In his book Crossbows, Frank Bilson begins with: “The overriding points to be striven for in good weapon design are, first, balance, which can be achieved only by trial and error, second, an apparent simplicity of line, which means ridding the surface of every irregular knob, joint, lever and unsightly angle, and third, accuracy in shooting, which can be affected by the kind of stock used, the groove the missile runs in, the angle at which the prod is attached to the stock, and the size, weight and fletching of the bolts.”
Bilson goes on to prioritise the groove in terms of straightness, friction, and dimensions with regard to accommodating the bolt – which he suggests can travel at up to 320kph. Get it wrong at this point, and everything else is just baggage. My priority, then, is the groove, for which I will use aluminium angle.
Butt design
Bilson’s next priority: “The shape of the butt (stock) also needs careful consideration. Some people hold the mistaken idea that the crossbow butt should be the shape of a rifle butt. This is quite wrong.”
That is also where I started, and what guided the shape for the original foam-board model. Bilson’s argument for this comes down to the trajectory required – while a bullet has a very flat trajectory due to its higher velocity, the crossbow bolt flies in a parabolic curve to get the same result. To achieve this, the butt must slide “much further down the shoulder so as to achieve the required elevation when aiming at [a] longer distance. This means that the crossbow butt must have quite a long and shallow curve compared to the short, deep curve of the rifle butt”.
I plan to use kwila for the butt of the stock. I designed the stock to be squared against the rest of the tiller, but Bilson’s comment regarding the curved end changed that. I will shape the kwila to match the curve.
This was the first departure from my foam-board model, but it wouldn’t be the last. The model got all the pieces in place; the research prioritised and corrected them. The prioritising – the most interesting part – went something like this:
1. the bolt length and therefore the tiller length, as determined by the Department of Conservation (DOC). Traditional crossbows used much shorter bolts. In his book , Sir Ralph Payne-Gallwey outlines bolt construction as being a total length of 12½ inches (31.75cm) and 2½ ounces (70.8g). A traditional medieval crossbow could not be used2. the rails that supply the groove for the bolt, and the accuracy of the bolt once released3. the nature and position of the prod. This needs to be sloping slightly upwards, thereby reducing the friction between the bowstring and the rails. Alongside this is the matter of the position of the bipod (a support or rest for the crossbow). I want it as near as possible to the prod to give the bolt maximum support and stability at the moment it leaves the crossbow. I have bought the bipod4. the trigger mechanism and its action – I am still unsure whether the one I have is suitable5. the shape and depth of the stock – I have to allow for both the curve into the shoulder and the depth of the bowstring draw mechanism. I had been considering geared fishing reels, before wondering whether I could get a small winch to fit within the depth of the stock without compromising it6. putting all this together without compromising its strength – particularly the perceived weak point around the trigger mechanism.
