Revised 14 June 2015
The crossbow played an important role in the late Medieval period. The crossbow was really the first hand-held weapon that could be used by an untrained soldier to injure or kill a knight in plate armour. The most powerful crossbows could penetrate armour and kill at 200 yards. Longbowmen could certainly penetrate plate mail (though perhaps not at such a great distance), but longbowman were generally highly trained soldiers. This meant that they were also expensive, and that they could not be replaced easily. (Many bowmen were recruited at a young age to master their craft.)
Anyone could use a crossbow though. Crossbows are easier to aim than longbows because the crossbowman doesn't have to use a hand to hold the string back while aiming. On a similar note, a crossbow can be loaded long before the bowman might need to shoot. In this way, the bowman would be able to shoot immediately if surprised. Crossbows require less upper body strength to operate as well. One can use both arms to span (draw back) a crossbow. Crossbows do, of course, come with a price. That price is in efficiency and in the firing rate. Longbowman could shoot 2-5 times more frequently in a given time than a crossbowman. Efficiency is a more technical problem.
Although it is impossible for any bow to be perfectly efficient, crossbows are particularly inefficient when compared to longbows. The reason for this is that the draw length and the lath (also called a prod) of crossbows are much shorter than those of longbows. So even though a crossbow may have more stored energy when spanned, the tips of the lathe do not have enough time to reach the maximum velocity that the amount of stored energy would otherwise allow. It is the lathe tip velocity that determines the speed of the bolt that is loosed. (Crossbows are not "fired", which is a term related to gunpowder.)
W.F. Paterson (1990) published data from Stephen V. Grancsay about an experiment comparing a longbow and a crossbow that was spanned with a cranequin.
|Type of Weapon||Draw weight||Bolt weight||Speed of bolt||Difference|
|Longbow||68 lbs.||2.5 oz||133.7 fps||Not much!!|
|Crossbow||740 lbs||1.25 oz.||138.7 fps||Not much!!|
This problem could have been alleviated with a longer draw length or a longer lath, but that would increase the weight and bulkiness of the crossbow, which are already two distinct disadvantages of crossbows. In the above example, it should be stated that the bolt loosed by the crossbow could have been heavier without experiencing much of a decrease in exit velocity. A heavier arrow loosed by the longbow would have had a significantly reduced exit velocity.
NOTE: through the use of modern engineering and advanced materials, modern crossbows are now much more efficient. The Excalibur Exomag has a draw weight of 185 pounds, and is able to send a bolt at 290 fps. The 165 pound draw weight Excalibur Exocet looses bolts at 270 fps, and the 150 pound draw weight Excalibur Vixen looses bolts at 250 fps. Special thanks to Excalibur Crossbows for the use of crossbow specs.
The following was written in response (edited slightly) to a question posed to me about the relative range and power of Medieval longbows and crossbows:
Although there are working examples of Medieval crossbows, there are no working examples of Medieval longbows, so a direct comparison between the two cannot be made. Hence, the only data I can draw on for longbows is either from historical evidence or from reproductions of Medieval longbows. It is my belief that while the range of longbows changed very little from the 11th. century through Medieval times, the range of crossbows certainly did increase. Historical evidence would indicate that in the hands of a well-trained longbowmen, distances of 250-350 yards were commonly attained. A few modern archers have regularly achieved distances of 350-450 yards with reproduction longbows. Inigo Simot loosed an arrow 462 yards 9 inches in 1914, and there is a claim of someone loosing an arrow 482 yards with a longbow.
At the time of the battle of Crecy (1346 C.E.), the English longbow almost certainly had a greater range than the crossbow used in field combat. Throughout the Medieval Period though, crossbows became more powerful. Sir Ralph Payne-Gallwey loosed a bolt from an actual Medieval crossbow spanned with a cranequin and achieve a cast of 490 yards. The ordinary 15th. century crossbow would likely cast a bolt 370-380 yards. These crossbows would surely outperform almost any longbow in terms of distance, but the accuracy of the crossbow at those ranges would likely be poor at best.
With range out of the way, power is an even more difficult subject to breach. In general, arrows weigh more than bolts, so they have a larger momentum (kinetic energy) at a given velocity. However, a late Medieval crossbow could loose a bolt at a higher velocity, thus overcoming the lower mass (the the kinetic energy being equal to the mass times the square of the velocity). Both longbows and crossbows were capable of penetrating all but the thickest plate maile armour, but my understanding is that the heavy crossbow was the main driving force leading to heavier and heavier plate maile armour. At point blank range, the crossbow almost certainly had greater penetrating power than a long bow. By the 15th century, and possibly earlier, it is safe to say that heavy crossbows (such as a windlass spanned crossbow) were more powerful than longbows. The common crossbow probably wasn't much more powerful though.