  We produce three-dimensional Monte-Carlo radiative transfer models of the
edge-on spiral galaxy NGC 891, a fast-rotating galaxy thought to be an analogue
to the Milky Way. The models contain realistic spiral arms and a fractal
distribution of clumpy dust. We fit our models to Hubble Space Telescope images
corresponding to the B and I bands, using shapelet analysis and a genetic
algorithm to generate 30 statistically best-fitting models. These models have a
strong preference for spirality and clumpiness, with average face-on
attenuation decreasing from 0.24(0.16) to 0.03(0.03) mag in the B(I) band
between 0.5 and 2 radial scale-lengths. Most of the attenuation comes from
small high-density clumps with low (<10%) filling factors. The fraction of dust
in clumps is broadly consistent with results from fitting NGC 891's spectral
energy distribution. Because of scattering effects and the intermixed nature of
the dust and starlight, attenuation is smaller and less wavelength-dependent
than the integrated dust column-density. Our clumpy models typically have
higher attenuation at low inclinations than previous radiative transfer models
using smooth distributions of stars and dust, but similar attenuation at
inclinations above 70 degrees. At all inclinations most clumpy models have less
attenuation than expected from previous estimates based on minimizing scatter
in the Tully-Fisher relation. Mass-to-light ratios are higher and the intrinsic
scatter in the Tully-Fisher relation is larger than previously expected for
galaxies similar to NGC 891. The attenuation curve changes as a function of
inclination, with R_(B,B-I)=A_(B)/E(B-I) increasing by ~0.75 from face-on to
near-edge-on orientations.
