This work is the outcome of an undertaking by the Mechanical Engineering Department to repair and if possible to improve a grab used by the Marine Laboratory. The grab was a type which has a good record of effective sampling on coarse hard sediment as reported by Smith & McIntyre (1954) and fully confirmed by our own experience. Its success in these difficult sediments is believed to be due to the initial penetration provided by the spring-propelled jaws and to adequate weight opposing the upward forces as the lifting cable closes them. Because of wear in the cocking mechanism the machine in question had become dangerous to operate and a number of parts had fractured. Improvements to the cocking/triggering mechanism were successful but it was felt that the repaired striking motion would remain prone to further failure by fatigue and wear. A fair degree of mechanical insight was necessary for effective operation and it was considered fairly laborious to use continuously, so the basic problem was reviewed from an engineering design standpoint with attention to the ergonomic and mechanical problems of achieving a quick turnaround in a small boat such as the laboratory's 10 m coble, and to ensuring durability.