Page 170 - Theory and Design of Air Cushion Craft

SES transverse dynamic stability 153

Calculation of craft trim

According to the method in Chapter 5, we can determine the craft trim at various
speeds based on four relationships from equation (5.12) and the other two equations
(5.13, 5.14) due to the deformation of the water surface caused by wave-making of the
craft. This method is rather complicated, especially for the calculation of deformation
of the water surface. We recommend the use of a simplified method for estimating
craft trim and for calculating the forces acting on the craft as follows:

1. In the case of a craft with bag and finger type skirt, the lift of skirt can be
calculated as
when (z b - r bl) ^0 L bs = Q ]
when (z b - t hi) < 0 L bs = P cS cl(z b - f bi)lcos(a b) J (4.14)
where L bs is the lift due to the bow skirt with bag and finger type, z b the gap between
lower tip of bow finger and sidewall base-line (m), t bi the inner draft of sidewalls at
bow (m), and a b the declination angle of bow fingers with horizontal plane, as
shown in Fig. 4.18 (°).
2. In the case of using a planing plate as the stern seal, the planing plate can be
calculated by the theory of Chekhof [43], i.e. as a two-dimensional planing plate
running in gravitational flow, and estimate the lift as follows:
when (z s - f si) ^ 0 L ss = 0 }
2
when (z s - f si) < 0 L ss = Q.5np wv lB ca,[l - Fr\ \n + 4)/2n] J (4.15)
where z s is the gap between the lower tip of the stern seal and base-line (m), f si the
inner draft of sidewall at stern (m), v the craft speed (m/s), L ss the lift acting on
planing plate (N), / the length of wetted plate (m), a s the angle of attack (°) (here
we assume the trim angle is zero, therefore the angle of attack is equal to the angle
between the plate and flow direction).
Since the wetted length is small and Fr is large, the calculation can also be
simplified to a two-dimensional plate in non-gravitational flow as follows:
2
L ss = Q.5np wv lB ca s (4.16)
3. In the case of stern seals of the double bag type (Fig. 4.19(b)), the lift acting on the
skirt can be calculated as
when (z s -; sl ) =*0 L ss = 0 1
when(z s — f si) < 0 L ss = p cB c\t si - z slcosec a s J (4.17)

Craft bottom

Sidewall
baseline

Fig. 4.18 Configuration of bag and finger type bow skirt.