I calculated attached polars when determining the benefit of the board.   These are reasonably close to actual measured performance with clean hulls in waves 1m or less.  In practice we have been able to get the boat speed up to windspeed when reaching but that is only in smooth water.

The darker line is how the boat is usually sailed - board down when on the wind and board up when true wind lower than 60 degrees.  The fainter lines are with the rudders alone.  Obviously both sets of traces correspond for a given windspeed below 70 degrees.

From these you can assess that with rudders alone best VMG is about 10 degrees lower than with the board.  Also the VMG with rudders alone is more than 15% slower than with the dagger board down.

The limit of the polars to windward is where the rudders are stalled.  That has some relationship with speed.  For example in 10 knots of wind (green traces) there is almost 20 degrees difference in pointing ability between board up or down.  If the rudders stall the boat rounds up and is in irons.  With the Aerorig the only way to recover is shunt back out or start a motor to regain enough speed to steer.  That is why it is necessary to make certain the boat has speed off the wind before coming back into the wind when completing a windward shunt.  It is also possible to get in irons on a downwind shunt if the boat is not brought far enough onto the wind before starting the shunt.  There is a significant tendency for the boat to be turned under the rig as the rig is sheeted in.  There is a large difference in  the location of the centre of effort on the sails from when they are foils to when they are slabs.  During a shunt the slab is presented to the wind and the CoE is a long way behind the mast.  Once sails are set and acting as foils the CoE is as close to the mast as we can allow it for the rig to feather jib windward if the boom sheet is released.

These days we do not try to go to windward without the board down.   It is simply a lot easier to sail to windward when the board is down. 

What this set of polars do not show is the gain in speed by lifting the board once off the wind.  We have actually measured an increase from 8kts to 11kts by lifting the board with the true wind abeam.  It is literally like releasing a sticky hand brake.  Remember this board is providing lift as soon as it is immersed.  The lift is simply a function of speed and comes with a corresponding increase in drag.  There is no point having negative leeway at the expense of speed.

We have not been able to verify the validity of these polars much above 15kts boat speed and 20kts of wind.  There is limited ability to reef the rig and keep the Aerorig balanced.  We have sailed with full rig to 25kts but the mast bends away and unloads the mainsail so rig loses efficiency.  Also wind over 15kts in open water makes waves that slap the cockpit and cabin sole and that increases drag markedly.

Rick

On 03/01/2016, at 9:31 AM, lucjdekeyser@telenet.be [harryproa] <harryproa@yahoogroups.com.au> wrote:

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