Nautical terminology is a law unto itself, heedless of the grammar of landlubbers. The domain of this law extends onto the land, in those places where ships are built and launched. In this case, the ways of the Sea cause a word that ordinarily seems plural to be used as a singular noun.
Small craft, such as your uncle's 75-foot sailing yacht, are often built upside-down (we won't consider those nasty fiberglass things you press out in molds), and far from shore. She can be turned upright when it's convenient, then hand-carried or trailered to the nearest puddle for launching.
But for a ship larger than a certain size, this is ludicrously impractical. Her hull must be built upright at the shipyard, in place, ready to be pushed into the water, where she will remain until she is broken up for scrap. Traditionally, such a ship is built on a special inclined drydock called a ways, or sometimes a building ways. Within the hearing of lubbers who can't get over that 's' at the end, or who think that the term refers to the pair of rails at the bottom of the dock, you will occasionally hear the term slipway used. During construction, the ship is said to be on the ways.
As the hull nears completion, certain of the junior shipyard workers are sent down to lubricate the bottom of the dock and the bottom of the ship. Traditionally, lard is used to grease the ways (you might grease the skids but I'd suggest you clean yourself up afterwards) although it is generally a bad idea to apply lard to a metal hull.
On the big day, the shipping company president and perhaps a couple of admirals make speeches, and then some high muckety-muck's wife stands on a platform, and gives the old girl a whack with a champagne bottle. In the case of Bess Truman, about a hundred whacks. Simultaneously, workers knock the chocks out from under the hull, and she slides down the ways into the harbor.
In case you're wondering, "ways" is its own plural.