As we know, double x chromosomes are never wrong and SWMBO pointed out that masts extend above the waterline and KEELS extend below it (remember Australia 2 infamous winged keel).
She's not right is she? 
Well, when foiling, the mast is actually above the water line...
If you were slogging along without getting onto a plane, I guess it would feel natural to call it a keel.
I think once you're foiling the whole deal is no longer a boat, it's an aircraft. So I'll go with vertical stabilizer.
This confusion is even worse for windfoiling. Sail mast or foil mast? Huh?
Some people use the word "strut" for the foil mast.
