I have put some detail together on the dagger board being used on the 18m proa in Melbourne.
The attached shows a recent windward track down Western Port with zoomed detail on one of the better shunts.
This proa has an Aerorig; under the hull rudders measuring 500C with 600D (end-plated at the hull to improve span efficiency when off line) and pivoting dagger board 550C with 2100 immersion. Board cross-section is as shown in attached.
The Aerorig has the limitation that the CoE cannot be moved forward of the mast unless the leading end of the boom is sheeted - a potentially unsafe practice. Hence in normal operation the boat has a strong tendency for weather helm given the large offset between drag and drive lines. All this means that a shunt on the wind requires coming into the wind to lose speed; release the sheet and swing the boom while resetting both rudders for the next tack; building speed off the wind to get steering then bringing back up on the wind to course of best VMG.
As it happens this track provides a good comparison of windward ability with board up versus board down as the first two shunts at the top end of the channel were made before the board was lowered. The performance of the board is not as noticeable on the boat as it is on the track because the boat does not point much higher with the board down. However the leeway is either zero or negative so it is easy to get to where the boat is pointing. Leeway with rudders alone is around 5 degrees. That is despite always setting the trailing rudder to counter the weather helm, meaning it also counters leeway to some degree.
On 01/01/2016, at 3:32 PM, Rick Willoughby rickwill@bigpond.net.au [harryproa] <harryproa@yahoogroups.com.au> wrote:
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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