Page 132 - Theory and Design of Air Cushion Craft

116 Steady drag forces

dvlv = 0.1 in general, or
Svlv = 0 if no effect of propeller wake on this drag;
v is craft speed (m/s), r the empirical factor considering the effect of shape, r = 5 tic,
where t is foil thickness, S T the area of wetted surface of rudders or foil-like append-
ages (m ) and q w the hydrodynamic head due to craft speed.
This equation is suitable for rudders or other foil-shaped appendages totally
immersed in the water.

Drag of shafts (or quill shafts) and propeller boss [35]

This drag can be written as :
R sh=C sh(d ll, + d 2l 2)q w (3.46)
where R sh is the drag of the shaft (or quill shaft) and boss (N), d } the diameter of the
shaft (or quill shaft) (m), d 2 the diameter of the boss (m), /, the wetted length of shafts
(quill shaft) (m), / 2 the wetted length of the boss (m) and C sh the drag coefficient of the
shaft (quill shaft) and boss. For a perfectly immersed shaft (quill shaft) and boss and
3
5
5.5 X 10 > R Cm> 10 , then it can be written:
3
C sh =l.lsin & h + rcC/ sh (3.47)
where /? sh is the angle between the shaft (quill shaft), boss and entry flow (for stern
buttocks), Cf sh the friction coefficient, which is a function of R e, where
R em = v(l l+l 2)/D
and also includes the roughness factor; for example, if /? sh = 10°-12°, with the shafts
(quill shafts) immersed perfectly in water, then we take Cf sh - 0.02.

Drag of strut palms
According to ref. 34, the drag of strut palms can be written as

= 0.75C pa (V<*)°" y h p (pJ2) v 2 (3.48)
* pa
where R pa is the drag of strut palms (N), y the width of strut palms (m) and d the
thickness of the boundary layer at the strut palms:
6 = 0.0l6x p (m)

where jc p is the distance between the stagnation point of water line and strut palms
(m), hp the thickness of strut palms (m) and C pa the drag coefficient of strut palms,
C pa ^ 0.65.

Drag of non-flush sea-water strainers

According to ref. 34, the drag of non-flush sea-water strainers can be written as

R Q = S 0C 0 (pJ2) v 2 (3.49)
where R 0 is the drag due to non-flush sea-water strainers (N), S 0 the frontal projected