Seppings was born to Robert and Lydia Seppings at Fakenham, Norfolk, on 11 December 1767 and baptised three days later. In 1782 he was apprenticed in HMNB Devonport, Plymouth. In 1800, when he had risen to be master shipwright assistant in the yard, he invented a device which greatly reduced the time required to repair the lower portions of ships in dry dock when compared with the laborious process of lifting then in vogue. His plan was to make the keel of the ship rest upon a series of supports placed on the floor of the dock and each consisting of three parts - two being wedges arranged one on each side of the keel at right angles to it, with their thin ends together, while the third was a vertical wedge fitting in and supported by the lower pair. The result was that it became possible in a comparatively short time to remove these supporting structures by knocking out the side wedges, when the workmen gained free access to the whole of the keel, the vessel remaining suspended by the shores. Soon, his creation became commonly known as "Seppings Blocks." For this invention Seppings received £1000 from the Admiralty, and in 1804 was promoted to be a master shipwright at Chatham.
At Chatham, in spite of the repugnance to innovation displayed by the naval authorities of that period, he was able to introduce important innovations in the methods of ship-construction. Quoted as saying "partial strength produces general weakness", he significantly improved the strength and seaworthiness of the Navy's fleet. He improved the design of the bow and the stern, but his greatest influence on ship design was in the introduction of 'diagonal bracing' into the construction of the hull. He first experimented with the idea in 1800 when he retrofitted the frigate HMS Glenmore with the trusses. The method was first introduced in new construction in 1805 with the construction of the 74-gun ship of the line HMS Kent.
The use of the diagonal trusses greatly increased the stiffness of the hull, improving a ship's sea worthiness and, more importantly, allowing longer hulls to be built without the drawback of excessive hogging (drooping of the hull at the bow and stern, where buoyancy is at its lowest). This meant that the rigging had better anchor points to take the force of high cross-winds, while shear forces along the hull were no longer borne by the oakum-packing of the timber planks which had leaked while they were squeezed and released like a sponge.
Seppings was also instrumental, along with other innovators, in the introduction of iron elements into ship construction, reducing the need for 'grown timbers', which were in increasingly short supply (grown timbers were structural elements, such as 'knees', that needed timber grown to shape in the living tree, in order to give the required strength).
These stronger designs offered better protection than the old forms to the crews against the enemy's fire, permitted a powerful armament to be fitted, and better allowed the ships to be kept on-station during bad weather, since the vessel would be more able to resist the stresses consequently reducing wear on the hull and subsequent leaks and salt water damage (especially insidious and dangerous to a ship where wood and iron were in contact).
It is with sincere regret we have to announce the death of [...] Sir Robert Seppings, F.R.S., M.R.I., &c., for so many years Surveyor of the British Navy. [...] He died in this town, on Saturday, the 25th instant, aged 72 years.