I am just a normal mortal aircraft engineer. Actually you don't need algebra for the basic understanding of the Reynoldsnumber effect. I try to attach some pictures to explain. In the first picture the lift coefficient is plotted over the Reynoldsnumber, in this special case for a windmill section (for general aircraft or foil sections the effect is not that strong). The second pictures shows the effect of Reynoldsnumber on drag coefficent. The Reynoldsnumber itself is the product of the fluids speed passing over the sail/foil/wing times its length divided by the viscosity of the fluid. The smaller the wing the lower the R.number and the slower you move the lower the R.number.
For the lift you can see, that below a certain R.number there is a drop off. In general this drop is off is stronger for thick sections and smaller for thin sections, hence the advantage for a soft sail. Friction drag may raise with low Reynoldsnumber because of the behaviour of the boundary layer, the air which is closest to the body. A common trick to make this boundary layer more stable is to make the surface rough, as done on tennis and golf balls. Again a fabric with sews and overlaping patches might be closer to a tennis ball, than a smooth airfoil.
Since BMWO wing is large and the boat is relative fast, it is less affected, than Ben Halls A-cat or a C-class. Another intersting point is, that the viscosity of water is different than that of air. The Reynolds number of the smallst rudder are larger than of a sail. That's way aircraft foils work fine for this application.
