My brown "Novathene" Polytarp sail is laid up with the warp and weft at 45 degrees to an imaginary straight line along the leach between the two spar tips. This orientation allows the fabric to distort freely in both directions, across the sail and down its length, opening up the leech and at the same time accommodating the variable luff curvature induced by the flex in the luff spar and boom without adversely affecting sail shape. The loose bias of this cloth shears and stretches to create a surprisingly fair curve with no induced shape sewn into the panels as we see in a modern sail.
The two spars are made from 4 scavenged windsurfer (sailboard) masts.
The luff spar is constructed with a sleeve joining the bases of two of the fibre glass masts resulting in the tapers running out to the tips top and bottom.
The boom is one mast sleeved into the other with the tapered end to the clew. The angle of intersection at the tack is 60 degrees making the sail roughly an equilateral triangle though to be traditional the Micronesians make their booms slightly longer that the main hull and the luff spar and mast slightly shorter and of roughly equal length.
You will notice that there no empirical rules in any of my commentary only a "rule of thumb" which irritates purists and control types terribly, this rule of thumb also applies to the hull shape.
I have made my connection between luff spar and boom at the tack connect by means of a very small jaw made of marine ply scarfed into the tube section of the boom which forms an open crutch against the luff spar. The jaw is held in place by the luff and foot tension exerted by the sail when it is bent on with all its supporting lashings to the boom.
I'll illustrate these details when I have more time.
I'm not to sure how I can expand any further on the Kiribati Dimple idea if it is not made clear from my chapter in my dissertation with comments contributed by some true experts in my later posts on the subject.
There is no science yet to support my hypothesis on the subject of hull asymmetry. I can only speak from experience the publication of which again has left my ideas open to derision and doubt from some surprisingly bright people.
"Hull asymmetry takes various forms throughout the Pacific. In Micronesia, where it was most highly developed, the backbone of the canoe is bent in two directions during construction. In other instances keels are straight but sides are built with parabolic cross sections, one side made rounder than the other and featuring a shallow concave indentation in the lee side of the hull below the waterline. Michael Toy designed the hulls Takapu and Toroa with this feature. He understood that asymmetric hull curvature works as a hydrofoil designed to counteract leeway. My personal experience is that, as the vessel gathers speed, it begins to make ground to windward to the extent that I always have to head below my objective so as not to over shoot the mark."
I drew this illustration showing the sections of the hull of Takapu.
The sections are derived from the plans as drawn in 1977 by Mike Toy. The keel is straight, the volume of the windward side is perhaps twice that of the leeward.
You will see that section 5, the mid bulkhead frame is narrower than the preceding section 4 (and following section six, remembering that the hull is a mirror of itself end to end)
I now also believe that the dimple reduces drag by mitigating the secondary, lee side shock wave and the feature also assists with maintaining directional stability in a following sea.
How much asymmetry? Not too much and not too little!
|Takapu the model: fish eye view|