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.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.Seppings innovations were probably influenced by the work of William May, shipbuilder for the Dutch Navy in Amsterdam and son of John May (shipwright).[8] 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).[9] Seppings was appointed joint Surveyor of the Navy alongside Joseph Tucker in 1813, and held that office till his retirement in 1832.
Plan of a ship (HMS Daedalus of 1826) incorporating the Seppings designed 'round stern', this strengthened the structure of the stern, made it more resistant to enemy fire and allowed more guns to fire directly aft and to the stern quarters. It was, however, unpopular on aesthetic grounds, being soon replaced by the 'elliptical stern', which was more conservative in appearance.
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