Just to stifle any myths that might arise from this discussion,
"shocks", a slang term for "shock waves", propagate in a medium at the
same speed as other waves propagate in that medium. Putting quibbles
(e.g. nonlinearities) aside, shock waves propagate with the speed of
sound in air. One may call a phenomenon associated with a bullet moving
in a gun barrel a shock wave, but that doesn't make it so. What happens
in a gun barrel has little to do with wave propagation in a homogeneous
medium like the atmosphere.
Another myth (the origin of which is unknown to me) is that the shock
wave associated with supersonic flight is closely related to the
vee-shaped bow wake associated with, for example, a duck swimming in a
lake*. Some textbooks have even enshrined this myth in problems at the
end of a chapter, giving the observed half angle of a boat wake and
asking the student to calculate the speed of the boat!
Boat wakes are much more complicated than shock waves in air. They were
first partially explained by Lord Kelvin, and the vee-shaped component
is called the "Kelvin wake", though as you will see from the photo
referred to below, there is much more structure to it than that. The
vee-shaped component is geometrical in nature. The Kelvin wake always
opens at 19.5 degrees half angle, regardless of the boat's (constant)
speed.
Derivation of the 19.5 degree vee can be found in a (small) number of
texts, and I didn't find a good one on the web in a cursory googling. I
did find a good posting** by a competent authority, Paul Filman, which
would make a good entry point for anyone who wants to pursue the
question of boat wakes farther. The whole story is, as Filman points
out, very complex, and some aspects of it are not understood.