The Windlass Wind-Up: Drum Diameter, Rope Layers, and the True Effort of Raising a Bucket

A hand windlass is usually sized as though the rope wrapped on the bare drum, so that the effort is set by the core radius for the whole lift. In service the rope stacks in layers, each new layer winding onto a larger radius than the layer beneath it, and the torque demanded at the crank grows accordingly. This design study computes the full effort curve for stated drum diameters, crank lengths, bucket sizes, and well depths, counting drum rotations and rope capacity layer by layer and allowing the suspended rope weight to fall as the rope is wound in. For the main case, a drum of 0.15 m diameter raising 15 litres of water from a depth of 15 m, the lift begins within a single layer and finishes on a second layer, the effective radius grows by 14.8 percent, and the crank force rises from 37.8 N at the start to 40.5 N at the end, through 28.77 drum rotations. At a depth of 10 m the same drum completes the lift on one layer in 19.65 rotations, with the force easing slightly from 37.0 N to 35.3 N as the suspended rope weight falls away. The study also reports a result that qualifies the usual concern: at the stated one-person force limit of 150 N and two-person limit of 300 N, no crossing of either limit occurs within the depths scanned for the principal drum and bucket combinations, because the crank advantage is large relative to the loads considered; the binding design questions are instead layer count and radius growth. A drum sizing rule is therefore given in two parts: a force bound, under which the effective radius on the final layer must not exceed 0.344 m for a 15-litre load under the one-person limit, and a layer rule, under which the smallest drum in the stated set that holds a 20 m lift within two layers has a diameter of 0.15 m, while a single-layer lift of that depth requires a diameter of 0.3 m.