If the ridged side is supposed to be the power face, that sharp ridge seems to be a rather primitive and overly aggressive means of preventing flutter via what we (somewhat confusingly) today call camber or dihedral. My first assumption was that the ridged side was the back face.
The ridge must degrade slicing and feathering the blade, and also must have some effect on rolling.
1 Canoe/Kayak Trailer
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