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Proa Notes
by Doug Taylor
I wrote the following in 1977 to document my experiences when
building and testing an experimental proa. The design followed about a
year of conceptual sketches and ruminations. I decided that it was time to
check my excursions of the imagination with a real test platform.
This design is of the Micronesian proa type with the ama, (outrigger float), always kept upwind. Tacking upwind is accomplished by changing ends; (the term "shunt" has been adopted). The single mainsail is like a genoa in that the luff contains a jibstay. The tack of the sail slides from one end of the main hull to the other on a small block pulled along a standing cable traveller. Two mainsheets are attached to the clew, one leading to each end of the hull. If a whisker pole is used, it must be disengaged when shunting. The hydrofoil, a surface piercing foil, is carried on the weather side of the ama. In section, the weather side is an arc; the lee side is flat. Steering is accomplished by shifting the center of lateral resistance. Longitudinal placement of the foil ranges the length of the hull by pulling the ama fore and aft on pivoting float booms (aka), like a pantograph. The hydrofoil tilts by means of universal joints at the connections of the booms to ama. Tilted inwards towards the hull at speed, the hydrofoil provides downward resistance to heeling. Let go out from the hull, the hydrofoil may provide upward lift to reduce ama resistance in light air. The float booms pivot on the weather side of the hull, beneath the weather side deck, which acts as an upper stop to their vertical travel. Connections at the ama, as mentioned above, permit the foil to intersect the waterline at 30 to 120 degrees.
SEA TRIAL NOTES, SEPT. 21, 1977 CLEAR LAKE, CALIFORNIA
Sailed Proa this afternoon after modifying the foil tilt arrangement. Temporarily abandoned the tilt adjustment line, and lashed a prominent tilt angle, (15 to 20 degrees in). Sailed about 2 hours. Can be steered as a regular proa with a big paddle, with the aka in perpendicular position. Steers well using only ama swing, could swing smoother, though. The akas bump along under the weather deck. Wind was about 6-7 knots. Noticed appreciable luff bend. Tack shift and clew shift sail works very well. A solid pole for foil tilt control would be good.
CONCLUSIONS TO DATE (1977)
In the design I did not much concern myself with the drag of the ama and foil. I had thought that the ama and foil pulled in toward the bow would result in a close hauled course. However, due to the drag, the actual effect is that a close hauled course is achieved with the akas about perpendicular to the hull. It can even pinch to luffing. Pulling the ama aft of midships slightly produces a steady windward beat without use of the paddle. Pulled way aft it produces a broad reach. Steering closer to a dead run requires steering with the paddle.
The aka thump and slide under the platform when shifting. Sitting in the canoe, on the after deck, helps, as opposed to sitting on the windward platform when sliding the aka forward and aft. The arrangement of the foil tilting lines did not work due to the pull being led to both outrigger shrouds. The ama would flop to either side rather than hold the desired angle. I abandoned this arrangement and simply lashed the outrigger to a selected angle. Lashed so, it will still slide fore and aft without appreciably changing the angle.
To shunt, I found that the best results were obtained by first pulling the ama forward (in the initial direction), and letting go the sheet before it could point up very high. Then I backed to the opposite end of the craft, pulling the sail tack line with me, securing it as soon as I sat down. Then, after setting the sheet for the new direction, I would adjust the ama for the new course. All can be done in about 20 seconds. This could be bettered with practice. The sail tack traveler works well, as does the double, two-purchase sheet arrangement. The strongest wind encountered was 6-7 knots. The ama lifted to its ski shaped bottom but no higher. The foil maintained a steady grip on the water, and I never had to hike out to counterbalance. In this mild wind the mast did bend to leeward some. On the next occasion that I sailed I first tightened the outboard (upper) shroud one notch and slacked the inboard (lower) shroud one notch. I tightened both (somewhat leeward) stays one notch.
POSSIBLE FUTURE IMPROVEMENTS
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