Ben
I did it but cannot recall if I published the comparison. There was not much difference in the polars. VMG does not change measurably.
From a sailing perspective having ww hull to lee is significant benefit in light winds because the boat automatically bears away when the rudders stall so it is self-correcting. Sailing ww hull to windward the boat rounds up when they stall and there is no way to recover other than shunting out of it.
The rudders stall around 2.5kts sailing either way around. In 5kt wind that occurs at 50 degrees to true wind. In present form with the two 900mm deep rudders the best VMG is at 55 degrees. So there is 5 degrees to play with. Momentary lack of concentration, wind shift or wave can cause it to round up.
In practice it is very easy to sail it at best VMG with ww hull to lee. With ww hull to windward it needs to be sailed freer than best VMG to avoid rounding up.
The polars show where the rudders stall and the angle for best VMG but they do not show the consequence of stalling the rudders.
The other factor I have not yet included in the VPP is the loss of span efficiency when the rudders lose the end-plate effect of the hull due to it being round section. This is most noticeable when sailing faster because there is a position of instability compounded by backlash in the two gearboxes in the steering linkages.
I have done a number of comparisons for different rudders and concluded that going to 1.2m deep but shorter and thicker would improve windward performance. However that does not completely overcome the rounding up issue but it occurs closer to the wind.
The rounding-up issue is simply something to be aware of. We have made lots of "Z" patterns over the Bay trying to sail to windward by getting in irons and needing to back out. It is nowhere near as inconvenient as trying to get some beach cats to tack through the wind - they are not intended to go backwards. This nuance with the proa is a blessing in light winds with ww hull to lee because the boat essentially sails itself.
Rick
On 14/07/2012, at 11:49 PM, bjarthur123 wrote:
rick,
you promised us a VPP polar for a harryproa sailing with the rig to windward a few months back. did i miss the post or is it sill forthcoming? i'm very curious to see the results, as it sounds like it could be faster for light winds.
thanks,
ben
Rick Willoughby
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