I think it is pretty apparent that upwind performance is hurt by a shallow keel, but my question is HOW MUCH. Is it a little bit of pointing and VMG lost, like 10%, or is it much more?
Most of these shoal draft keels lack a NACA profile (haha I think even the standard fin keels generally lack this but less so) and are pretty thick, so I wouldn't be surprised if the difference in VMG is pretty high. But nautical engineering seems to be an art as much as a science sometimes...
I'll take a whack at your question, "HOW MUCH" on shoal vs deep draft.
If you're comparing the same boat with shoal/deep draft options, I'd guess it would be no more than 5 degrees, average. I base this on many years with a full keel centerboard boat that has 4' and 8' of draft with a simple flat plate bronze board.
There's little difference(if at all at) in how the boat feels board up vs board down. But you can often see the increase in your GPS track over the ground. Sailing to windward, a 5 degree average windward increase, sometimes more, sometimes less, is what I see.
Another thing I base my guess on, my friend has a J35. It points higher than my 1961 centerboarder
. How much higher? I'd guess in 15 knots, sailing alongside, about 10 degrees. That's a lot.
Does that mean if I bolted his 7' foil shaped deep keel on my 1961 yawl, I'd point 10 degrees higher? No.
It's a combination of all the design differences in the two boats, not just one factor like draft(although sailboat hype may want you to believe it's one "new" advance).
Expect a boat with deep draft to sail a few degrees higher than the same with shoal draft.