On 24 December 1807 he witnessed the wreck of the frigate Anson in Mount's Bay, when over a hundred people died, and this disaster led him to devote his life and patrimony to the discovery of some means for saving lives at shipwrecks.

He spent much labour in attempting to devise a lifeboat, but produced no satisfactory results, and turned his attention to the "Rocket" lifesaving apparatus, an early form of the Breeches buoy.

The idea occurred to Manby in February 1807, after the grounding of HMS Snipe a mere 50 yards off shore in a storm and the subsequent loss of 67 lives, and in August he exhibited some experiments to the members of the Suffolk House Humane Society.

[4] The advantages were that the rocket was much lighter and more portable than the mortar; that the cost was much smaller; that there was little risk of the line breaking, because the velocity of a rocket increases gradually, whereas that of a shot fired from a mortar was so great and sudden that the line was frequently broken; the whole of Trengrouse's apparatus could, moreover, be packed in a chest four feet three inches by one foot six inches, and carried by vessels of every size, while Manby contemplated the use of the mortar only on shore, and the safety of the vessel depended therefore on the presence of an apparatus in the vicinity of the wreck.

The government ordered twenty sets, but afterwards preferred to have them constructed by the ordnance department, and paid Trengrouse £50 compensation.

[8] He is buried in the churchyard of St Michael's Church, Helston, and is commemorated in the naming of Trengrouse Way, a main thoroughfare in the town.