Practical Ship Hydrodynamics

Resistance and propulsion 93
only be 25% to 30% of the total resistance. In model scale, this part is even
smaller. Therefore the measurements of the model resistance must be very
accurate. Resistance of planing hulls strongly depends on the trim of the
vessel. Therefore a careful test set-up is needed to ensure that the model is
towed in the correct direction. The most important problem, however, is the
accurate determination of the wetted surface and the wetted length which is
needed to compute the frictional resistance for both the model and the ship.
The popular use of side photographs are not adequate. Preferably underwater
photographs should be used. In many cases, the accurate measurement of
trim and sinkage may be adequate in combination with hydrostatic computation
of wetted surface and length. As the flotation line of such vessels
strongly depends on speed, proper arrangement of turbulence stimulation is
needed as well.
Depending on the propulsion system, planing vessels will have
appendages like rudders and shafts. For typical twin-screw ships with shafts,
one pair of I-brackets and one pair of V-brackets, the appendage resistance
could account for 10% of the total resistance of the ship. As viscous
resistance is a major part in the appendage resistance and as the Reynolds
number of the appendages will be small for the model in any case or the
appendage may be within the boundary layer of the vessel, only a crude
correlation of the appendage resistance is possible: the resistance of the
appendage is determined in model scale by comparing the resistance of
the model with and without appendages. Then an empirical correction for
transferring the appendage resistance to the full-scale ship is applied. In
many cases, it may be sufficient to perform accurate measurements without
any appendages on the model and then use an empirical estimate for the
appendage resistance.
ž Craft with hydrofoils
Hydrofoils may be used to lift the hull out of the water to reduce resistance.
Besides classical hydrofoils which are lifted completely out of the water and
are fully supported by foil lift, hybrid hydrofoils may be used which are
partially supported by buoyancy and partially by foil lift, e.g. catamarans
with foils between the two hulls. When performing and evaluating resistance
and propulsion tests for such vessels, the following problems have to be kept
in mind:
– The Reynolds number of the foils and struts will always be very low.
Therefore the boundary layer on the foil may become partially laminar.
This will influence the lift and the frictional resistance of the foils in a way
requiring special correlation procedures to compensate at least partially
these scaling errors. The uncertainty level is still estimated as high as 5%
which is definitely higher than for conventional craft.
– Cavitation may occur on the full-scale hydrofoil. This may not only
cause material erosion, but it will also influence the lift and drag of
the foils. Significant cavitation will certainly occur if the foil loading
exceeds 105 N/m 2 . With configurations not fully optimized for cavitation
avoidance, significant cavitation is expected for foil loadings in excess of
6 Ð 104 N/m 2 already. Another important parameter is the vessel’s speed.
Beyond 40 knots, cavitation has to be expected on joints to struts, flaps,
foil tips and other critical parts. At speeds beyond 60 knots, cavitation
on the largest part of the foil has to be expected. When model testing