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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?

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  • $\begingroup$ Ever seen the sails Chinese junks used before the 20th century? $\endgroup$
    – Zeiss Ikon
    Oct 15, 2021 at 16:30
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    $\begingroup$ If you want rigid sails, consider a junk (a type of ship, very popular in the Chinese sphere of cultural influence). But the entire point of a wingsail is that it is not rigid. (And anyway, all sails behave like wings on most points of sail. Sails rely on drag only when the ship is running downwind.) $\endgroup$
    – AlexP
    Oct 15, 2021 at 16:32
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    $\begingroup$ You're also going to be able to build a much lighter structure using fabric-covered spars & ribs than even with balsa wood - which isn't all that strong parallel to to the grain. (A problem with any wood construction.) Look at the wings of early (pre-aluminum & composites) airplanes. $\endgroup$
    – jamesqf
    Oct 15, 2021 at 16:57
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    $\begingroup$ @Thesaurus Rex: Planks and canoes? (Really bad puns avoider here :-)) The issue is stability: just trying to stand up in a canoe is an exercise in balance: I hate to think what a tall plank would do. Then if you're going to sail any way but pretty straight downwind, you'll need a keel of some sort. To get a wooden structure that's both light enough and strong enough, you'd probably need some kind of plywood - ever tried to work with say a 4x8 sheet of 3/8 plywood in a good wind? Then maybe with an outrigger on your canoe, it would sort of work. $\endgroup$
    – jamesqf
    Oct 18, 2021 at 2:08
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    $\begingroup$ I want them to stick their viking swords into the water, realize there is lift to be had and then proceed to raid in wing-sail-hydro-foil contraptions made from swords, hide and light wood :-) $\endgroup$
    – bukwyrm
    Oct 19, 2021 at 13:29

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It is a keel

If you really do mean a canoe, then you're putting all this furniture on top of it makes the center of gravity over your head. The sail will be under water and your canoe will be upside-down. So you need to have an outrigger on them. Now you've got a good old fashioned kai 'opua.

Kai 'opua outrigger canoe (Kai 'Opua outrigger)

One thing you will sacrifice with "pre-industrial materials" will be longevity. The sail won't be something you roll up and stow away in the winter. Likely, it will be thatched together palm fronds or another large leaf. You will be knitting these together before each trip and they may last a couple days. Maybe you can roll up and tie a spare bundle of leaves to the outrigger.

As you can see, the sail is an inverted triangle and when you want it stowed, the boom just lifts up to the mast. You will get mixed results with leaf sails however.

As you said, leather is another option for a sail. It would be more permanent and durable, maybe a bit heavier too. Seal skin or another animal should suffice.

In the end, I believe your answer is 'yes.'

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