The Vortex Sail, Variable Geometry Proa

by Malcolm Smith

Photos by Jane Smith

Model 1B

Model 1B on the water

I recently revisited Model 1 to try out some modifications to the rig and hull. The mast supporting the main spar was lengthened, raising the rig. The sail area was increased by deepening the sail in the centre, making it more triangular in planform. I made a new ama with more volume than he original one, and removed the hook shaped hydrofoil, replacing it with two short vertical foils.

Now when the boat is tacked, the new forward end of the spar is dropped down towards the water. The sail is now a mixed flow sail rather than a pure vortex sail. It looks slightly more conventional, and has something in common with the lanteen, crab claw and Gibbon's style of rig.

When I sailed the modified model, I found that the raised C of G, and the incresed overall weight caused the short hulled, low freeboard boat to pitch badly. So I then increased the length of the main hull to 530mm, and increased the freeboard at the ends. With the main hull modified, the model now sails very well.

Giving the main spar some rake has had a dramatic effect on the location of the centre of effort (CE). It now remains much closer to the geometric centre of the sail, rather than moving well forward as it did when the sail was horizontal. It also tracks upwind much better than it did before, when variations in wind strength and direction would cause the CE to shift wildly.

The model will sail upwind at an angle of 45 degrees to the true wind, which is not bad for such a small model. Downwind it is still very fast.

 

Model 1B viewed from windward

 

Model 1B viewed from leeward

 

Postion of sail and crossbeam for upwind sailing

 

Postion of sail and crossbeam for downwind sailing

 

The model still tends to take off if hit by a strong gust

Model 3 Photos and Notes