Making Full Speed

making full speed

Remanufacturing gives an old vessel a new mission.

This article was prepared by staff writers in collaboration with outside contributors.

Ships have been cutting through waves for thousands of years. Long ago, hull design moved from art to science. Yet, ship designers continue trying to find ways to get smaller vessels to sail along on top of the waves instead of plowing through them.

One concept, the well-known hydrofoil, behaves like an aircraft wing to lift a hull out of the water. Another, the surface effect ship, or SES, rides on a cushion of air generated by a big downward blowing fan.

Both have drawbacks. Hydrofoils don't work well until the ship reaches high speed; during medium- and slow-speed operations, the underwater planes just get in the way. They add greatly to the ship's drag and can double its draft, creating a hazard in shallow or rocky coastal waters. SES vessels, on the other hand, tend to ride hard in heavy seas.

With a variety of specially built research ships, the U.S. Navy has worked on these challenges for three decades. Recently, the Office of Naval Research shifted its attention to lifting bodies. They generate lift from buoyancy as well as from their hydrodynamic shape. Hydrofoils, in contrast, generate lift only by moving through the water. Thus, lifting bodies can raise a hull out of the water at slower speeds than hydrofoils can.

Before going to the yard for its latest remanufacturing, the surface-effect vessel SES-200 spent some of its time under way at speeds in excess of 40 knots.

When the Office of Naval Research decided that it needed a vessel 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. Before the project could begin, however, ONR had to be sure that the hull was up to the task.

To do that, ONR contacted the National Center for Remanufactured Resource Recovery at the Rochester Institute of Technology. In the past, experts there had come up with ways to recapture value from obsolete industrial and automotive components.

Sometime after her original 1979 launch, the 110-foot SES-200 was cut in half and lengthened by 50 feet. Her original propellers were replaced with water jets that could drive the ship at more than 40 knots. At the time of her deactivation in 1990, she was the Navy's only operational SES.

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.

A simplified model of the full hull depicts the location and orientation of two of the four lifting bodies that would be added eventually to the SES-200.

"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 hull arrangement of the reconfigured ship resembles that of a SWATH vessel, Berg said, which stands for "small waterplane area, twin hull." This design uses lifting bodies submerged beneath twin hulls to lower water drag on the vessel. But where a SWATH vessel uses two lifting bodies, the modified SES-200 would use four, Berg explained.

There is a drawback common to these designs, Berg observed. "From a structural standpoint, all the loads are concentrated in just a couple of regions of the vessel. A traditional hull has its loads distributed more evenly," he said.

Using Mechanical software from ANSYS Inc. of Canonsburg, Pa., Berg's group began modeling the complete structure of the ship—its hull, decks, stringers, and bulkheads, as well as the lifting bodies and their struts. "The work became very complex geometrically," Berg said. But, analyses confirmed that the hull structure designed for one purpose could be adapted economically to something quite different.

This is the view of a lifting body looking inboard (top) and outboard (bottom). Red indicates high stress in the lifting body mounting and hull.

The first big full model took about 15 hours to solve initially, Berg said. "We got better at meshing and learned to use half models and even quarter models," he said. Quarter models could be solved in three or four hours. "At first, we went overboard with tight meshing. Then we learned to use larger meshes wherever we could," he said.

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.
"We had never done an analysis of this size before," Berg said, "and never modeled anything like a large marine vessel."

The initial modeling was done with models of SES bulkheads and frames imported from Pro/Engineer 2000 software from PTC of Waltham, Mass. The remanufacturing center did a lot of modeling manually, as well, using ANSYS products.

"Working from AutoCAD drawings, we dimensioned the model, then added stringers. We skinned it, loaded it, meshed it, remeshed it, rebuilt it, and so on," Berg said. "The second time around we got smarter. We started with a skin of the hull as a closed volume created in Pro/E. Then, we added the two dozen bulkheads as slices turned into planar surfaces. This cut the model building time from a month to a week." AutoCAD is the trademark of Autodesk Inc. of San Rafael, Calif.

This simplified mesh of the aft starboard hull includes bulkheads and stringers. A lifting body (in red) can be seen below the hull.

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.