Surface effect ship

A Surface Effect Ship (SES) or Sidewall Hovercraft is a watercraft that has both an air cushion, like a hovercraft, and twin hulls, like a catamaran. When the air cushion is in use, a small portion of the twin hulls remain in the water. When the air cushion is turned off ("off-cushion" or "hull borne"), the full weight of the vessel is supported by the buoyancy of the twin hulls.

The SES has two advantages over a hovercraft for open sea operation: it is more resistant to slipping sideways when acted on by air or sea, and it can use water jets for propulsion since the inlet nozzles are always covered by water.

The United States Navy initiated the SES model test program in 1960. By 1963, a 10-ton test craft called the XR-1 was designed and built to test the surface effect concept. The first version of the XR-1 used fixed plywood seals at the and ends of the captured air bubble section. A jet engine providing 1700 pounds of thrust was used for main propulsion. A separate 110 horsepower (82 kW) gas engine was used to drive lift fans, and also to power out-drive units that were used for off-cushion slow-speed maneuvering. During the first year of testing, the XR-1 was fitted with more powerful main propulsion and flexible fore and aft seals were tested as well.

The XR-1 was also used to test the concept of water-jet propulsion. The ship was modified in 1964 and equipped with gas turbine engines driving the water jet propulsors. The ship's weight increased to 17 tons.

Rohr Industries became involved in SES history in 1970, taking over the XR-1 test program for the Navy. Rohr continued to use the XR-1 to test propulsion and seal concepts. Their eventual goal was to develop a 3000-ton SES capable of utilizing vertical launch missile systems and anti-submarine helicopters. The proposed ship was to be capable of 80 knots in sea-state 6 conditions. The proposed main power units were to be 4 40,000hp gas turbine engines driving water jet propulsors. The 3000-ton ship development contract was canceled in 1980.

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