Definition of a wingsail, from Wikipedia:
A wingsail, twin-skin sail or double skin sail is a variable-camber aerodynamic structure that is fitted to a marine vessel in place of conventional sails. Wingsails are analogous to airplane wings, except that they are designed to provide lift on either side to accommodate being on either tack. Whereas wings adjust camber with flaps, wingsails adjust camber with a flexible or jointed structure (for hard wingsails). Wingsails are typically mounted on an unstayed spar—often made of carbon fiber for lightness and strength. The geometry of wingsails provides more lift, and a better lift-to-drag ratio, than traditional sails. Wingsails are more complex and expensive than conventional sails.
Basically, I want to have a pre-sailing culture try to invent a way to harness the wind for their canoes. But why would one use cloth, where the wind would simply move the fabric and not the boat? Maybe something stronger would give a stronger effect. As sort of an in-joke among friends, I want them to experiment with a "sail" made of thin wooden planks; the wood itself can be lightweight, but no more than balsa. The density of balsa wood is 0.11 gm g/cm3, itself significantly lighter than carbon fiber's 1.75–1.93 g/cm3, though in terms of tensile strength carbon fiber is anywhere from 149 - 745 times stronger (depending on the density of the particular balsa)
Hoisting the "sail" in a square-rigged fashion, they launch it at their bay in a decently windy day, resembling a smaller Viking longboat with planked sails, lashed and jointed tightly together until they form a single surface. Pointed directly downwind, how would this craft fare in speed against the same boat with a regular linen sail?
Let's also say that, acknowledging the problem of stowing the sail away when the wind doesn't cooperate, they give the mast the ability to rotate parallel to the wind's direction. Unaware of Bernoulli's principle, the shape and angle of the sail might take on the properties of a primitive wing.
Or...does it at all? Is this arrangement even structurally sound enough, or would the sail be pulled apart by the winds or through some other mechanism of which I'm unaware? If they really thought there was merit behind it, they could also experiment with other materials like sealskin -- do modern materials have to be used in the creation of a wingsail, or would pre-industrial materials also work decently enough to be practical?