Thanks to Paul for posting those excellent videos.
I was looking at the 2nd one - Atum in 11knts breeze. It got me thinking about the whole T-foil thing. Now I may well have got this wrong, but I believe the theory goes that the downward displaced water under the hull rises again behind the transom, and in doing so exerts a force on the T-foil resulting in a bit of forwards propulsion. But looking at the video, I'm not sure I buy that. I think Atum's T-foil is pretty close to the bottom of the rudder blade - is that right? So presumably something approaching a metre under the water's surface. It doesn't look like the hull could possibly be doing much to the water 1 to 2 metres below the hull skin, I would say 50-60cm at the very most. So unless it's a + Foil with the element close to the surface, all it can be doing is taking a bit of displacement out of the hull, thus exchanging a bit of hull form drag for a slightly more efficient foil form drag, plus the extra skin friction of the foil. Can somebody enlighten me?