Surface effect ship

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A Surface Effect Ship (SES) or Sidewall Hovercraft, is a watercraft or vessel 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 clear 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 as 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 fore and aft 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 40,000 hp gas turbine engines driving water jet propulsors. The 3000 ton ship development contract was canceled in 1980.

Two experimental SESs, both of around 100 tons, with 80 foot length and 40 foot beams, were developed for the US Military in the 1960/ 70s; the SES100A and the SES100B. They both attained speeds in excess of the program goal of 80 knots (x 1.15 for MPH) The SES 100B, was built by Bell Aerospace/Textron Corporation at the NASA Michoud Assembly Facility. Driven by 2 semi-submerged, supercavitating controllable, reversible propellers, achieved speeds in excess of 96 knots (approximately 110mph), in the Gulf, outside of St. Andrews Bay, Panama city, Fl. The speed was verified by the USAF "ARIS" radar. It also fired the Navy's first vertically launched missile (an SM-2)while doing 60 knots. It was also used to test automatic ride control and high speed collision avoidance and navigation systems. The 100A ship was used to test waterjet propulsion, variable geometry inlets and automatic ride control systems. The 100A and 100B were both capable of operating almost completely out of the water on an air cushion with only 18 inches of sidewall, the propellers or te waterjet inlets entering the water. The 100 ton SES 100B was built for the US Navy as a 1/10th scale model to test the feasibility of using hovercraft and other Surface Effect Ships in combat situations. The top-secret specifications called for a ship capable of entering a combat zone at a speed of over 80 knots and disgorging a tank and 100 soldiers onto a beach, turning back out onto water and escaping at high speed. Unlike the hard sidewall SES's, the full skirted AALCs, JEFF A&B were the prototype vehicles for the LCAC's used by the Navy with such success in the Persian Gulf War.

(The 3000 ton SES concept was never designed to go up on the beach, like a landing craft. The AALC (Amphibious Assault Landing Craft) and the SES were very different concepts from the beginning. The AALC was a pure hovercraft that is capable of high speed insertion of troops and vehicles onto a beach. As a hovercraft, the LCAC, derived from the AALC has no rigid hull parts that extend past the air cushion. It, therefore, can operate across the beach. The SES has rigid side hulls that enclose the air cushion, permitting water propulsion but allowing it to only go to the shore when conducting amphibious operations. Design concepts for a 6000+ ton cargo SESes that Rohr and Ingalls proposed relied on the idea of the ship loading and unloading from beach, lighter, causeway or conventional dock facilities.

Currently SESs are used, primarily as small ferries The Russian Navy built a 600 ton combatSES (DERGATCH).It;s status is unknown. Royal Norwegian Navy built 9 MCM's SES, of which 6 are in service.

The Norwegian Navy have also one prototype Littoral Combat Ship SES, KNM [1], the Skjold class. KNM Skjold's service speed exceeded +55 knots (102 km/h). This prototype was also a year in the US for testing in the US Navy. The Norwegian Navy have orderered 5 more vessels from the ship yard-UMOE Mandal