DSK wrote:

My question defines an area bounded by for sides, a quadrangle, I
suppose I should said "plane geometric shape" rather than "triangle",
sorry for the confusion.

Walt wrote:

OK. If you mean "polygon" please say "polygon".

Isn't a triangle a polygon?

Yes it is.

Anyway, what is the point in using plane geometry to analyse sails? They
are not flat.

Bingo. We've got a winner. Sails aren't triangles. Or Quadralaterals.
Or any other object from plane geometry.

One way to maximize sail area would be to make the sails more spherical,
with boundaries defined by the limits of the above-mentioned polygon.

Cool. Then you could make them out of leather from your spherical cow.

//Walt