Monday, 31 October 2011

Crossbow Workout

The crossbow got a good work out yesterday at the Celtic Harmony Samhain Festival.
We were running an archery have-a-go. The bow shot nicely, but at one point the string flew off, I added a few twists to effectively shorten it and increase the bracing height and re-fitted it.
A few of the club members had a go with it and found it easy to handle. One of our ladies in period costume had a go, I should really have taken a pic but didn't have my camera to hand.
Experimenting at home shooting through the chronometer shows it's not particularly fast and the actual kinetic energy imparted to the bolt is similar to the last Yew bow I made.
E.G 80# at 12.5" ends up about the same as 40# at 28".
For those who like figures the kinetic energy is calculated as 1/2 x mass x Velocity squared.
The figures need to be in kilograms, and metres per second to give an answer in Joules.
personally I dislike units like Joules as they mean nothing to me , can anyone tell me what a Joule feels like? An old fashioned imperial unit like a horse power at least give you some feel, as you know what a horse looks like.
The figures below are in 'grains' which is what arrows are usually weighed in and feet per second which is what arrow speeds are usually measured in. You can see it's a weird mix of units and if you haven't heard of 'grains' before, a grain is about the weight of a grain of barley.
The heavy bolt (11/32") in the picture is 440gn @ 125fps = 20.7 Joules of kinetic energy.
A light bolt (5/16") 150gn @ 170fps = 13.0 Joules of energy.
The heavy bolt goes pretty slow, but being 4 times heavier ends up having much more energy.
It would be interesting to plot loads of different weights and see which actually gives most energy.
Here's a couple of extra pics. The front view shows how the string sits level, it also shows how I was a bit sloppy making the prod as the left end curves up a bit more abruptly than the right. Still it was just a quick experimental bow to try out the Laburnum, the other pic shows the bolt clip which is Ash and looks a bit 16th century in style.


  1. Excellent stuff Del - imitation is the sincerest form of flattery so I have made my own version and posted in Primitive Archer. Not easy tillering these short bows is it?

    Stan (backgardenbowyer)

  2. Indeed, it's not easy to see the curve on something so short.
    I thonk you were right to get a longish draw, it gives a bit more time to get the bolt moving.
    I'm going to make another prod for this one sometime.

  3. You said "the bolt in the picture" but there is no bolt pictured on any of the pages I viewed. People always seem to leave out the bolts.

  4. If you look at the previous blog entry you will a picture of the bow cocked with the bolt in place.