In 1839 he was engaged in preparing schemes for a railway between Exeter and Plymouth, running over Dartmoor. At the time sufficient funds could not be raised, but an alternative coastal line was afterwards carried out by I. K. Brunel. In 1843 he made plans for docks at Birkenhead, which he defended before parliamentary committees against hostile local influence. The contest was long protracted, and the incessant labour served to shorten Rendel's life; his published evidence forms a valuable record of engineering practice of the period. In 1844–53 he constructed docks at Grimsby; in 1848–53 extensions of the docks at Leith; in 1850–53 docks at Garston on the Mersey, with extensions of the East and West India and the London docks. As constructor of the Grimsby docks he was one of the first to apply W. G. Armstrong's system of hydraulic machinery for working the lock gates, sluices, cranes, etc. For this work he received a grand medal of honour at the Paris Exhibition of 1855.