[3][4][5] Joining the navy as a cadet on 13 December 1859, when fourteen, Grenfell passed out first from the Britannia, and gained as sub-lieutenant the Beaumont Testimonial in 1865.
On 31 December 1876 he was made commander, and on 1 May 1877 was appointed, on account of his linguistic attainments, second naval attaché to the maritime courts of Europe.
He was the first to direct the Admiralty's attention to the night-sighting of guns; and about 1891, on the introduction of the incandescent electric lamp, he invented his "self-illuminating night sights for naval ordnance".
Grenfell was also one of the first to suggest the use of sight-scales marked in large plain figures for naval guns, and advocated, though without success, the adoption of a telescopic light for day use.
[6] In April 1877, Grenfell read before the Institution of Naval Architects a paper advocating the trial of Hermann Gruson's chilled cast-iron armour in England, and in 1887 he published Grüson's Chilled Cast-iron Armour (translated from the German of Julius von Schütz: de:Julius von Schütz).