Wednesday, 19 October 2011

Grain/gram Scales, Crossbows, Speed and Power

I've just bought myself some digital scales for weighing arrows and suchlike. As I've started making a crossbow prod from Laburnum my thoughts strayed to the comparative speed and power of the crossbow vs the longbow.
there is a lot of daft speculation on the subject and it's largely irrelevant as the two weapons are/were so different.
In medieval times crossbows with wooden and composite prods were out ranged by the longbow, the introduction of the steel bow gave the crossbow extra range and power but at the expense of weight and speed of shooting, thus it became a defensive weapon or effectively a snipers weapon.
Doubtless there will be plenty who will argue with this synopsis.
Anyhow I shall illustrate with some facts and figures using my 75# longbow and my 275# crossbow (both on the website)
The Longbow shoots an arrow weighing 28.93 grams at 166 feet per second.
The Crossbow shoots a bolt weighing 54.55 grams at 158.7 feet per second, (the slower speed may surprise you).
Because the crossbow prod has heavy steel limbs it is slightly slower and shooting a lighter bolt doesn't increase the speed much.
If I convert the figures to kilograms and metres/second to get a figure for kinetic energy in recognised units (Joules) I get 37 J for the arrow and 63 J for the bolt.
(Using the formula 1/2 x mass x velocity squared)
You see the bolt is going slightly slower, and will have less range* but has almost twice the energy when it hits home.
Of course the crossbow is only a representation of a 'light sporting bow' and my longbow isn't a full 'Warbow' but it illustrates that range and power aren't necessarily the same and that you have to match the weigh of the projectile to the bow to maximise the energy as well as the range, it also shows the limitation of the crossbow.
To get real long range from a crossbow they had to go to immense draw weights.
E.G. Sir Ralph Payne-Gallwey refurbished a siege crossbow, draw weight 1200 pounds (that's over half a ton!) It would shoot a bolt 460 yards.
The other factor of course is draw length, the crossbow having a much shorter draw. In the above example the string only came back 7".

The point of all this is the primitive crossbow I'm going to make is hopefully going to be about 70-80# draw weight, pulling back about 8-9" but it will doubtless be less fast and less powerful than my longbow.
The prod is made with the tips curving up slightly to help the string lift up over the stock, I've also used a bit of wood with a natural deflex, this will help to avoid over straining such a short bow, and allow me to get a bit more draw length.
This is all just done by guesswork/experience, so I fully expect it to explode the the tiller, especially as I haven't used Laburnum before.

Blimey I've trimmed about 3/16" off the width and put a loose string on it. It winched back to 70# on the tiller very quickly at just a few inches deflection, it looks scary as hell.
I think I'll clean it up and maybe glue some rawhide or leather onto the back for extra safety before doing too much more. I thnk it could gould go bang rather spectacularly.
It looks odd having such a short bow on the tiller. I'll post a pic in a day or so.

*As a point of interest a crossbow bolt is a bit more aerodynamic than an arrow. there is an optimum ratio of length to diameter and a short bolt is closer to this. There was a 'Scientific American'article on this many years ago. This shows the cover of the relevant issue (Jan 1985).

1 comment:

  1. 'Explain More'
    You'll have to say what needs further explanation.