Still, a nearly 300 pound slab of bronze sheet surely does something down there? I think it does but I'm a hopeful type so I've been doing some experimenting out there. I'm a general tweaker when sailing and pay as much attention to the telltales on my mizzen as the mainsail so of course I'll raise the board off the wind, totally if I sense leeway is eliminated deep off such as on a broad reach.
But it's when the wind is forward of the beam the board should come into play of course. So I've spent some time sailing alone (few would have the patience for this) trying to discern centerboard effect. My unscientific fun has been sailing with the board lowered and raised while "on" the wind. Mostly with full sails, the auto pilot steering an easy compass course (the boat will nearly sail itself in these conditions but the AP maintains the direction, or the orientation of the hull to wind). This gives me time to feel and watch.
While this shot is a little more off the wind, and in plenty of it, it still illustrates the first and simplest thing a centerboard should achieve. Cutting down on the boat slipping to leeward. I have a suspicion that some that have told me they can't tell the difference board up, board down, may not be aware that while the sails are trimmed and drawing nicely, they're slipping to leeward at a greater rate board up.
Afterall, the board is about 5/8" thick, 18" at the cord and reaches nearly 4 feet below the long keel. The upper area of the board is in the keel disturbed flow but at least, say half, 2 feet, is riding alone in clear flow.
I believe I'm able to see this illustrated in the readout from the GPS. Simply, sailing a course close hauled board up, sails trimmed, AP holding a compass course and observing actual GPS track. After a time, lowering the board shows a changing of GPS direction to windward, as you would expect. Little changes though in "feel" as the angle of the sails to wind stays much the same, as does the actual magnetic compass in the binnacle.
From there, the optimist in me could take over. As this fairly apparent direction improvement shows (I'll stab at about 10 degrees?), I can sense a change in speed. More that the boat "may" begin to feel more weatherly, and point a bit higher, This could result in needing to trim now slightly luffing sails, IF, this is so, it could only be the result of "some" lift from the board, despite the old foil less shape and angle.
Does anyone have any experience or data about keel centerboard boats, especially the older styles?