Grasping at straws? Speaking of straw.

"Flogging a dead horse." That's an old cavalry thing, right?

Actually, no. The following is from "When A Loose Cannon Floggs A Dead Horse There's The Devil To Pay (yes that's a book title) by Olivia A Isil, p.38
"'Flogging a Dead Horse".... (an excercise in futility):
The band of variable calm in the Alantic Ocean-roughly in the area of the Canary Islands-is known as the horse latitudes. They take their name from the Spanish 'Golfo de las Yeguas (Gulf of Mares). It is thought that the Spanish name stems from a comparsion between the unpredictable nature ohf the high strung Arabian mare & the capricious nature of the wind in the area. In the days of sail, when a sailor signed up for the duration of a voyage, it was customary to pay him 1 month's advance-but sailor's money never lasted long in rollicking port towns. Once their advanced wages had been spent & the ship had put to sea, sailors felt as though that they were working for nothing. Because it took approximately 1 month to reach the horse latitudes frommost ports in England, sailors bega the tradition of calling that 1st month at sea the "dead horse month". To mark it's end, the crew celebrated by stuffing a canvas likness of a horse & marching it around the deck with great pomp & ceremony. The symbolic representation of the "dead horse" was then hauled aloft to the yardarm & cut adriftinto the sea, as the sailors chanted, "Old Man [Captain], your hose must die!" Admiral William Smyth suggested that flogging a dead animal into activity was an excercise in futility as trying to get a wholehearted work commitment out of the ship's company while they were working off the dead horse month."

The Great Plains Indians would flog their near dead mustangs as a last resort to to get them to rise again & get the last mile from their horse before it actually would drop dead underneath them. Not an Army practice.

No you didn't say steam Jim (don't hang your argument on that), but steam was the 1st type of power generally used in the chain of events leading towards the dimise of sail, to have said otherwise or a different fuel type would have put the era even further into the future, farther from the usage of the phrase "blow the man down".

Barry