Design of a Small Cantilevered Sheet: The Sail of Velella velella

Abstract

The upright sail of the sailing hydrozoan Velella velella is supported by a very thin cantilevered sheet of colorless and transparent chitinous material. The skeletal material is a layered fibrous composite that is similar structurally to arthropod exoskeleton; but the appearance and mechanical properties (breaking stress, breaking strain, and stiffness or Young's modulus) are more similar to vertebrate hyaline cartilage. Since the homologous perisarc of some sessile hydroid species is both stiffer and stronger, the Velella skeletal material probably has not been selected evolutionarily for extreme strength or stiffness. Several specific design features make this thin cantilevered sheet of relatively floppy material a suitable support for Velella's permanent sail. The sail sheet is thicker than the rest of the skeleton, and is further reinforced by two overlapping patterns of raised ridges. The sheet is triangular-to-semicircular, and this tapering shape provides a larger cross section of material at the base to resist the greater bending moment there. A three-dimensional curve at the insertion line between sail and float provides more flexural stiffness, further reducing the tendency for the sheet to fold at the base. Consequently, the sail bends smoothly and progressively under an increasing load and quickly returns to the upright position when unloaded, rather than curling or kinking at the bottom. This, plus some tilting of the whole animal, may reduce stress on the sail in heavy gusting winds.