The Vortex Sail, Variable Geometry Proa

by Malcolm Smith

Photos by Jane Smith

Model 2


Model 2 was radio controlled.


Model 2 on the water.


Model 2 had a hull length of 1.2 metres and a main spar length of 2.4m. With this model I attempted to simplify the design of model 1A by using a fixed cross beam and by configuring it as an Atlantic proa (ama to leeward), to reduce the overall width.

I leaned that the centre of effort of the horizontal vortex sail moves a long way forward when sailing upwind, further than I had expected it to. As a result, model 2 had huge lee helm and the forward rudder would stall out in any reasonable strength of wind. Unlike model 1A with it's variable geometry, this variant was not self balancing.

Narrowing the overall beam was also a mistake. Model 2 looked like being much easier to capsize to windward than model 1.

Having the weight of the rig supported by the short ama was another mistake. The ama sat very low in the water and caused a lot of drag, which no doubt also contributed to the lee helm

On the positive side, the model did actually sail and go to windward, and it was surprisingly fast and easy to shunt.

In conclusion, the model worked, but had lots of bugs, and its performance was very pedestrian.

Model 1B - Photos and Notes