In World War II, the Army knew it would have to fight globally, and that most of its theaters of operation would be overseas. And not only would it have to provide its own massive fleets of ships to move men, equipment, fuel and supplies, it would have to operate ports, both traditional and austere. And ports mean tugs. Worse still, it would have to move those tugs overseas. Major seagoing vessels would still require a traditional tugboat, but many operations that Army Transportation Corps Harbor Craft Companies would be tasked with could be performed by smaller vessels. And so the Army set out to design a small tug that could be mass produced, cheaply and quickly. More importantly, it had to be easily transported to overseas theaters.
It came up with a rather brilliant design. Rather than building a traditional hull, the so called Sea Mule was really four pontoons bolted together. The two rear pontoons each housed an 8 cylinder Chrysler gas engine, and the two forward pontoons each housed a fuel tank of about 700 gallons. No pilot house or other deckhouse was provided. A simple stand for the wheel and throttles was on the deck.
Since the Sea Mule was made up of four pontoons, the unassembled craft could be shipped to the theater of operations via rail, or truck, or more efficiently loaded in the holds of freight carrying vessels. Once in theater, the crews could quickly assemble the craft on site with little more than hand tools and a welding kit, and soon thereafter begin operations.
Chrysler was the main producer of Sea Mules, but other companies also built some, in a variety of sized and configurations. About 8000 were delivered, but they are virtually unknown today, though I hear a fellow up in Washington has restored one.