  We present and analyse high-quality SCUBA 850- and 450-um images of 14 local
spiral galaxies, including the detection of dust well out into the extended
disk in many cases. We use these data in conjunction with published far-IR flux
densities from IRAS and ISO, and millimetre-wave measurements from ground-based
facilities to deduce the global properties of the dust in these galaxies; in
particular temperature and mass. We find that simple two-temperature greybody
models of fixed dust emissivity index beta=2 and with typical temperatures of
25 < T(warm) < 40 K and 10 < T(cold) < 20 K provide good fits to the spectral
energy distributions. The dust mass in the cold component correlates with the
mass in atomic hydrogen and the mass in the warm component correlates with the
mass in molecular hydrogen. These results thus fit the simple picture in which
the cold dust is heated predominantly by the interstellar radiation field while
the hot dust is heated predominantly by OB stars in more active regions,
although we argue that there is some mixing. The mean gas-to-dust ratio is
120+/-60, very similar to that found within our own galaxy and roughly a factor
of 10 lower than that derived from IRAS data alone. The gas-to-dust ratios in
the warm, molecular component are on average higher than those in the cold,
atomic component. We compare our modelling results with similar results for
more luminous spiral galaxies selected at far-IR wavelengths and find that
whilst the total dust mass distributions of the two samples are
indistinguishable, they have significantly different dust temperature
distributions in both the warm and cold components. We suggest that this
difference...(abridged)
