Contents:|	Ride Quality & Seakeeping of Small-Waterplane-Area Twin Hull Ships
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|	he possibility of becoming seasick is viewed by most passengers as a risk and a drawback of travelling by sea, whether they are considering a relatively short ferry ride or a long cruise. When a ship is described as providing good ride quality, that is understood to mean the ship is not likely to make a sizable fraction of the passengers seasick. All of the existing SWATH passenger ferries, casino ships and dinner cruise vessels have demonstrated excellent ride quality, equal to that for much larger monohull ships. One of the best real-world comparisons available is that for the Japanese SWATH ferry Seagull 2, which is similar in size to SI’s Super 4000 class ferry Cloud X. Seagull 2 carries up to 410 passengers at a speed of about 30 knots. In July and August of 1991 Seagull 2 operated on the same route near the mouth of Tokyo Bay as a 67.5 m long, 17-knot monohull ferry that can carry up to 800 passengers. During that period the operator of the 2 ferries kept record of the incidence of motion sickness on each voyage. The chart below, incorporating data presented in 1993 at a Symposium on High Speed Marine Vehicles, shows that the incidence of seasickness on the SWATH ferry was considerably lower than on the much larger monohull. Scientific study of motion sickness began in earnest in World War II with the goal of improving the performance of troops being transported to battle in amphibious craft or aircraft. A key finding is that people seem to be most sensitive to vertical accelerations along the head-to-toe axis. At sea, such accelerations result from simple heaving motion of the ship, from vertical motions induced at the bow and stern by pitching, and from vertical motions at the port or starboard deck edge due to rolling. Beyond this, it has been determined that the proportion of people who become sick depends not only on the magnitude of the acceleration but also on the frequency of the vertical motion. The most commonly used criteria are the curves published by O’Hanlon and McCauley1. These curves are based on systematic experiments conducted in the early 1970’s with a group of young men, unacclimatized to motions, who were subjected to sinusoidal vertical motion of various amplitudes at a series of single frequencies. Results of these experiments are often expressed in terms of the acceleration level found to cause 10 percent of the subjects to become physically ill when subjected to such motions for a specified time interval. Figure 1 shows the O’Hanlon criteria curves. According to the O’Hanlon data people have significantly less tolerance to vertical motion in the 0.2 to 0.16 Hz (5 to 6 seconds) range than at higher or lower frequencies. This is in accordance with experience on ships at sea. SI evaluates the ride quality of each SWATH ship design by predicting vertical accelerations at key locations on the ship and comparing the acceleration magnitudes against the O’Hanlon & McCauley criteria. If the SWATH vessel will be equipped with an active motion control system then the system is assumed to be operating for purposes of predicting ride quality. Results of a typical ride quality evaluation using the O’Hanlon criteria for 2 hours exposure are shown below for the case of vertical accelerations at the bow.
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