3.3.4 Conclusions
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       After observing the various drogues in the water channel, it was apparent that a cone or a small parachute drogue will collapse when the towline goes slack, as will occur each time the boat passes through the trough of a large storm wave. This behaviour results from the fact that the mass of water in the wake behind the drogue continues to move forward after the towline force has dropped to zero. This wake can collapse and even tumble the drogue.

        There is a long history of drogue failures under storm conditions. It is probable that the alternative filling and collapsing is a major cause of these failures. In a single storm, a drogue can be subjected to as many as 10,000 cycles. The very large parachute drogue and series drogue do not behave in this manner; however, the small conical elements of the series drogue are subjected to some cyclic motions. A section of the full-scale series drogue was subjected to a fatigue test which will be discussed in a later section.