How it Works: How to Caulk a Wooden Boat

As a kid, hanging around the Concordia shipyard in Padanaram in New Bedford, Frank Rapoza was fascinated by the way boats were caulked. Everyone said, Why learn a dying art – after all, fiberglass is the future. Undaunted, Frank convinced one of the old-timers at the yard to teach him his craft, and today, what with the building of so many large wooden vessels up and down the eastern seaboard, Frank’s services are in great demand.

Whether working on a Herreshoff 12½-foot sloop or Old Ironsides, the caulking technique is essentially the same. Strands of cotton – or in the case of large vessels, cotton and oakum (hemp fiber coated with pine tar) – are driven into the seams of a hull. Each cotton strand ranges in diameter from half-inch to three-quarters of an inch, and in the case of smaller craft, a single strand is all that is needed.

The trick is to feed or tuck the strands into the seams so that when all are driven home, they are the same distance below the surface of the planking. Caulkers use a mallet and various caulking irons, which resemble chisels.

Frank says the tools developed in American shipyards are superior to European versions because of the variety of nationalities represented in the workforce. Craftsmen from all over the world would compare notes, appropriate the tools with the best qualities, and thereby advance the state of the art.

Caulking requires both hand-eye coordination and the patience of Job, as you can only advance along the seam one width of the iron at a time. It also requires earplugs; the constant ringing of the irons just inches from the ear can easily cause hearing damage.

Over the years, Frank has worked on the Constellation in Baltimore and the Amistad, the replica slave ship that was the subject of the Steven Spielberg movie. He’s also worked on both Juno and Rebecca, two large schooners built by the Gannon and Benjamin Marine Railway in Vineyard Haven.

In order to caulk a vessel the size of Amistad, four or five strands of cotton and oakum are driven into the seam using a beetle, which resembles a sledgehammer, and hawsing irons, which are larger versions of the caulking irons. One person holds the iron, and one wields the beetle. After all the strands are driven home, one finishing strand is put in using smaller irons.

A sixty-foot vessel such as Rebecca can take several weeks to caulk, one the size of the whaling ship C.W. Morgan in Mystic can take several months. A good caulking job should last fifteen years or so before it requires some spot repairs. With regular maintenance it should last from twenty to thirty years.

Frank claims that someone once asked him if he would caulk the floor of his house. He respectfully declined. But it occurred to me that there must be other places where a classic, time-tested, and relatively inexpensive method for preventing leaks could be of great value.

Like the Big Dig.