Adam Clark Estes provides a beginner’s guide to Navy-strength rum:
The Royal Navy’s successful invasion of Jamaica in 1655 had a lot of terribly negative outcomes. The commanders ended up in the Tower of London. Many of the English sailors fell sick or starved. A lot of Spanish settlers died. But there was one undeniably positive outcome: rum.
After that fated invasion, the Royal Navy started giving its sailors daily rations of domestically produced rum instead of the French brandy they’d been receiving. (“Domestically produced” meaning produced on the captured island of Jamaica, of course.) Referred to as a “tot,” this ration of rum measured about half a pint and was given to sailors around midday. The order used to distribute rum rations—”splice the mainbrace” — got its name from one of the most difficult repair jobs aboard it the ship. It remains a euphemism for having a drink today.
In order to ensure that the rum hadn’t been watered down, the sailors would “prove” the spirit’s strength by pouring it on gunpowder and then trying to ignite it. If it lit up, they knew that the alcohol content was greater than 57 percent. If it did not, the rum was considered “under proof.” This is where the term alcohol proof comes from, though it means something slightly different today.
The Royal Navy later tweaked the formula of the rations after the rum had been proved by adding some water and a bit of lime juice to combat scurvy. This healthy cocktail became known as grog after the 18th century British admiral Edward Vernon, better known as “Old Grog” for the waterproof grogram cloak he wore at sea.
Over the course of the past three centuries, Navy-strength rum has become the stuff of legend. The deep brown spirit made its way around the world, often in oak grog barrels with brass letters that read “The Queen God Bless Her,” or “The King God Bless Him” depending on the reign. Sailors used copper cups of various “measures” to portion out the grog. Since it took little more than molasses to make rum, the Royal Navy had no trouble keeping the kegs full.