The type of engine he used for ships was a side-lever design, in which a beam was mounted alongside the cylinder. This saved on height in the cramped engine rooms of steamers. His first marine engine was built in 1815, of 17 h.p., and fitted to a Thames steamer named the "Richmond". In 1823 a Maudslay engine powered the "Lightning", the first steam-powered vessel to be commissioned by the Royal Navy. In 1829 a side-lever engine of 400 h.p. completed for HMS "Dee" was the largest marine engine existing at that time. The marine engine business was developed by Henry's third son, Joseph Maudslay (1801 - 1861). He had trained in shipbuilding at Northfleet and, with Joshua Field, became a partner in his father's firm, trading as Maudslay, Sons and Field of North Lambeth. In 1838, after Henry's death, the Lambeth works supplied a 750 h.p. engine for Isambard Kingdom Brunel's famous SS "Great Western", the first purpose-built transatlantic steamship. They patented a double cylinder direct acting engine in 1839. They introduced some of the earliest screw propulsion units for ships, including one for the first Admiralty screw steamship, , in 1841. By 1850 the firm had supplied more than two hundred vessels with steam engines, though the firm's dominance was being challenged by John Penn's trunk engine design. They exhibited their engines at the 1862 International Exhibition. In 1825 Marc Isambard Brunel began work on the Thames Tunnel, intended to link Rotherhithe with Wapping. After many difficulties this first tunnel under the Thames was successfully completed in 1842. The tunnel would not have been possible without the innovative tunneling shield designed by Marc Brunel and built by Maudslay Sons & Field at their Lambeth works. Maudslay also supplied the steam-driven pumps that were so important for keeping the tunnel workings dry. In 1791 he married Bramah's housemaid, Sarah Tindel, and they had four sons together: Thomas Henry, the eldest, and Joseph, the youngest, subsequently joined their father in business. William, the second, became a civil engineer and was one of the founders of the Institution of Civil Engineers. Near the end of his life Maudslay developed an interest in astronomy and began to construct a telescope. He intended to buy a house in Norwood and build a private observatory there, but died before he was able to accomplish his plan.