'Face cup' is is a terrible phrase because misleading. There's camber and there's cup. Cup is
a large camber change over a very short chord length at the trailing edge. Slowly varying camber at
the trailing edge will not have the effect of cup. Why? You have to understand something
about (hydro- or air-) foil theory. A prop blade is a hydrofoil. An airplane wing is an
airfoil. Cupping the trailing edge is like the effect of dropping the aileron on a wing, the lift
on the wing is increased. You can't get the same increase in lift by smoothly cambering the
trailing edge of the wing. The leading edge of an airfoil is completely different than the trailing
edge. At the leading edge, the angle of attack produces the main component of lift.
Camber in the foil section also produces lift. With a surface-piercing
prop, the leading edge must be very thin to reduce drag because each blade enters the water every revolution.
Now, an eddy is created any time there's flow past a sharp leading edge. Just move a paddle in the
water and you'll see what I mean. An eddy shed by an edge is nothing but drag, and that's
what we want to reduce. You can only reduce that by reducing the angle of attack. The
paddle's a rather flat hydrofoil, you can feel the side force (the 'lift') as you move it at nonzero angle of attack.
You can get rid of both the eddy and the side force by moving the paddle blade parallel to the direction
of travel (zero angle of attack). If you put camber in the blade of a paddle or prop then you'll recover
the lift. So 'face cup' is not cup, it's gently-varying camber. If you camber the leading edge
right (with a pitch gauge, and being able to calculate the camber you need) then you can reduce the
leading edge drag and make up for the lost pitch with camber. There's a myth that anything that increases top
speed reduces acceleration, and vv. Correctly cambered, a prop will show an increase in both top speed and
acceleration. That's what I've known how to do since 1978. Tracy Hawkins and Mike Schubert
are running props made like that in F1Sport. Roddy Foreman ran one at Kankakee 2011 in SST 60 (until he broke a piston skirt). Hans is running one
in SST45 now as well. All the factors mentioned above affect the location of 'center of pressure', the average position
on the blade where the side force ('lift') is focused. A center of pressure too near a very thin leading edge
poses an obvious danger, but center of pressure location affects performance as well. PTR, Progress through research,
is Alex Hledin's motto at PTR. It's a damned good motto, we practice ptr too. And it does make a difference to understand physics!

'Dr. Joe'
mccauleyandson.com