The Coanda/Vortex Sail, Variable Geometry Proa

by Malcolm Smith

VP4 Concept Design

VP5 - Concept Design with Cant Angle Control

Model 1A - Photos and Notes

Model 2 - Photos and Notes

Model 1B - Photos and Notes

Model 3 - Photos and Notes

Model 4 - Photos and Notes


The Coanda/Vortex Proa Concept

The Vortex Sail, Variable Geometry Proa, or 'Vortex Proa' for short, is a sailboat design concept which has evolved slowly since about 1990. The aim of this design is to lower the centre of effort of the sail as far as possible, and to separate the sail and hydrofoil transversely, so that the lift vector of the sail acts directly through the centre of lateral resistance of the hydrofoil, thus negating the heeling moment.

To achieve this aim, the sail is arranged such that the mast is nearly horizontal, and the air flows over the top of the sail, rather than around the sail. The idea is that the sail acts as a 'vortex generator' and produces what is commonly referred to as 'vortex lift'.

Conventional Sails

In a conventional sail arrangement, the air is intended to flow around the sail with streamlines parallel to the water surface. The fluid mass in the airstream local to the sail is deflected sideways as the flow tends to follow the curved surface of the sail. This tendency of a fluid to follow the curved surface is known as the coanda effect. The resultant momentum change produces a reaction force on the sail. The reaction force has a component normal to the airsteam (lift), and there is a second component along the direction of the airstream (drag). There is also additional drag due to friction.

At the tip of any sail or wing, the air will tend to bleed from the high pressure side if the sail to the low pressure side by flowing around the tip. This results in a spiralling, funnel shaped flow pattern downstream of the tip which we call a 'vortex'. The effect of the high pressure air leaking into the low pressure area is to reduce the amount of lift the sail produces. The airflow near the tip has a spanwise component which is directed outward on the the pressure side, and inward on the low pressure side.

Now the greater the chord of the sail is in proportion to its height, or span (the lower the aspect ratio), the more time the air has to bleed across, and the greater the loss in lift. To make up for the loss we can increase the angle of incidence of the low aspect ratio sail until it achieves the same amount of lift as the high aspect ratio sail, but at the cost of higher drag. We are able to increase the angle of incidence of the low aspect ratio sail because it will not stall as easily as the high aspect ratio sail. The drag due to the angle of incidence of the sail is called 'induced drag', and sometimes is refereed to as 'vortex drag'.

Vortex Generators

Although we use the term vortex drag, it is not the vortex flow that causes the drag. Rather, it is the pressure leakage flow around the tip, that sets up the vortex. In fact, vortex generators placed along the leading edge of a wing or along a mast, are sometimes used to improve the lift of the wing or sail by creating vortices to mix higher velocity air above the foil surface with the flow near the surface. They are also used in general fluid mixing applications.

A vortex generator is usually a low aspect ratio foil or flat plate placed at an angle of incidence to the airstream, but can be just any old lump on the surface which will disturb the flow. In order to generate a vortex, you must deflect the airstream normal to the flow direction so that the pressure difference on either side of the vortex generator will cause the airflow to spill over the top, thus starting a vortex. Deflecting the airflow results in a momentum change and, like it or not, you get a lift force and a drag force.

Most vortex generators consist of a flat vertical plate. As the airstream attempts to flow around the tip, it will separate from the foil, leaving a cylindrical eddy on the low pressure side. Because the flow is separated, the lift to drag ratio of the vortex generator is pretty poor compared to a high aspect ratio foil.

Canted Sails

By canting a sail at an angle to the horizontal plane, it is possible to reduce (or increase) the heeling moment. Follow this link for a fuller explanation of the principle of the canted sail.

The Coanda/Vortex Sail

Where the original vortex sail concept differs from the usual vortex generator is that it is intended that the airstream spilling over the top of the foil remains attached to the low pressure side. To achieve this, I have configured the vortex sail as a very low aspect ratio airfoil which is cambered in the spanwise direction rather than along the chord as is normal. In reality, it is just a cambered sail with a mast rake of somewhere between 60 and 90 degrees to the vertical. On the models built so far, the rig is configured as a long spar, the centre of which is freely attached to the top of a short mast. Rigid battens are fixed to the spar to hold the sail in shape.

On later designs, I have use a a flat plate type sail which is a true vortex generator configuration with fully separated flow. No models have yet been buit using this configuration however. The main reason for moving to this configuration is not (I think) for better performance, but because it is a more practical configuration, allowing the sail to be reefed and furled by rolling it around the luff wire. It may also be a cheaper and simpler arrangement to construct.

In either case, the sail is placed to leeward of the hull and is canted to reduce or negate the heeling moment.

Theoretically, a vortex sail is not a particularly efficient sail, but there are advantages to using this configuration.

1. Because there is no heeling moment, the sail can be relatively large for the size and weight of the vessel, thus producing a large force.

2. Because of the downwash produced, the sail takes advantage of ground effect, which helps to improve its efficiency.

3. The sail's lift vector has a vertical component which reduces the displacement of the hulls as the wind speed increases, thus reducing the hull drag.

The diagram below shows the general arrangement of the hull/sail combination as applied to a proa. The double ended proa configuration is ideally suited to this rig as it allows the sail to be a constructed as semi rigid airfoil which changes tack by 'shunting' (reversing direction). Also, the outrigger, ama or log hull, call it what you will, acts as an effective counterweight to the rig which is suspended out to leeward on a movable cross beam. The hydrofoil which counteracts the leeway force can be located on the ama far to windward of the sail, allowing the sail force vector to be applied at the shallowest possible angle. The variable geometry enables the sail force vector acting through the centre of effort (CE) of the sail to be aligned with the centre of lateral resistance (CLR) of the hydrofoil. In theory at least, the arrangement should be self balancing and movable rudders should not be necessary except for manoeuvring.


Diagram showing general principle of operation when sailing to windward.

Model Testing

I have tested the Vortex Proa concept using models, both free running (model 1), and radio controlled (models 2 and 3). My initial concept was to use a horizontal vortex generator as a sail, and models 1A and 2 had the main spar arranged parallel to the water surface. I am now investigating mixed flow variations of the concept with varying degrees of mast rake to improve the efficiency of the rig (model 1B and 3). The more raked version of the sail is now closer in principle to a delta aircraft wing.

Model 1A - Photos and Notes

Model 2 - Photos and Notes

Model 1B - Photos and Notes

Model 3 - Photos and Notes

Model 4 - Photos and Notes

VP4 - Concept Design

VP5 - Concept Design with Cant Angle Control