The Vortex Sail, Variable Geometry Proa

by Malcolm Smith

Photos by Jane Smith

Model 4


Model 4, launched June 2003, Bowral, NSW, Australia.


Model 4 Data


Main hull



Radio controlled, 2 channel.

Total Displacement = 0.6kg

Max hull spacing = 0.58m

LWL = 0.9m

WLB = 0.04m

Disp = 0.45kg

LWL = 0.26m

WLB = 0.03m

Disp = 0.15kg

Main spar length = 1.4m

Max depth (chord) = 0.3m

Area (approx) = 0.75sqm

Model 4 is a rudderless design. Three ideas were to be tested with this model:
1) Sail Steering. The boat is steered by varying both the crossbeam angle and the rake angle of the sail. In a similar fashion to model 3, the sail is shunted by tilting the main spar either side of the mid horizontal position. I found with model three that as the sail was shunted,the boat would turn in the direction in which the sail was tilted, in a similar fashion to a steerable kite. I decided to use this feature as the primary steering method, and eliminate the rudders. The sail is most efficient when raked fully forward, so tuning the boat to sail in a straight line with the sail raked near to as much as possible (leaving a few degrees in reserve) is important for best performance. In the limited testing so far, I haven't managed to tune it to my satisfaction. However, the sail steering concept has proved to be a very effective method for steering this type of boat.
2) Heel Control. The model features a sensor paddle, which can be seen on the photo above, attached to the windward side nacelle. The nacelle is attached to and rotates the crossbeam and thus the paddle is at an angle of incidence to the water flow. The paddle is attached by a wire to the sail in order to control the sail's cant angle. When the boat heels to windward, the paddle becomes immersed and is pulled outboard (assuming the boat is moving forward) and the cant angle of the sail is decreased, causing the boat to heel to leeward again. Theoretically it should be possible to tune the boat to sail on just the main hull, with the paddle skipping across the water surface. In the limited testing so far (sailed twice in very light and variable winds), I haven't been able to achieve this level of control, but the paddle sensor system does seem to be effective in controlling the windward heel. I have found however, that varying the cant angle of the sail effects the steering, so steering can become a bit erratic in gusty conditions.
3) Oblique Planing Ama. Based on the theory that the boat was designed to lift the ama clear of the water by heeling to windward, I decided to attempt to simplify the design by using a simple fixed planning surface attached the leeward end of the cross beam as the ama, rather than a more complex rotating ama as used on earlier models. I had assumed that I would be able to balance the boat so that there was very little immersion of the ama in light conditions, thus minimising the drag of the ama in non planing mode. In practise this proved difficult, and at low speeds the drag of the ama is many times that of the main hull. Due to this, I have abandoned any further testing until I have replaced the planning ama with a rotating narrow displacement type ama.

Model 4 detail.


Not much wind. The oblique planing ama causes a lot of drag in these conditions.



Model 4 heeled to windward, sensor paddle controlling the heel angle.

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