Manuel Swartz, the well-known boat builder, near Steamboat Wharf, has just built for Capt. Manuel Sylvia, of Nantucket, a fine large cat-boat, which is to be named the “Helen,” and will be sailed by Capt. Sylvia. The boat was measured by the custom-house here this week and is 30.8 long; breadth 13.6; depth 4.5, and is to be equipped with a 10 h. p. Victor gasoline engine, and all modern gear. The boat is in every respect a fine product of the boat-builder’s art.