I have created a bow-drill for the purpose of starting fires.
The wooden shaft's diameter is about the diameter of a typical male thumb, and is approximately 11 inches in length.
The bow string is standard nylon string.
I know that it is typical for the string to travel up and down the drill shaft as the bow oscillates, however, almost without fail, my bow string travels up more than it does down and eventually works its way off the drill shaft.
How do I reduce the bow-string travel that results in detachment from the drill shaft?