The track only shows pointing ability.  Without the board, the pointing ability is poor but speed a bit faster.  Angle between tracks worse than 120 degrees.   Best VMG with the board is between 100 to 110 degrees but in a narrow channel it makes sense to pinch up a bit to make the best of each shunt.  The board increases VMG by about 15% over rudders alone.  

There is a good deal of history now in the development of this boat.  It originally had only two under hull rudders 900 deep and 450 chord.  These gave reasonable pointing ability however they were not built strong enough for the loads and draft was 1.3m.  Pointing was not as good as with current arrangement and top speed when reaching was lower than now because the rudders are always immersed.

The current rudders can take beaching but suffer abrasion damage on rocky ground - we know this because the boat has been deliberately and accidentally grounded many times now.  

The board has also been replaced since the first trial board.  There were two dagger boards built before my involvement with the boat.  They were initially Tom Speer bi-directional sections formed over a hollow aluminium louvre core.  One was strengthened and section modified closer to the existing section with much sharper edges than a Speer section.  That first board failed a couple of months ago due to the hollow core crushing then board bending - that was sailing at 12 to 15kts into a lumpy 3m swell in 15 to 20kts of wind.  The new board has a marine ply core.  It has a good safety margin at 20kts, a speed I would not expect to see going to windward on this boat.

I like the combination of stout rudders and efficient dagger board.  Draft is less than 1m with board up.  The rudders have ample authority above 3kts motoring or sailing.  The board offers high lift to drag ratio so improves windward ability.  Lifting the board off the wind provides a significant increase in speed.  

Two big rudders near the beams can provide similar performance but the structure required to carry the huge loads adds considerable weight.  The renamed Blind Date , now Compaen, has been refitted with rudders that appear to be up to the task.  Maybe Nol Twight will publish some detail on these.

Loads on rudders and dagger boards for a boat weighing more than a tonne and capable of speeds around 20kts are not trivial.  The design of the structure to resolve those loads needs some care.  For example the board on the 18m proa generates a lateral force of 44kN at zero angle of attack when moving at 20kts.  The force is centred 1m below the surface so is producing significant bending in the board.  It does not surprise me that 100ft monos doing 30kts have problems with dagger board construction. 

One other recent development with the 18m proa is installation of hydraulic steering to enable re-orientation of the wheels.  Given the early stage of installation we are happy with the results.

Rick