  Debris discs are often modelled assuming compact dust grains, but more and
more evidence for the presence of porous grains is found. We aim at quantifying
the systematic errors introduced when modelling debris discs composed of porous
dust with a disc model assuming spherical, compact grains. We calculate the
optical dust properties derived via the fast, but simple effective medium
theory. The theoretical lower boundary of the size distribution -- the
so-called 'blowout size' -- is compared in the cases of compact and porous
grains. Finally, we simulate observations of hypothetical debris discs with
different porosities and feed them into a fitting procedure using only compact
grains. The deviations of the results for compact grains from the original
model based on porous grains are analysed. We find that the blowout size
increases with increasing grain porosity up to a factor of two. An analytical
approximation function for the blowout size as a function of porosity and
stellar luminosity is derived. The analysis of the geometrical disc set-up,
when constrained by radial profiles, are barely affected by the porosity.
However, the determined minimum grain size and the slope of the grain size
distribution derived using compact grains are significantly overestimated.
Thus, the unexpectedly high ratio of minimum grain size to blowout size found
by previous studies using compact grains can be partially described by dust
grain porosity, although the effect is not strong enough to completely explain
the trend.
