@Ronald Raygun
The exactly-at-windspeed scenario is a bit of a crazy special case ...

small snip

Therefore the presumption that the wheels must generate 1hp
and the prop must therefore provide 10lb of thrust is a bit silly,

You did notice that it was *you* who brought up the "crazy special
case" of exactly at wind speed. :-)

It is your own special case that has brought into play the seemingly
odd (but true) case of the 100% efficient propeller which can produce
10lbs of force on 0hp.

The way the term "efficiency" is applied in any given situation can be
a bit arbitrary. As I previously mentioned, propellers and fans use
the term very differently because they have very different
applications and thus very different definitions of "work". What is
most important is that the the term and it's relevent formula be used
consistently throughout any given. We can't use one definition at 2x
wind speed and another at exactly wind speed. I have been consistent
in my application.

It just so happens that the standard propeller efficiency formula
turns up looking a bit odd (but still valid) in static conditions.