I'd like to share the ideas that led to a fast bowdrill.
Drill Well, first of all I had done some previous work on a fast hand drill. I think I got it down to under 12 seconds. For that it is pressure, pressure, pressure all the way. So one idea was to use the setup for hand drill and convert it to bowdrill. You need a drill that will deliver good punk at high pressures. Elder is the best I know, so that was the chosen wood.
Drill tip Again from hand drill I knew that in general having a hard outer ring and soft centre got the best results. I reasoned this should also be true of the bow drill. The reason for this I think is that you get all the punk from the highest velocity part of the drill tip - the outer edge without wasting energy also producing punk from the centre where the high pressure/ low velocity will more likely produce a coarse, inferior punk. I could create a ring drill tip like this by 2 methods. A hollow/ pith filled elder stem will do this naturally. I could also form it from a carved elder drill made of solid wood by hollowing out the centre of the drill tip between drilling attempts.
Notch The notch was just deep enough to quickly fill up with hot punk that did not need to fall through cool air a long way to the bottom. Also the apex of the notch was just beyond the "tramline" formed by the spinning, hard outer rim of the drill tip. Therefore all the punk was concentrated in a very small area at the notch apex, so enabling a quickly forming coal.
Bearing block Lowest friction I could manage. I opted for a metal cone, which I oiled with cycle oil.
Top of drill For the carved solid elder drill this was straight forward - I just sharpenned the tip and waxed it with candle wax and leaves. For the hollow elder tube however I had to be inventive as a wooden ring would have been extra friction. I eventually succeeded by finding a nicely fitting metal ring collar and seating a screw with a round head in that metal collar. The slot in the head caused some extra friction but it worked OK as a compromise.
Bow A sturdy, long, straight bow was my choice - about as long as my arm could usefully use. I think it is about 1 ft 9 inches long.
Bow string Green paracord was good and strong. However, a key point was good control of the tension. I put about 6 turns round the drill and wound a tensioner into the cord at one end for final adjustment of tension.
Pressure. I put a lot of weight through the drill.
Hearth - ordinary commercial pine.
That's about it really.