Bow Drill string slipping
If you find your bow drill string slipping, it helps to have a bow with some curvature, so you can pull the string tighter with your hand as your grasp it. That is, place your hand in from the end of the bow a little ways as you use it, and then by clenching your hand you can instantly make the string tighter.
Also check if you are using slippery twine or string.

Drill shape
Try using a drill with a slightly narrow part in the middle of the drill. The string wraps around the drill in the center, so this will have two effects:
1. It helps prevent the string from slipping up or down along the drill
2. A smaller diameter where the string is wrapped will actually spin the drill faster. To understand this, imagine a car wheel spinning. Notice that the center looks like it is barely moving, whereas the outer edge appears to be spinning quite fast in comparison. So if you speed up the spin of the center (the smaller diameter part) the larger diameter part (the outer edge) will spin a lot faster. Should save you some effort.

Survival Fire Bow Drill

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Bow Drill Miscellaneous Information and Tips

Bow Drill
Miscellaneous Information and Tips