Anonymous wrote:
Eric: no the boards are not toed in. I guess I shall just have to develop my biceps.
I'm not sure what Eric meant there. Drawing a line from the mid point of the LE to the mid point of the TE of the board, it's at an angle of 1.6˚ to the centreline of the boat, so even though the flat outer face of the board is parallel to the centreline, there is actually that amount of toe-in. But this makes the boards harder to get up and down - in theory - because they are feeling more water pressure on them. In practice, it doesn't seem to make any difference. You use the weight of the board to get it down, when it's on the windward side, and the floatation to help to get it up as you tack, but in both cases, you do it when there is no hydrodynamic lifting force being generated.
Bonjour Annie and David
I will take as support the figure joined.
For the cambered sail the figures shows that, in this example, the angle of attack that gives a 0 lift force is - 3°.
So to have no hydrodynamic drift (or opposed drifts between the two lowered dagger-boards) while motoring or sailing downwind the dagger-board should be toed in by 3°.
If you don’t toe in the dagger-board, when you are leaving your morning with a significant current, when you try to lower the dagger-board, you’ll have a lateral force, a drift, that will apply an horizontal torque (the dagger-board can not rotate into the case) on the dagger-board that tends to squeeze the dagger-board (as when you want to rise or lower a dagger-board in a dingy while sailing windward).
On other issue is that, when you are sailing headwind with both dagger-board down, if the dagger-board are parallel, if the drift is, let say 5°, the windward dagger-board (if down) will have an incidence of -5° + the 3° of offset and it will work « inverted » from - 8°. The profile doesn’t work properly at negative angle and it creates drag.
On an asymmetric profile, the reference cord is the cord associated with the O lift. That cord should be parallel to the boat axis.
For SibLim it might be possible to twist the dagger-boards in there cases by having a difference in thickness in the HDPE wedges. I don't know what profile was chosen but normally a few degrees of toes in should be enough.
Eric