3.4.1 Introduction
Back    Forward    Index

         An important objective of the program is to develop a reasonably accurate method of estimating the load imposed on a specific boat and drogue system by a breaking wave strike. From the tests reported in reference 1, it was determined that the drogue load was made up of three major components:

  1. The inertia load as the boat is decelerated by the drogue.
  2. The horizontal component of the buoyancy force when the boat is riding on the sloping wave face.
  3. The hull drag force resulting from the boat being pulled through the fast-moving water of the breaking wave crest.

A diagram of these forces is shown on Figure 10.

        The tests described in this section are specifically directed toward defining the hull drag during a breaking wave strike. The result will be used as input into a computer simulation of a boat/drogue system during a breaking wave strike. The computer program will be documented in a subsequent section of this report.

        In a typical event, the boat will be pulled up to the wave crest by the drogue and then pulled through the moving water of the crest. For a wave with a wave length of 300 feet, the water in the breaking crest will be moving near wave phase speed or 39 ft/sec. Computer simulation indicates that a drogue may decelerate the boat to a speed of approximately 15 ft/sec. For a 30-foot boat with a water line length of 22 feet, this relative velocity of 24 ft/sec would represent a speed/length ratio (knots/ water line length) of 3, far above the hull speed. There are no published data on the drag of sailing yacht hulls in this speed range. Therefore these tests were run to obtain drag values to use in the computer simulation. Since the goal of the computer program is to predict gross loads and general behaviour only, the accuracy sought in the drag measurements is on the order of + 10%.

Top