  We present optical and near infrared observations of the type IIb supernova
(SN) 2011fu from a few days to $\sim300$ d after explosion. The SN presents a
double-peaked light curve (LC) similar to that of SN 1993J, although more
luminous and with a longer cooling phase after the primary peak. The spectral
evolution is also similar to SN 1993J's, with hydrogen dominating the spectra
to $\sim40$ d, then helium gaining strength, and nebular emission lines
appearing from $\sim60$ d post-explosion. The velocities derived from the
P-Cygni absorptions are overall similar to those of other type IIb SNe. We have
found a strong similarity between the oxygen and magnesium line profiles at
late times, which suggests that these lines are forming at the same location
within the ejecta. The hydrodynamical modelling of the pseudo-bolometric LC and
the observed photospheric velocities suggest that SN 2011fu was the explosion
of an extended star ($\rm R\sim450$ R$_\odot$), in which 1.3 $\times 10^{51}$
erg of kinetic energy were released and 0.15 M$_{\rm \odot}$ of $^{56}$Ni were
synthesised. In addition, a better reproduction of the observed early
pseudo-bolometric LC is achieved if a more massive H-rich-envelope than for
other type IIb SNe is considered (0.3 M$_{\rm \odot}$). The hydrodynamical
modelling of the LC and the comparison of our late-time spectra with nebular
spectral models for type IIb SNe, point to a progenitor for SN 2011fu with a
ZAMS mass of 13-18 M$_{\rm \odot}$.

Solar and Stellar Astrophysics;High Energy Astrophysical Phenomena