Turns out that Abe Lincoln held patent 6469. From the Patent Office:
Abraham Lincoln had a strong interest in new technology and is the only U.S. President to hold a patent.
As a young man, Lincoln took a boatload of merchandise down the Mississippi River from New Salem to New Orleans. At one point the boat slid onto a dam and was set free only after heroic efforts. In later years, while traveling on the Great Lakes, Lincoln’s ship ran afoul of a sandbar. These two similar experiences led him to conceive his invention. Lincoln received Patent #6,469 for “A Device for Buoying Vessels Over Shoals” on May 22, 1849.
The invention consists of a set of bellows attached to the hull of a ship just below the water line. On reaching a shallow place, the bellows are filled with air and the vessel, thus buoyed, is expected to float clear. The invention was never marketed, probably because the extra weight would have increased the probability of running onto sandbars more frequently. Lincoln whittled the model for his patent application with his own hands. It is on display at the Smithsonian Institution National Museum of American History.
In 1858, Lincoln called the introduction of patent laws one of the three most important developments “in the world’s history,” along with the discovery of America and the perfection of printing. During the Civil War, he took a personal interest in new weapons, advocating the adoption of ironclad ships, the observation balloon, the breech-loading rifle, and the machine gun. Lincoln declared that “The patent system added the fuel of interest to the fire of genius.”
Notice that this patent is under the kids section of the Patent Office. Machine guns, ironclads and “new weapons” is what Abe liked. Should we be surprised?
The actual patent, dated May 22, 1849, is here. He describes an invention to lift boats over shoals.
More info here.