Logistics check

[Aldarion]

Can somebody check these numbers?
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For a two-week march, a 15 000 men force (two legions plus auxilliaries) requires 288 400 kg of grain for soldiers' rations - 115 360 kg for a standard 6 000 men force. Horses and mules significantly increase this – while light horses used by scouts and rangers can subside entirely off the grass, large war horses require a supply of food, around 15 kg per day, while normal war horse requires 9 kg per day, and same for pack or draft horse, while mule would require 7,5 kg per day. Further, each cavalryman has to have a war horse, a riding horse and a pack horse. 2 000 cavalry in usual legion thus requires 6 000 horses, of which 800 heavy war horses, 1 200 average-sized war horses, 2 000 riding horses, and 2 000 pack horses or mules. Assuming mules instead of pack horses, these would require a total of 59 000 kg of fodder per day or 826 000 kg for two weeks. In normal conditions a third of figure for animals is barley, which means animals will require 275 000 kg of barley over two weeks, for a total of 390 360 kg over two weeks. If there is no possibility for grazing at all, total requirements for a legion come for 941 360 kg over two weeks.

A standard pack horse or a mule can carry a maximum of 120 kg over any distance. For a two-week march, a mule would carry 105 kg of barley for itself, leaving 15 kg free. Ridden cavalry horses would carry 34 kg of barley, unridden cavalry horses 68 kg of barley, and mules 84 kg of barley. Since soldiers can and do carry 14 to 17 days worth of food with themselves on the march (this includes animal guides), barley carried by mules would be that required by animals themselves. Horses themselves would carry 204 000 kg of barley. As such, a 6 000 strong legion on a two-week march would have 737 360 kg of barley carried by pack animals, requiring 6 145 mules. If grazing is available as an option, barley carried by pack animals is 186 360 kg, requiring 2 219 mules. Additional 600 mules are required to carry non-perishable equipment (tents, digging supplies etc.), one per decuria. Each heavy cavalryman also has two servants, although they are not always taken on campaign. These 1 600 assistants share 160 mules, and further 1 120 mules carrying fodder. Overall non-grazing requirement of a legion comes to 8 025 mules; if grazing is available, legion can make do with 2 979 mules.

Army accompanied by pack animals can make 25 km per day in flat terrain, or 350 kilometers with above numbers; with ox carts, number is 20 km per day. Army with no carts and wholly mounted can move at pace of 52 km per day, and separate cavalry units can cover 64 km per day. A legion on a paved road may move 40 – 50 km per day regardless of weather. Oxen however would require no fodder, as they can obtain food by grazing; this however requires significant time, and may not be useful in mountainous areas. In flat areas, two mules can pull a wagon carrying 660 or cart carrying 500 kg. Two mules would require 210 kg of fodder for two weeks, leaving 450 kg for a two-week march or 240 kg for four-week march. As such, a legion would require 868 – 2 092 wagons for two week march (280 – 350 km) or 1 626 – 3 922 wagons for four-week march (560 – 700 km). These would require 1 736 to 7 844 mules – former number being mere one fifth of number required in mountainous areas. If using carts – of lesser capacity but more agile in hard terrain – numbers are 380 (with grazing) – 1 504 (no grazing) carts for two-week march and 2 330 (with grazing) – 9 217 carts (no grazing) for four-week march. Four-week march is standard which provincial officials have to set aside for an army, but supply dumps are set at two-week distances from one another. All numbers are increased in practice by some 10 – 20% in order to compensate for losses during the march, and do not include servants' mules.

Above limitations however only really apply for armies that have more than one legion present, as an individual legion is small enough to feed itself through foraging when in enemy territory. Even so, foraging is one of riskiest activities of an army, as foraging parties could be attacked, or else fail to acquire enough supplies. When being fed by foraging, each 15 000 – strong army will be 10 – 15 miles away from the closest similarly-sized army; usual recommendation is 20 miles. If supplied by logistical network instead of foraging, five legions (40 000 men) is usual upper limit of what can be deployed in a single army for any extended period of time, although up to 60 000 is possible, and larger armies may be deployed by joining up forces for a single battle.

Water transport is far more efficient than overland transport. Large sailing ship can carry 900 tons of grain, with 600 tons for hulk-type ship and 300 tons for cog-type ship. Smaller ships used for riverine transport carry 20 to 30 tons.

Equipment carried by a soldier is generally 50-60% of body weight. Assuming 150 lbs average, this means 75 lbs of equipment. Full plate weights 40 – 60 lbs (18 – 27 kg) with 50 lbs average, while heavy infantry half-plate weights 15 – 25 lbs (7 – 11 kg), with 22 lbs average. Weapons weights are 5 lbs (poleaxe), 2,4 lbs (arming sword), 10 oz / 486 g (dagger); as such, total weight carried by an infantryman is 30 lbs / 13,6 kg. This leaves 45 lbs for food and other accessories.

 

[Yora]

That's some serious amount of calculations. I admit I am lazy, so I am asking this instead:

What do you need these numbers for?

 

[Aldarion]

That's some serious amount of calculations. I admit I am lazy, so I am asking this instead:

What do you need these numbers for?

I'm planning on writing military fantasy, so I'd rather avoid things such as mechanical horses and teleporting armies.

 

[Pemry Janes]

The more general numbers of army size, speed, and the like seem right to me. How much do you figure a single soldier needs per day? Also, don't forget they'll need water. At least to drink on the march, since they'll almost certainly need a source of replenishment for that while they camp. That's going to cut into how much food and the like they can carry.

And they're going to want to have spare capacity to take on loot.

 

[skip.knox]

It all sounds reasonable, but mainly as a theoretical marker. There are, as you know full well, a hundred variables that can make a dramatic difference in the numbers, including weather, terrain, and whether the forces move through friendly or enemy territory. Then there's the possibility of illness, local famine, and other catastrophes.

IOW, in the actual writing, I'd look more at the variables than the constants. You can slow down your army to suit the plot. You can't really speed it up, unless they abandon their supply train or the cavalry abandons the infantry in order to flee, that sort of thing. If the story just requires the army move in orderly fashion from A to B, then go ahead and say they traveled for two weeks without incident, or some such. No one is going to question it. Well, there's this guy Aldarion who might. ;-)

 

[Aldarion]

The more general numbers of army size, speed, and the like seem right to me. How much do you figure a single soldier needs per day? Also, don't forget they'll need water. At least to drink on the march, since they'll almost certainly need a source of replenishment for that while they camp. That's going to cut into how much food and the like they can carry.

And they're going to want to have spare capacity to take on loot.

Spare capacity appears automatically as food and water stores are expended. As for water, I have not found any numbers for it, at least not in Byzantine manuals and stuff derived from them. I did find book "The Logistics of Roman Army at War", I will have to check it out.

IOW, in the actual writing, I'd look more at the variables than the constants. You can slow down your army to suit the plot. You can't really speed it up, unless they abandon their supply train or the cavalry abandons the infantry in order to flee, that sort of thing. If the story just requires the army move in orderly fashion from A to B, then go ahead and say they traveled for two weeks without incident, or some such. No one is going to question it. Well, there's this guy Aldarion who might. ;-)

Aye, but to know what impact variables will have, you have to have at least some idea of constants.

 

[skip.knox]

In accounts I've read--medieval accounts don't usually say much about logistics--I've never seen any chronicler say the army ran out of transport. The more common event is to run out of supplies. This doesn't need necessarily to be food. It can be shoes, cloaks, horses and pack animals, and so on.

FWIW, Guy Gavriel Kay does a fine job of incorporating logistics into his Children of Earth and Sky, especially in the later chapters. It's a good example of how potentially dull details like logistics are eminently readable when they are integral to the plot.