Judging a ship by its anchor: Bronze age stone weight anchors as indicators of ship size

Abstract

Three scale models of a one-hole stone anchor found at the Uluburun shipwreck site were tested in the sea, with their dragging force history being monitored and recorded. It was found that on a sandy seabed the measured holding power of an anchor is about 37 % of its dry weight, and about 49 % of its dry weight on a rocky seabed. The results were applied to the Ma‘agan Mikhael II replica ship, and the forces acting on the ship, in different sea conditions, were compared to the holding power of the anchor. Wind velocity of 31.1 knots (16 m/s) and a current velocity of 2.9 knots (1.5 m/s), were chosen as representing extreme sea conditions. The forces acting on an anchored Ma‘agan Mikhael II ship under these conditions were calculated and extrapolated for ships of different dimensions, but with the same structure and proportions as the Ma‘agan Mikhael II using the replica scaling relations. These were compared to the holding power of the anchor, and the results can create a model that estimates the maximum size of the ship (as represented by its cargo capacity) by the mass (weight) of the anchor. This model estimates the size of a ship using a single one-hole stone anchor.