In order to predict the required propulsion power for a ship reliably and
accurately, it is not sufficient to only evaluate the resistance of the hull and the
propeller performance in open water alone. Interaction effects between hull and
propeller can even be a decisive factor in ship powering prediction and design
optimization. The hull-propeller interaction coefficients of effective wake
fraction, thrust deduction factor, and relative rotative efficiency are traditionally
determined by model tests.
Self-propulsion model tests consistently show an increase in effective wake
fractions when using a Kappel propeller (propellers with a tip smoothly curved
towards the suction side of the blade) instead of a propeller with conventional
geometry. The effective wake field, i.e. the propeller inflow when it is running
behind the ship, but excluding the propeller-induced velocities, can not be
measured directly and only its mean value can be determined experimentally
from self-propulsion tests.
In the present work the effective wake field is computed using a hybrid
simulation method, known as RANS-BEM coupling, where the flow around the
ship is computed by numerically solving the Reynolds-averaged Navier–Stokes
equations, while the flow around the propeller is computed by a Boundary
Element Method. The velocities induced by the propeller working behind the
ship are known explicitly in such method, which allows to directly compute the
complete effective flow field by subtracting the induced velocities from the total
velocities. This offers an opportunity for additional insight into hull-propeller
interaction and the propeller’s actual operating condition behind the ship, as
the actual (effective) inflow is computed.
Self-propulsion simulations at model and full scale were carried out for
a bulk carrier, once with a conventional propeller, and once with a Kappel
propeller. However, in contrast to the experimental results, neither a significant
difference in effective wake fraction nor other notable differences in effective
flow were observed in the simulations. It is therefore concluded that the
differences observed in model tests are not due to the different radial load
distributions of the two propellers. One hypothesis is that the differences are
a consequence of the geometry of the vortices shed from the propeller blades.
The shape and alignment of these trailing vortices were modeled in a relatively