  We present new and stronger evidence for a previously reported relationship
between galactic spiral arm pitch angle P (a measure of the tightness of spiral
structure) and the mass M_BH of a disk galaxy's nuclear supermassive black hole
(SMBH). We use an improved method to accurately measure the spiral arm pitch
angle in disk galaxies to generate quantitative data on this morphological
feature for 34 galaxies with directly measured black hole masses. We find a
relation of log(M/M_sun) = (8.21 +/- 0.16) - (0.062 +/- 0.009)P. This method is
compared with other means of estimating black hole mass to determine its
effectiveness and usefulness relative to other existing relations. We argue
that such a relationship is predicted by leading theories of spiral structure
in disk galaxies, including the density wave theory. We propose this
relationship as a tool for estimating SMBH masses in disk galaxies. This tool
is potentially superior when compared to other methods for this class of galaxy
and has the advantage of being unambiguously measurable from imaging data
alone.
