So, I looked at Prince Harry's Wikipedia entry, because I couldn't remember how old he was. This led me to the entry for his batty father, and on to the entry for his highly eccentric granddad.
What I didn't know was that his granddad, as a Lt. Cdr., had commanded the sloop HMS Magpie in 1950.
Sloop? In 1950? Ah, the Black Swan class were little corvette-like convoy escorts, which the Brits termed 'sloops', as they weren't fast enough to keep up with fleet operations. Apparently small warships designed for operating independently of the fleet were all termed "sloops" at that time: gunboats, merchant-hulled escorts, and the like.
But that wasn't the oddest use of the term "sloop": The Royal Navy even has one that won't move at all, on account of being rather firmly attached to the Earth's crust.
Huh. I did not know that.