I'm writing a fantasy story and the main character, a young woman, must defend her farm from bandits when her brothers are conscripted. One of them left her a crossbow from a previous battle he fought in. My question is, what sort of armor, and at what range, would a standard medieval crossbow be effective at?
The crossbow is one that can be loaded by hand (without a crank) and light enough to carry. If she were to shoot a bandit in chain mail, at say, 30 meters, would she likely be able to incapacitate him? Or would she need to be much closer? What about a breastplate? What about leather? Thank you for any insight you can provide. I ask here because wikipedia and youtube have many conflicting accounts.
For a hand-spanned crossbow (using both hands, with a stirrup for the foot), the limit for most fit strong people would be a draw weight of about 200lb. For a typical Medieval European crossbow, the power stroke (i.e., the distance over which the string is drawn, i.e., the draw length minus the brace height) is about 6"/15cm, so the energy of the bolt would be about the same as an arrow from a 50-60lb conventional bow, perhaps about 50J delivered to the target at short range.
Such a crossbow would have little chance of penetrating armour, other than the very lightest of armour. It might be able to penetrate the thinnest iron plate armour (less than 1mm thick, such as gauntlets, and thin arm and leg plates) if the bolt hits square-on. It might be able to penetrate a buff coat. But even in those cases, the bolt might not penetrate deeply enough to have any much effect. it can't be depended on to inflict an incapacitating wound through any armour, even the lightest armour. Mail should stop it, any breastplate should stop it.
Of course, such a crossbow would be capable of incapacitating or killing an armoured opponent, by hitting them where they are unarmoured.
(Energy required for armour penetration from Alan Williams, The Knight and the Blast Furnace, Brill, 2003.)