Gettysburg, Day 4: Horsepower, Hay, and the Limits of Logistics

In the previous post, we established the notion that an artillery battery’s combat power is directly related to the ammunition on hand, and can, theoretically, be expressed in hours and minutes of “staying power” in battery engaged in battle. But such quantifies but one (the last one) of the three questions posed with logistics. What of the resources expended to move supplies and the time involved?

Again, working backwards on the questions, we next have to consider the requirements for moving a chest of ammunition. A chest for a 12-pdr Napoleon weighed, by the book, 484 pounds. Let’s round that up to 500 pounds… because … well you know, round numbers. But 500 pounds is significant for another reason. In a word, horsepower. Allow me to steal this illustration for Wikipedia:

I’m not going down into the story of James Watt here, but just know there is an interesting history, steeped in 18th and 19th century technology, that brought us to the measure we know today as horsepower. Suffice to say, one horsepower is the ability to lift 550 pounds. But we need to translate “lift” to “pull” here. The Army’s manuals indicated a healthy horse could pull between 600 and 650 pounds, in addition to its own weight.

I’ll try to keep the math simple, with no decimal places or fractions to fiddle with. At 500 pounds (484 to be exact), we see the need to have one horse in order to move one ammunition chest. And we can’t split horses into parts, so it is a one-for-one allocation. And we are going to need that spare horsepower anyway, because in addition to the weight of the chest, one has to factor in the “overhead” of the vehicle used. If we want to move three chests as part of a caisson, we’d need six horses to pull 3,811 pounds. I think those numbers were chased down in a posting from many years back, if you want to check those figures. Even putting four chests on a standard wagon will require at least four horses. Not to mention extra manpower expenditure transferring chests from wagon to caisson.

The great thing we have with this figuring is a clean discussion of how many “resources” are required per unit of supply. Um… not so fast… the teamster says. Those horses must eat. A good rule of thumb is that a moderately active horse will eat between 1-2% of its body weight per day. A heavily worked horse increases that up to 3%. During the Civil War, the army sought out healthy horses between 1100 and 1200 pounds. So a well worked horse in our “one chest equals one horse” equation would need between 24 and 36 pounds of food per day. I would actually go with 25 pounds as a practical guide here – not over-feeding the horse, while keeping it healthy.

The Cheetos munching crowd will say, “horses eat grass, problem solved, right?” Nope. Not by a long sight. First off, horses have diets. They eat some grasses, sure. But not all grasses are equal in nutritional value. Nor are all grasses healthy for the horse. For an army on the move, there might not be a lot of fields left for grazing. Better to bring 25 pounds of sustenance for the horse, just to be sure. Likely that is a mix of hay and grains. So you see we have more overhead to factor. At the same time, we see yet another planning equation emerging – 25 pounds of hay provides one ammunition chest.

The other problem with “just let them graze” is the nature of the horses’ digestive system. Gluttony is bad for horses (as it is with humans… but you know). If left to its own, a horse will eat in small increments. We call that grazing, of course. For normal healthy grazing, the animal’s keeper would need to allocate several hours per day. Even if not out in the field grazing, a good teamster would not simply dump thirty pounds of hay out there for his horse to consume in one go. Small increments, spread out over the day. Bottom line, unless you want horses who cannot pull the required 500 pounds plus of payload, you better feed them well and in the proper manner.

That means the logistician has another set of measures to consider – one horse will need four to six hours per day for meals, if kept healthy, not rushed. Wow! That really cuts into the work day. Good news, horses only need four or five hours of sleep. Still, we see another governing factor in play here, as keeping the horse on healthy habits means only getting at most 16 hours of work out of the animal.

I know… there’s a war on and we can’t have all the luxury letting these horses munch and sleep in… well, I hear you. And that’s one of the reasons why the Civil War was so destructive on the horse population. The logistician who didn’t make allowances for those healthy practices would soon find another problem square in the face – finding more horses to replace the worn out and dead animals.

Still, we go back to the measure of sixteen hours of horse-work per day. A horse can pull a wagonload between 10 and 15 miles per hour. Factor that into the equation. One horse hauling one ammunition chest for sixteen hours can be expected to cover 160 and 240 miles. For planning purposes, we narrow that down to 180 miles as a good day figure. There are a lot of additional considerations of course, as supply wagons rarely operated singly. And convoys tend to operate at much slower speeds than the normal cruising rate of any vehicle. The figure of 60 to 80 miles is more accurate, historically. Indeed, that fits with known supply train movement rates in the Gettysburg campaign (specifically those dispatched by the Federals).

So now we have some rough numbers to throw around in regard to the first two questions. On the Federal side, the main supply depot is in Washington, D.C., some 85 miles give or take. One day’s work for a horse pulling a wagon in a convoy. Horses, of course, were allocated as part of teams. Given a four horse team, a supply wagon could haul four ammunition chests on that trip, with sufficient payload to spare for food for the horses in the team (preferably so as to not impose that overhead on additional teams, but we know that was not always possible). In that march, the wagon team gets 3.4 miles per pound of horse fodder. Those four chests of ammunition could keep a Napoleon gun on the battlefield at the preferred slow, deliberate rate of fire for four hours.

Circle back to the earlier post when we determined a gun’s four chest allocation would allow it to remain in action, at the slow, deliberate rate, for four hours. It takes one day to move those four ammunition chests up from the depot in Washington. The horses pulling that wagon on which those chests are carried each require 25 pounds of hay. So by rough estimations, the cost of keeping one gun in action for one hour at the slow, deliberate rate is 6.25 pounds of hay.

That’s for one gun. The Federals had 372 guns in the Army of the Potomac. Maybe not all at Gettysburg. But you get the picture. Now I’m not going to say the Federals would be firing every gun at a constant rate of a round every couple minute for every hour of the battle. Rather to say that each gun had about four hours worth of “shooting time” from its allocation of “valued rounds.” And the minutes within those hours would be husbanded accordingly.

Of course, we know that Henry Hunt had a wonderful, yet unauthorized, ammunition train supplementing that allocated to the batteries. That came into play on July 3, allowing the Federals to sustain “shooting time” for its guns despite being heavily engaged for the previous 48 hours, in some cases. But at some point even that was going to be depleted. The good logistical plan would have a complete replenishment each day. If not, there would be limits as to how much combat power could be projected from the batteries. If the situation limited how much the cannon could “speak” then infantry or cavalry movements would likewise be governed.

And we have yet to speak once here of how many crates of .58-caliber ball ammunition was needed for the infantry. Whole different set of equations in order to figure the “shooting time” for an infantryman working out of his cartridge box. Likewise, figure on the pounds of hardtack or other food stuffs needed to feed the soldiers. There are planning factors and equations that one cold derive there too. Oh, and don’t forget that every horse pulling guns or serving as a mount for the cavalry troopers will need the same 25 pounds or so of food per day. All of this is adding up to more and more supply wagons needing to make that one day trip out from the depot in order to just sustain the combat power in the field.

Criticism at the time, and certainly in the century that has followed the battle, has at times faulted Meade for not aggressively pursuing the Army of Northern Virginia starting on July 4, 1863. I think that criticism is often made without thought to the logistical situation that Meade faced. When we quantify the situation in terms of numbers of wagons needed to keep the Army of the Potomac in the fight, per day – indeed perhaps down to the pounds of hay required just to move the needed supplies – there is something to be said for Meade’s hesitation.

2 thoughts on “Gettysburg, Day 4: Horsepower, Hay, and the Limits of Logistics

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