An Anchor Sizing and Scope Rule for Small Fishing Boats: Holding in Sand and Mud at 20 and 30 Knots of Wind

This simulated study derives an anchor sizing and scope rule for a 6.0 m open fishing boat, by setting the load the weather imposes on the boat against the holding force an anchor of a given weight can develop in sand and in mud, with holding reduced at short scope. Wind load is computed from a stated windage area and the square of wind speed; a stated current of 1.0 knot adds a constant load of 14.94 kgf; and the steady total is doubled by a stated surge allowance to give a design load of 66.90 kgf in 20 knots of wind and 113.18 kgf in 30 knots. Holding is modelled as anchor weight multiplied by a bottom factor at a scope of 7:1 — 25 in clean sand and 10 in soft mud — multiplied further by a scope factor of 0.40 at 3:1, 0.75 at 5:1, 1.00 at 7:1, and 1.15 at 10:1. At a scope of 7:1 the anchor weight required is 2.68 kg in sand and 6.69 kg in mud at 20 knots, rising to 4.53 kg in sand and 11.32 kg in mud at 30 knots. Scope is worth more than weight in this model: at 30 knots in sand, shortening the scope from 7:1 to 3:1 raises the required weight from 4.53 kg to 11.32 kg, exactly the weight that soft mud demands at full scope, while in mud at 3:1 the requirement reaches 28.30 kg, beyond what such a boat can reasonably carry. The rule is therefore twofold: size the anchor for the worst bottom and the worst weather in the table that the boat will actually face, and treat scope as the first remedy when holding is in doubt, because veering more rode multiplies the holding of the anchor already aboard. The study is simulated, not measured.