One of the advantages of a Yagi is its directivity. When mounted
horizontally, its horizontal pattern is quite directional, but the
pattern in the vertical plane is relatively broad -- the vertical
pattern of a typical HF Yagi in the forward direction, in fact, isn't
much different from a single dipole. If you mount the Yagi vertically,
the two patterns swap so the horizontal pattern ends up very much
broader than when mounted vertically.

A vertically mounted Yagi is vertically polarized, and vertically
polarized radiation reacts differently with the ground than horizontally
polarized radiation. So rotating a moderately or very high yagi from
horizontal to vertical will result in loss of low angle radiation unless
your ground is very highly conductive.

These effects can be very easily be seen by modeling.

Roy Lewallen, W7EL

John Ferrell wrote:
Why do we always mount our HF Yagi's in the horizontal plane?
Antenna maintenance would be much easier for those of us with tilt
over towers if the antenna were in the horizontal plane when down for
maintenance!

John Ferrell W8CCW