Leaner, Meaner Aircraft Carriers

Building an Aircraft Carreir Hull is No Small Matter

In shipbuilding, some things never change...When its first steel-hulled ships were being built in the late 19th century, the Navy was a tough customer. Not only was the shipbuilder required to conform to the 'tests of steel' prescribed by the Naval Advisory Board (we now call it "certification"), but he also had to agree to pay a "penalty per ton" for any excess weight he built into those ships.*

Today the Navy is designing its next generation aircraft carrier (on the drawing board as CVNX), and once again weight, and hull steel certification, is on everyone's mind.

Ship hulls are in some of the most demanding environments imaginable for any material. They have to withstand changing temperatures — from the tropics to the Arctic. Pounding ocean water subjects a ship's hull to constantly changing load and torque, causing complex and often severe stress states. And, in a fighting ship, there is the potential for additional trouble. You get the idea.

The new aircraft carriers now being built must lighten up to meet design goals. One answer might have been to shave off a pound here and a pound there, which would have called for a redesign of the entire Nimitz class hull. Not an option.

But, a new high strength, low alloy steel — called HSLA-65 — is on the market and both the shipyard as well as the Navy** believe that the new carriers can shed a few pounds if this steel is used in the carrier hulls. Preliminary calculations indicated that if it was used in hull plate, it could provide equal or greater service life than the traditional high-strength steel, but be thinner, and therefore weigh less. The same would be true for the hull's interior supporting structures.

HSLA-65 is stronger and tougher than conventional steel, and has proven itself in commercial bridges, pipelines and other ship above-deck structures. The question is: can it stand up to the demands of an aircraft carrier's hull?

And so, the rigorous certification testing of this new steel began.

"That was in 1995," says Julie Christodoulou, current program officer at ONR on the project. "All steels must conform to the American Society for Testing and Materials (ASTM) standards, but the Navy's requirements are more stringent. HSLA-65 is currently undergoing seven of the eight tough certification tests required. This summer, the last, the toughest, and the most rigorous — the grillage test — will take place. In the next two months we'll know if it makes the cut."

"Saving as much as 2000 long tons may not sound like much in an 80,000 ton carrier," says Dave Edwards of ONR's Commercial Technology Transfer office. "But carriers generally "grow" at a rate of about 100 tons/year, and this will give the ship an additional 20 years of "normal" growth for new weapons, aircraft, sensors, communications and maintenance equipment. For CVNX, HSLA-65 seems the way to go."

* Alas, we'll never know what happened to poor shipbuilder John Roach - who was driven into bankruptcy by politics and Congressional investigation. His ships were finished by the Navy, who did not note their final weights.

**The testing and certification of the new steel for use in carrier hulls is a collaborative effort between the Office of Naval Research, Naval Sea Systems Command and the Navy's Program Executive Office on Aircraft Carriers.

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