I actually went back and forth with Paul Larsen and co a bit on their blog comments about this idea. Essentially, back when they were using their larger foil, before they started hacking it down, I told them that they would incur too much drag if they could not control ventilation. In my head, this is because the atmosphere will just keep pouring air down into the wake as fast as the foil can shed it, and this is slow for some reason.
I only say it is slow because on a moth, if you suck air down onto the foil, it climbs up into the low pressure area and sits there until, little by little, it gets scrubbed off. In the interim, lift is reduced, and drag markedly increased.
So I could not see how any foil with a continual connection to the atmosphere would ever go very fast without a LOT of power pushing it.
In the end, they put a fence on it, and it went fast, which I took as vindication for my position. I realize my thinking is very simplistic, and poorly founded in theory, but in interviews Paul has since said that the fence made the difference, and they never changed it a bit after glomming it onto the foil because it worked.
I have no idea whether the base ventilation has anything to do with the lift producing part of the foil, or the fence. But bubbles assuming shapes in low pressure areas is pretty much what kept them from going fast for a long time, I think.
Now that I think about it, a ventilated base bubble open to the atmosphere must have Cp=0 or nearly so. In reality, the bubble will assume whatever shape it has to in order to achieve this Cp=0 over its length.
The closest way to simulate such a bubble in XFOIL is to essentially turn off the wake viscous dissipation with a very large Kdl (I'm using 8.0 now), and shape the rear of the airfoil to get very nearly Cp=0 in the near wake.
I player around with this idea, and came up with the attached BV02. It's heavily based on transonic airfoil technology, with a deep center "spar" section with little loading, and loading in the front and back. The L/D is a ridiculously huge 20:1. This is even with a generous 6.8% thickness, and min(Cp) = -0.18, which should be good to 65 knots, or 80 knots with some modest sweep. Yowsah!
