Page 131 - Theory and Design of Air Cushion Craft

Underwater appendage drag 115

0 0.5 1.0 1.5 2.0 Fr,=v/Sgl c

Fig. 3.30 Residual drag coefficient of sidewall as a function of LJB^ and Froude number.

3.10 Hydrodynamic momentum drag due to engine
cooling water

In general, the main engines mounted on SES have to be cooled by sea water which
is ingested from Kingston valves or sea water scoops mounted at propeller brackets,
via the cooling water system, then pumped out from sidewalls in a transverse direc-
tion. The hydrodynamic momentum drag due to the cooling water can be written as

R™ = /> w V. } G W (3.44)
is the hydrodynamic momentum drag due to the cooling water for engines
where R mvf
(N), Fj the speed of inlet water, in general it can be taken as craft speed (m/s), and g w
the flow rate of cooling water (m/s).

3,11 Underwater appendage drag

Drag due to rudders, etc.

Drag due to rudders and other foil-shaped appendages, such as plates preventing air
ingestion, propeller and shafts brackets, etc. can be written as [34]:
(3.45)
R t=C f[(\+Sv/vY(l+r)S Iq v
where R T is the drag due to the rudder and foil-shaped propeller and shaft brackets (N),
C fr the friction coefficient, which is a function of Re and the roughness coefficient of the
rudder surface. In this case Re = (vc/u) where c is the chord length of rudders or other
foil-like appendages (m), dvlv is the factor considering the influence of propeller wake: