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Making Full Speed


Making Full Speed
Making Full Speed

Ship designers continue trying to find ways to get smaller vessels to sail along on top of the waves instead of plowing through them. Recently, the Office of Naval Research (USA) shifted its attention to lifting bodies. They generate lift from buoyancy as well as from their hydrodynamic shape. Lifting bodies can raise a hull out of the water at slower speeds than hydrofoils can. When the ONR decided to prove the concepts behind lifting bodies, it opted to save money by converting an existing research vessel. By remanufacturing and upgrading the decommissioned surface-effect vessel, SES-200, the Navy is saving about $11 million from the $18.5 million cost of a new ship. The conversion is now under way at a shipyard in Hawaii.

ONR contacted the National Center for Remanufactured Resource Recovery at the Rochester Institute of Technology. According to Joel Berg, a senior staff engineer at the remanufacturing center, the Rochester lab needed to determine if the ship's modified hull could withstand the load passed to it through new struts and lifting bodies. Among the most important analyses were those focusing on the radically different stress and modal factors in the redesigned hull. Conventional ships are designed for strength longitudinally, from stem to stern. The main stress on such a ship comes from cargo and the force of waves passing beneath the hull.

For the converted SES, adding struts and lifting bodies changes the stresses completely—in effect, rotating them 180 degrees. These new transverse loads—known as squeezing or prying loads to distinguish them from the hogging and sagging loads seen by a conventional craft—were likely only a secondary consideration at the time of the original ship's design. "This meant that the vessel's transverse bulkheads would have to be redesigned," Berg said. "The hull stresses generated from the struts would have to be reduced by linking the lifting bodies in some way." The struts were considered in both 20- and 25-foot designs and, in each case, they were about 30 inches thick.

The full program took two years. Most of the modeling was done in the first half of 2000. The first set of models took three or four months to build. Revisions went faster. The remanufacturing center did not come up with the basic design; that was the Navy's. The geometry for the lifting bodies came from the Hawaiian shipyard. Naval architects will refine the center's feasibility study and structural analysis. "We were more of a litmus test with these analyses," Berg said. The center is currently evaluating alternate configurations of the lifting bodies and their attachment for stress. It also conducted a modal analysis to identify natural frequencies that could be excited by hydrodynamic loads.While most of the SES-200 is fabricated of marine-grade aluminum, Berg is analyzing composite structures for the Office of Naval Research as well. The lifting bodies, for example, while now aluminum, could eventually be made from composites to shape them into the advanced geometries that are likely to develop.

Mechanical Engineering Magazine Online, 12 December 2001  

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