  In this contribution, we present the results of the application of a
parameter space reduction methodology based on active subspaces to the hull
hydrodynamic design problem. Several parametric deformations of an initial hull
shape are considered to assess the influence of the shape parameters considered
on the hull total drag. The hull resistance is typically computed by means of
numerical simulations of the hydrodynamic flow past the ship. Given the high
number of parameters involved - which might result in a high number of time
consuming hydrodynamic simulations - assessing whether the parameters space can
be reduced would lead to considerable computational cost reduction. Thus, the
main idea of this work is to employ the active subspaces to identify possible
lower dimensional structures in the parameter space, or to verify the parameter
distribution in the position of the control points. To this end, a fully
automated procedure has been implemented to produce several small shape
perturbations of an original hull CAD geometry which are then used to carry out
high-fidelity flow simulations and collect data for the active subspaces
analysis. To achieve full automation of the open source pipeline described,
both the free form deformation methodology employed for the hull perturbations
and the solver based on unsteady potential flow theory, with fully nonlinear
free surface treatment, are directly interfaced with CAD data structures and
operate using IGES vendor-neutral file formats as input files. The
computational cost of the fluid dynamic simulations is further reduced through
the application of dynamic mode decomposition to reconstruct the steady state
total drag value given only few initial snapshots of the simulation. The active
subspaces analysis is here applied to the geometry of the DTMB-5415 naval
combatant hull, which is a common benchmark in ship hydrodynamics simulations.
