  We investigate the effects of the competition between exchange ($J$) and
dipolar ($D$) interactions on the magnetization reversal mechanisms of
ferromagnetic nanotubes. Using first atomistic Monte Carlo (MC) simulations for
a model with Heisenberg spins on a cylindrical surface, we compute hysteresis
loops for a wide range of the $\gamma=D/J$ parameter, characterizing the
reversal behavior in terms of the cylindrical magnetization components and the
vorticity parameter along the tube length. For $\gamma$'s close to the value
for which helical (H) states are stable at zero applied field, we show that the
hysteresis loops can occur in four different classes that are combinations of
two reversal modes with well-differentiated coercivities with probabilities
that depend on the tube length and radius. This variety in the reversal modes
is found to be linked to the metastability of the $H$ states during the
reversal that induce different paths followed along the energy landscape as the
field is changed. We further demonstrate that reversal by either of the two
modes can be induced by tailoring the nanotube initial state so that vortices
with equal or contrary chirality are formed at the ends, thus achieving low or
high coercive fields at will without changing $\gamma$. Finally, the results of
additional micromagnetic simulations performed on tubes with similar aspect
ratio show that dual switching modes and its tailoring can also be observed in
tubes with more microscopic dimensions.
