Traditional Pacific Island sails – analysis by Graham Sanborn.

I created a program to help me understand the traditional sails of the canoes of Pacific Islands. These sails seem very effective, yet very simple. So I wanted to understand these sails better. My program currently renders only triangular sails with straight edges, similar to the sails of the Marshall Island canoes of Waan Aelõñ in Majel (Canoes of the Marshall Islands).

The example shown in these videos is an equilateral triangle sail that has straight edges. The top spar is inclined at an angle of 10° from vertical. This example is supposed to be a sail that is filled with wind and the main sheet is pulled enough that the sail makes a 90° arc of the cone. The point of attachment of the main sheet is on a line that would be on the centerline of the canoe’s main hull.

The first 3 videos show the relationship between the sail and the cone that the sail is a slice of. The last 3 videos present a simple analysis of the sail’s shape. The first of these shows the maximum camber and the position of the maximum camber as a percent of the chord’s length. The next shows the angles of the airfoil chords (the “twist angle”) relative to the forward direction of the hull. The last shows the angle of the leading edge of the sail in the horizontal plane relative to the forward direction of the hull.

Most of the triangular sails of the Pacific Island canoes appear to be completely lashed to the top and bottom spar. When they are, it appears that these sails form a conical shape when they fill with wind. The shape is probably not exactly conical. But I suspect that the shape is close enough that a cone is a reasonable approximation. It appears that the airfoil of this sail is created by the conical shape. It is known from a field in mathematics called conic sections that horizontal slices in an inclined cone form elliptical, parabolic, or hyperbolic shapes, depending on the amount of inclination of the code and the shape of the cone. In this case, the cone dips below the horizon, so the conic sections are hyperbolas. Therefore, the airfoils are short hyperbolic arcs.

The sails of the Pacific Islands are fascinating. But I cannot find any technical information about these sails on the internet. I cannot find any analyses of their aerodynamic properties or their design principles. There do not appear to be many how-to guides to help people who want to make them or hints to know if we are using them correctly. Therefore, I have decided to investigate them a little myself.

The relationship between the sail and the cone:

The airfoil shapes, twist angle, and leading edge angle: