Automatically prevent double-counting displacement

DELFTship forum Feature requests Automatically prevent double-counting displacement

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    • #35328
      N J Cook

      I have been using Delftship for International 2.4mR design. This is a mini metre rule sloop. The class has a minimum displacement that depends on waterline length. Optimum performance is obtained by getting as close to the minimum permitted displacement without going under. So correct calculation of displacement is vital.

      I have already figured out that the fewer stations there are, the fairer the hull is. So a convenient way to obtain the “bustle” and long stern overhang is to generate two hulls, one passing through the other. This avoids the need to define creaselines or add extra stations to define the complex shape of the “bustle” in the stern. In the attached design, “Hull A” provides the long stern overhang and “Hull B” provides the “bustle” and rudderpost. Also the keel sticks up inside “Hull B” so that it can be moved fore/aft, with its internal ballast, to align LCB with LCG. The top of the keel has to be closed off because, although inside “Hull B”, the program says it will fill up with water.

      When “Hull A”, Hull B”, keel and rudder are all ticked for hydrostatics, the indicated displacement is 292kg … but this is counting the below waterline volume of “Hull A” and that part of the keel that lies inside “HullB”. When hydrostatics is switched off for “Hull A” the indicated displacement drops to 216kg … close to the real value, but missing some displacement from “Hull A” in the stern and still double-counting the volume of the keel inside the hull.

      Of course, the keel-hull intersection can be found and the part inside the hull deleted, but this effectively fixes the keel, preventing easy fore/aft adjustment.

      All these problems would disappear entirely if the program automatically recognised when one part lies inside another and automatically discounted its effect on displacement and on leaks.

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