How Sails Work

Sails use the hollows (curves) in the edges to

set up tension through the sail, the depth of

the curve is a percentage of the edge length

(the longer the edge the bigger the hollow).

As the corners of the sail are tensioned the

edge hollows try to straighten pulling the

fabric tight.

Hypar's (Four sided twisted sails)

The twisted shape in a sail is not just for

looks, by adding the high and low points into

the sails the shape of the sail also pulls tension through the fabric by the high points trying to pull

the fabric up and the low points trying to pull the fabric down.

This shape also reduces wind loading on the sails by creating a pitch which has less suction force

acting over it, as opposed to a flat sail.

Triangles

A triangle relies purely on the edge hollows to

pull tension through the sail, and thus have

higher setup tensions. The other disadvantage

with triangles is that by the time the hollows

are put into the edges, the amount of sail left is

reduced and thus not a great deal of shade is

achieved, more often than not triangles are

used for visual effect rather than shade.

Source: http://www.peninsulashadesails.com.au/products/shade-sails/how-sails-work