the peak motions especially in resonant conditions. Motions can be reduced generally by 20% with active ride control, but up to 50% depending on sea state. The 112m WPC when compared to the 96m and 98m WPC can offer improvements of at least 25%, but up to 50% depending on sea state. Although wider hull separation increases GM (and stability), it does not lead to an increase in motions, but rather reduces motions quite significantly in the right conditions. tonnes 1000 http://www.rolls-royce.com/marine/default.jsp http://www.rolls-royce.com/marine/default.jsp

Table of Contents Feed for the Digital Edition of Marine Log - September 2007

Marine Log - September 2007 Contents Update Inside Washington High Speed Design Heads into New Waters The National Maritime Salvage Conference & Expo Guide China Aims for the Top Azipods: Just Add Ice Rigdon Scores Newsmakers Tech News Contracts Events Website Directory InfoDirect Ml Marketplace Last Word

Marine Log - September 2007

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