A very large part of my scepsism against the flaps below the boom (and even the angled up booms) is derived from this :





Sailing is basically harvesting energy from the wind. If the wind at a lower altitude above the water contains less energy per kg (or volume if you will) then there is less energy to harvest there (less sail drive to produce)

Now compare the energy content at 1 mtr. above the watersurface to that of say 4 mtr above the water surface. The latter being at the centre of the mainsail luff when the boom is 1 mtr above the water.

In laminair winds (below 5 knots wind speed at 10 mtr altitude) the ratio is 1/15th, meaning that 15 times more energy is to be harvested at 4 mtr altitude then at 1 mtr altitude.

In turbulant winds (above 5 knots wind speed at 10 mtr altitude) the ratio is still a tad lower then 2/5th meaning that 2.65 times more energy is to be harvested at 4 mtr altitude then at 1 mtre altitude.

Of course a peice of sail area can never harvest more energy then 100% of the energy content of the given altitude layer (the real theoretical ratio is actually max 52 , but I won't bother you guys with that). So basically the same area of cloth at 4 mtr altitude can be several times more inefficient and then still outproduce the flap in the amount of harvested energy/saildrive. So why have the area (that comes out of the total) being "wasted" in the flap that way.

The only reason to justify that is when the flap reduced the overall drag of the design to such an extent that this amount is larger in absolute terms then the same cloth area placed higher up can compensated by produced sail drive.

I also contest that possibility but that is for a later posting.

By the way, the wind can only blow in two mutually exclusive forms, one being the laminair flow pattern and the other being the turbulant flow pattern. The transition from one to the other is abrupt and complete. The typical borderline between these two forms is found on average at 5 knots of wind (as measured at 10 mtr altitude).

The graph were developped by me based on data found in "High Performance Sailing" by Frank Bethwaite and various documentations that were used during my courses on fluid dynamics. The flow approximation is a 5th order fluid approximation. Reynolds numbers are not of interest at this time as we have explicitly modelled the laminair and turbulant flows. Of course the Reynoulds number is normally used to distinguish between these two situations, but experimental data had already fixed this transition at 5 knots of windspeed at 10 mtr altitude so using the Reynolds number at this time would be pointless.

Wouter