Page 145 - Theory and Design of Air Cushion Craft

128 Steady drag forces

Fig. 3.40 Sketch of skirts with bag and jetted extensions.

Fig. 3.41 Rigid stern seal with the function of controlling the air gap.

trials on water in 1970, it was found that a large amount of drag also acted on the
stern, and as a result it was difficult for the craft to pass through hump speed. This
may be traced to the following reasons:
(a) Suppose the stern seal was balanced hydrodynamically so that the lift moment
(about point B in Fig. 3.43) due to the rear part of the seal would be greater
than that due to the fore part of the seal. The stern seal plate would assume a
negative angle of attack with the flexible nylon cloth taking the form of a con-
cave bucket as shown by line 2 in Fig. 3.43. This would lead to a large amount
of stern seal drag and it would be difficult for the craft to take off.
(b) On the other hand, if the hinge of the stern seal were moved to the rear with a
longer nylon cloth, then it would take up the form of line 2 in Fig. 3.43. Here
although the lift moment of the fore part would be greater than that of the rear
part, forming a positive angle of attack, the planing surface is discontinuous,
which would lead to a large amount of drag. In such a case the drag of the
stern seal would be so large that it would be impossible to take off. The nor-
mal form of the stern seal is as shown in line 1 of Fig. 3.43 with proper length
of nylon cloth to combine with the proper position of the hinge. In this case it
is easy for the craft to take off.
6. The SES version 719 with bag and finger type skirt for bow seal and twin bag for