He invented Dennett's Life-Saving Rocket Apparatus for saving shipwrecked crewmen in 1832, and was made custodian of Carisbrooke Castle.
Dennett's apparatus "resembled the old skyrocket", but had "an iron case instead of a paper one, and a pole eight feet long instead of a mere stick"; it weighed 23 lbs.
A ship's crew off Bembridge, in the Isle of Wight, having been saved by means of Dennett's rocket, the board of customs had the apparatus supplied in 1834 to several coastguard stations.
[4] A short time before his death, Dennett was appointed (apparently as some recognition of his services as an inventor) custodian of Carisbrooke Castle.
i–v) short accounts of various antiquities found in England, and read a paper on the barrows of the Isle of Wight at the Winchester congress of the association in 1845.