Bandwidth of a yagi is largely controlled by the diameter of the elements. Generally, the larger the diameter, the larger the bandwidth. However, the yagi is also helped by very slightly tapered elements.
The impedance matching network may also affect the SWR bandwidth.
The boom conductivity does not affect bandwidth, as well explained by Phil.
The boom is like two end-fed right-angle fan-dipole elements for some completely different frequency added to the driven element. Elements of a fan dipole for a very off frequency typically do not take much of the Rf current away from the on-frequency resonant element of a fan-dipole.
My guess is that some RF current will travel down the boom (and back), ...
The elements of a Yagi are dipoles. If you look at a dipole at any instant, the voltage at one tip will be some value, and the voltage at the other tip has the same magnitude, but opposite polarity.
So let's say at some instant, the left side is at +50V and the right side is at -50V. Consider these relative to the soil at the base of the mast, if you like. ...