The rudder, on most boats, would be too far from the mast. You want to shunt
the lightening strike directly to ground (water). If your ground cable or
wire makes any bends/turns of less than ~6" radius, it will jump to
something else. Also if your ground strap runs parallel to other wiring or
life lines, it will arch or jump across.

There is no problem having a copper plate attached to your hull. The
lightning discharges though the edges of the plate, not the surface.
Therefore, you should not fair in the edges or file them down smooth. Leave
them sharp.. There is a formula for computing the linear measurement of the
plate edge perimeter. I found most of my information in the ABYC rules.

Steve
s/v Good Intentions