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Some pretty ingenious people have tried their hand at novel ways to ease the labor-intensive process to design, loft, and build speedy hull shapes. Examples include tortured plywood, constant camber, cylinder molded, the Kelsall KISS method…. And so forth. All these efforts try to balance fast hull shapes with less man-hours to build them. I hesitate to propose yet another approach but I recently used one to design and construct both the amas and main hull of a 34 foot trimaran. It did save a lot of time, produced a fast shape, and it was compatible with vacuum bagging carbon fiber/nomex honeycomb hull sides. I will try to explain….
Say you want to design an easy to build hull shape. If you look at across sections of good hull designs they often look a lot like a segment of an ellipse or sometimes a parabola. So, we could start by cutting a quarter ellipse out of stiff paper. Make it the right size to form a mid ships hull section. To use this same ellipse cut out to make other hull sections, you need to carry the ends of the ellipse segment somewhat past the keel and sheer positions of the midship section. Call this the “master template” Now, imagine the range of adjustments you can make as you sweep the master template along the length of the hull: you can adjust the position thwartships, move it up or down, and rotate it about the centerline. You can also choose which part of the elliptical segment you want to use at any given hull station. For instance, at the bow where there is very little volume, you would want to use the flattest part of the ellipse section and orient it vertical and close to the centerline. At the stern you may want to use the “curviest” part and rotate it outboard somewhat to increase volume and make a flatter run. After some trial and error on a sheet of drafting paper you can end up with a very attractive set of hull lines that are about 99% as good as drawing the traditional orthogonal set of waterlines, buttock lines, and sections.
So far you might say, “this is no big deal, I saved only 5 hours of drafting time”. But now it’s time to build a male mold and the real savings become apparent. Imagine a hard chine catamaran hull. It has three lines that have to be lofted; the keel, the chine, and the shear. Going to our lines drawing that consists of a family of elliptical sections, what if we superimposed a hard chine hull into elliptical sections so the shear and keel of the two hulls coincide and the chine is located to be tangent to all the elliptical sections in the bilge area. Now for the mold…. We first build the hard chine hull full scale with all the correct tangent points to the elliptical hull sections. It is planked with thin plywood and over planked with 2 inch Styrofoam insulation board. Next make a fairing tool with the exact shape of the ‘master” ellipse scaled up to full size. It needs to be about 8 inches wide and just like the original paper cutout it needs to extend several inches beyond the keel and shear of the hull sections. Now glue some sheets of 40 grit sandpaper to the fairing tool and start rubbing it lengthwise (fore and aft) up and down the hard chine mold. The foam will quickly sand away until you contact the tangent points. There is only one orientation of the tool at any given section that contacts sheer, keel, and chine at the same time. In spite of your best efforts to screw this up, you will produce a perfectly faired round bilge hull shape that is about as good as any you can draft the traditional way. Add a layer of glass to the foam and an epoxy tooling compound and you are ready to go into production – saved perhaps two hundred hours!
The next part of the story is how to mold the hull sides but that is a different subject. I can send pictures of all this once I figure out to post them!