The sidewall hovercraft, in common with all hovercraft, can exhibit uncomfortable ride in the form of large vertical accelerations in fairly moderate sea conditions The most critical conditions are met in short, steep, head seas A major research project was conducted at University College London to investigate this ride problem and to develop a means of reducing motions to acceptable levels. The motivation for the study was the need to increase the British market for sidewall craft (referred to henceforth as surface effect ships or SES) by increasing passenger comfort and reducing crew fatigue to levels comparable to the primary competitors of SES, such as planing craft, fast catamarans and hydrofoil craft For success in this venture it was essential that modifications to current craft designs did not significantly change the main advantage of SES, namely, very low fuel consumption at cruising speed It was also considered necessary to ensure that a system developed to improve ride quality should be capable of being retrofitted to existing craft when developed to full scale.
A wavebelt testing, previously used by Clayton and Tuckey (1983) for simulating amphibious model hovercraft motions, was available for adaptation to SES tests The type of motion in response to waves that is associated with hovercraft is not apparent on catamaran craft, so it is reasonable to assume that the au cushion is primarily responsible for the adverse behaviour. Information published by Kaplan and Dams (1978) and Doctors (1976) indicated that the problem was associated with the elastic properties of the air within the cushion volume A large model SES, of mass 120 kg and length 3 7 m, was supplied by Vosper Hovermarine Ltd (now Hovermarine International) and the wavebelt rig modified to enable pitch and heave motion tests to be undertaken without the sidewalls being immersed in water.
On the basis of the results obtained by Clayton and Webb (1986, 1988), a study of suitable solutions was made. It was clear that an active control device was required if the dynamics of the craft were to be substantially unproved To obtain adequate magnitude of the control input for SES motion control it is necessary to use hydrodynamic lifting surfaces, or dynamic control of the skirt geometry, or dynamic control of the flow into or out of the cushion space.
With the test rig and facilities available, practical development of a hydrodynamic device was not possible. The active control of the skirts on the craft could give very large control input, as cushion pressure and centre of pressure location can be influenced by skirt movement. However, substantial amounts of power would be required to achieve this with existing skirt systems. The development of a new skirt design to alleviate this problem was felt to be a poor solution, as many full scale open sea trials are required to develop a skirt system which has proven resistance to chafe and local tearing and thus possesses a useful working life.