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Title: Design of a Small Cantilevered Sheet: The Sail of Velella velella 
Author: Francis, Lisbeth
Date: 1985-01
Publisher: University of Hawai'i Press
Citation: Francis L. 1985. Design of a small cantilevered sheet: the sail of Velella velella. Pac Sci 39(1): 1-15.
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
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
ISSN: 0030-8870

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