A bi-directional cambered dagger board is well suited to a proa. It can be designed to maximise VMG such that its working range for AoA is very narrow. Realistically leeway need vary no more than plus/minus one degree. That means angle of attack is near zero. This enables the edges to be very small radius meaning there is negligible vortex shedding from trailing edge and no risk of ventilating at either edge. So there is little benefit in having it mounted through the hull.
The working range in terms of pointing ability of such a board is quite small. It can be stalled if the boat is pinched up into the wind due to the fine leading edge. If left down off the wind it will act as a brake, hauling the boat sideways to windward due to the camber. In my experience it is unusual to see a sailing boat making a course higher on the wind than it is pointed.
Rudders are a different matter. They need a wide working range. Typically plus/minus 20 degrees. That means they need a large radius leading edge, which is not suited to being bi-directional. Rudders have much greater potential to ventilate when flow detaches at high angle of attack. It makes sense to have them under the hull. In this case they need to be solid with ability to be undamaged when grounded or they have some collision protection system.
On 01/01/2016, at 9:04 AM, "robriley@rocketmail.com [harryproa]" <harryproa@yahoogroups.com.au> wrote:
Of course under the hull they cant ventilate the same and that will add some percentage of efficiency.
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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