Photo 1 drawing various elliptical hull-cross-sections
I have had a number enquiries and criticism regarding the shape and appearance of my MK 2 catamaran. So as a response, I have designed a conventional catamaran-hull which, I gather from these comments, comprises an elliptical cross-section hull, narrow beam, greater freeboard and knife-shape bow, relative the MK 2 hull. I used a method of design that I described in the June 1998 Installment for the circular cross-section MK 2 hull. The conventional hull comprises an elliptical midship-section whereas the waterline curve, number of stations and spacing between them is the same as the MK 2 hull - so that a comparison can be made between it and my circular-section hull.
This elliptical section was selected from a number of ellipses drawn full size as shown in Photo 1, which also shows an easy way to draw an ellipse. Pertinent features of the ellipse - it has a major and a minor axis with their centrepoints forming a right-angle cross at what is known as the centrepoint of the ellipse. It has two foci points which lie on the major axis equidistant from the centrepoint. The distance of a foci from the centrepoint is calculated by the square root of the (major axis squared minus the minor axis squared). The ellipse is drawn using a string of length equal to that of the major axis, with each end fastened to a foci point and stretched tight by a pencil at the apex formed by the string - as in Photo 1. ( in the photo I used a length of fibreglass measuring tape because, unlike string it does not stretch). I selected the inner-most ellipse of the three shown on Photo1 as the midship section for this cat design - labelled EllipseXcat. It has a major axis of 700mm, minor axis of 300mm and foci points 632mm from the ellipse centrepoint. I produced a template of this curve as shown below on the left-hand side of Photo 2.
Photo 1 drawing various elliptical hull-cross-sections
Photo 2 - Sections of the conventional hull EllipseXcat and template.
A full-size outline of the hull may now be produced by for example; firstly using a router guided by the template to cut out the half hull-sections from 12mm particleboard followed by mounting them on a building frame at 500mm centres as shown Photo 1 of the March 2003 installment. A visual outline of the hull maybe obtained by laying four stringers over the sections and extending them forward of section 1 to produce a suitable bow block outline. This setup provides the basis for producing a plug-mould for fibreglass construction of a hull - as shown in the June 1998 installment. But, before proceeding with this, there is much to be gained by using naval-architecture hull-design software to obtain displacement, draft and perspective views of the design. In the following I have use HULLFORM software and entered, vertical and horizontal offsets scaled off the section lines in Photo2. Photo 3 shows a perspective outline of EllipseXcat and Photo 4 shows its hydrostatic parameters.
Photo 3 - perspective view of EllipseXcat (produced by my very old MSDOS version of HULLFORM)
Photo 4 - Hydrostatic data of EllipseXcat and other craft, produced with HULLFORM
MCT provides an answer to why catamarans pitch-pole - for example comparing the MCT of 7.7 for the 14 foot dinghy and 1.4 of WR5600 which is a MCT typical of 14 foot off-beach catamarans. These cats generally have sailrigs of the same size as the dinghy but have bow-buoyancy which is a fraction of the dinghy - so they pitch-pole.
My MK 2 hull has a relatively buoyant-bow and greater MCT than conventional cat hulls.
I acknowledge that my MK 2 cat hull is not nice to look at, but as a result of this design exercise and much thought I have concluded that it is a much more seaworthy hull than conventional cat hulls - I shall present my case for this conclusion in the next installment.