Page 192 - Introduction to Naval Architecture

178 RESISTANCE

TYPES OF RESISTANCE

When a moving body is near or on the free surface of the fluid, the
pressure variations around it are manifested as waves on the surface.
Energy is needed to maintain these waves and this leads to a resistance.
Also all practical fluids are viscous and movement through them causes
tangential forces opposing the motion. Because of the way in which
they arise the two resistances are known as the wave-making resistance
and the viscous or frictional resistance. The viscosity modifies the flow
around the hull, inhibiting the build up of pressure around the after
end which is predicted for a perfect fluid. This effect leads to what is
sometimes termed viscous pressure resistance or form resistance since it is
dependent on the ship's form. The streamline flow around the hull will
vary in velocity causing local variations in frictional resistance. Where
the hull has sudden changes of section they may not be able to follow
the lines exactly and the flow 'breaks away'. For instance, this will occur
at a transom stern. In breaking away, eddies are formed which absorb
energy and thus cause a resistance. Again because the flow variations
and eddies are created by the particular ship form, this resistance is
sometimes linked to the form resistance. Finally the ship has a number of
appendages. Each has its own characteristic length and it is best to treat
their resistances (they can generate each type of resistance associated
with the hull) separately from that of the main hull. Collectively they
form the appendage resistance.
Because wave-making resistance arises from the waves created and
these are controlled by gravity, whereas frictional resistance is due to the
fluid viscosity, it is to be expected that the Froude and Reynolds'
numbers are important to the two types respectively, as was mentioned
above. Because it is not possible to satisfy both the Froude number and
the Reynolds' number in the model and the ship, the total resistance of
the model cannot be scaled directly to the full scale. Indeed because of
the different scaling of the two components it is not even possible to say
that, if one model has less total resistance than another, a ship based on
the first will have less total resistance than one based on the second. It was
Froude who, realizing this, proposed that the model should be run at the
corresponding Froude number to measure the total resistance, and that
the frictional resistance of the model be calculated and subtracted from
the total. The remainder, or residuary resistance, he scaled to full scale in
proportion to the displacement of the ship to model. To the result he
added an assessment of the skin friction resistance of the ship. The
frictional resistance in each case was based on that of the equivalent flat
plate. Although not theoretically correct this does yield results which are
sufficiently accurate and Froude's approach has provided the basis of
ship model correlations ever since.