This is what I think of first when I think of a Rappel Spool.This spool is based on the spool Bill Cuddington is testing in the Depths of the Earth photograph. I scaled all the dimensions from that photograph, then my father had it made out of stainless steel.
This spool is heavy, mainly because the spool portion has such a thick wall (about 0.33 in). The coil rope guides keep the rope running properly, although I broke one off once and had to weld it back on. The safety bolt is probably not necessary. I normally use two or three wraps, and get a very smooth rappel.
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©2007, Gary D. Storrick