Page 202 - Introduction to Naval Architecture

188 RESISTANCE

Eddy making resistance or viscous pressure resistance

In a non-viscous fluid the lines of flow past a body close in behind it
creating pressures which balance out those acting on the forward part
of the body. With viscosity, this does not happen completely and the
pressure forces on the after body are less than those on the fore body,
Also where there are rapid changes of section the flow breaks away from
the hull and eddies are created.
The effects can be minimized by streamlining the body shape so that
changes of section are more gradual. However, a typical ship has many
features which are likely to generate eddies. Transom sterns and stern
frames are examples. Other eddy creators can be appendages such as
the bilge keels, rudders and so on. Bilge keels are aligned with the
smooth water flow lines, as determined in a circulating water channel,
to minimize the effect. At other loadings and when the ship is in waves
the bilge keels are likely to create eddies. Similarly rudders are made as
streamlined as possible and breakdown of flow around them is delayed
by this means until they are put over to fairly large angles. In multi-shall
ships the shaft bracket arms are produced widi streamlined sections
and are aligned with die local flow. This is important not only for
resistance but to improve the flow of water into the propellers.
Flow break away can occur on an apparently well rounded form. This
is due to die velocity and pressure distribution in the boundary layer.
The velocity increases where the pressure decreases and vice versa.
Bearing in mind that the water is already moving slowly close into the
hull, the pressure increase towards the stern can bring the water to a
standstill or even cause a reverse flow to occur. That is the water begins
to move ahead relative to the ship. Under these conditions separation
occurs. The effect is more pronounced with steep pressure gradients
which are associated with full forms.

Appendage resistance

Appendages include rudders, bilge keels, shaft brackets and bossings,
and stabilizers. Each appendage has its own characteristic length and
therefore, if attached to the model, would be running at an effective
Reynolds' number different from that of the main model. Thus,
although obeying the same scaling laws, its resistance would scale
differently to the full scale. That is why resistance models are run
naked. This means that some allowance must be made for the
resistance of appendages to give the total ship resistance. The
allowances can be obtained by testing appendages separately and
scaling to the ship. Fortunately the overall additions are generally