Arto
One board in the middle. It swings up and down longitudinally and is braced against the centre longitudinal beam of the deck. When up it parks under the deck. The board pivots on one end at the deck level. It has a stop to prevent the top swinging above the deck on the shear pin end but if it hits something and the shear pin goes there is room in the deck slot for it to swing up level with the bottom of the lee hull before it damages the deck. Overall it is 3m long. The brace comes out at 40 degrees 150mm below the water level. So load paths are simple and straight forward although the board sees quite high bending loads.
It is symmetrical fore and aft but cambered. It has relatively sharp edges because its angle of attack does not change much. So there is no noticeable vortex shedding.
The board is either up or down. Off the wind it is swung out of the water to get best speed rather than pointing ability. We discovered by chance that there could be merit in being able to swing it off centre longitudinally a little when sailing because then it is possible to shift the CLR and balance the boat better. So this is a possible solution to the lack of adjustment with the aerorig i.e. shift the CLR rather than the CoE to get balance. That is much the same as Rob promotes with the twin rudders. Leave the trailing one down and lift the leading one. Overall my preference for improving balance is the schooner rig because it gives the ability to sail forward out of irons rather than having to backtrack. I do not know if a big rudder provide enough lateral resistance when not moving to give the boat lee helm.
Rick
On 10/03/2014, at 7:25 PM, Arto Hakkarainen <ahakkara@yahoo.com> wrote:
Is the daggerboard symmetrical? And do you have one or two boards?Arto
On Sunday, March 9, 2014 8:06 AM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
I have added a couple of proa videos on Youtube.
This link shows the 18m proa in light wind sailing with new dagger board down:
http://www.youtube.com/watch?v=Q7Bx9J_tM7w
It gives an indication of pointing ability compared to an older style mono. The board is 2m deep and 500mm chord. It has a thickness of 80mm and a camber of 6%. The proa tracks true when sailing around 60 degrees to true wind. Higher than this there is a little leeway and below this the board is creeping the boat to windward. The board is therefore lifted off the wind.
This one shows the proa at full power on the electric drives:
http://www.youtube.com/watch?v=IUTefb7-7ME
In calm conditions and clean hulls it will reach 8.5kts but it could only do that for 20 minutes. At the 2 hour rate it does 5 knots.
Dean and I continue to play with his model proa:
http://www.youtube.com/watch?v=6G7irmQgTPI
It is now about as developed as needs be to scale up. It is well balanced and responsive. Also gets to windward quite well. In strong winds we have had both hulls flying by increasing the sail cant but there is no independent sheet control on the model to allow the cant to be continuously adjusted when sailing.
Rick
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