The hundred or so freighters that make up the Westcott’s customers each carry millions of dollars’ worth of commodities like coal, iron ore, grain and limestone. With freight this valuable, time costs a lot, so barring a catastrophe, boats stop only to load or unload cargo. Mail — and anything else a ship might need — must be delivered on the fly, while the mail boat and the freighter are moving.

One warm morning in late May, the mail boat chugged out to meet the Rt. Hon. Paul J. Martin, a 739-foot freighter steaming up the Detroit River. The big ship slowed but did not stop, and Capt. Bill Redding pushed the tire-clad bow of the 45-foot J.W. Westcott II snug against the sheer steel side of the Martin, keeping pace with it.

Canada Steamship Lines was painted in white across the red hull, and from above the “S” in “Steamship” a man in a hard hat and red jumpsuit lowered a dirty white bucket on a black rope. Two bags, filled with six cans of Planters mixed nuts and seven boxes of K-Cup coffee, were quickly tied on by the Westcott’s deck hand and hoisted aboard the freighter.

Twenty-four hours earlier, the captain of the Martin had called to order the nuts and coffee, knowing that they would be delivered to his ship when it passed the Westcott office, just southwest of the Ambassador Bridge on the Michigan side of the Detroit River.

Sailing two months on and one month off, “sailors on contemporary merchant ships are basically trapped,” said Mr. Hogan, sitting in front of black-and-white family photos and hockey posters in his office. He recently read that Maersk, the largest container shipping company in the world, was experimenting with using drones to deliver to its ships. But drones are expensive and small, so Mr. Hogan is confident that it will always be cost effective to deliver bulky, low-value cargo like coffee or paper towels by boat.

About one-third of the Westcott’s business comes from its contract with the Postal Service, which calls the boat its only floating ZIP code. The contract was just renewed to 2021 and pays for the Westcott to deliver United States mail to any vessel free of charge. Another third comes from transporting passengers, pilots, crew and contractors at $90 per person. The remaining third is from delivering non-Postal Service packages and other goods. (The mixed nuts and coffee delivery ran $16.) “If we were one entity — just a grocery guy, just a pilot boat — I don’t see that this would have continued,” Mr. Hogan said.