” A submarine can go faster under water than riding on the surface - this is the theory. And so Dreher designs to keep the spoon blade just at the surface.

If you keep the blade deep then it is more effective.

The C2 has extra buoyancy in the blade and it tends to bob up and down a lot. By keeping the oar deeper, if the oar stays under the surface it doesn’t have chance to catch air in front of it or mound water in front of the blade which is better.”

So there you are, the answer about why Dreher sweeps and sculls are designed to sit squared at the water’s surface depth.