PROJECT WINDRIGGER - June 1998 installment

by Ian E. Smith, 12 Sproxtons Lane, Nelligen, NSW 2536, Australia
e-mail to: smithvanaalst@bigpond.com

Back to Yacht Research Homepage | Previous page | Next page

WINDRIGGER MK 2 SECTIONS LOCATION

Top view:

Referring to the top view of the Windrigger MK 2 hull design drawing, to assist this explanation the section nearest to amidships should be numbered section 7. The curve of the maximum beam between section 1 and section 7 is determined by the equation of the ellipse shown on the drawing. Please do not obtain these dimensions by scaling off the drawing as it will result in much labour in fairing the resultant hull.

Also, the scale shown on the drawing is incorrect.

The half-breadths shown in the table below, are maximum half-beams of each section calculated by solving the equation of the ellipse following substitution a x-value for that section measured from section 7. The centre-offset figure is the result of subtracting a half-breadth from 350. This establishes the centre for scribing part of the circle of 350mm radius which forms half of the bottom and topside shape of a section. This centre is located on the maximum beam horizontal line and at a distance from the vertical centreline equal to the centre offset figure.

Similarly, the corresponding deck half of the section is produced by scribing a 600mm radius centred on a line 350 mm below and parallel to the maximum beam horizontal, and at a distance from the vertical centre-line equal to the centre offset figure.

TABLE 1
Section no.Distance from station 7Half breadthCentre offset
7 0350 0
6 500346 4
51000335 15
41500314 36
32000283 67
22500237113
13000164186
In manufacturing the marine-ply bulkheads for the MK 2 hull, these centre offset figures were used to locate the pivot-point of a radius arm attached to my router prior to cutting-out each of the bulkheads with the router. Note that these figures were changed to take into account the diameter of the router bit and the thickness of the hull laminate. The mould-frame sections of the strip-planked plug were also cut-out this way.

To facilitate discussion of this method of hull design, I have named it CONSTANT CROSS-SECTION CURVATURE or simply CSC. This method is not limited to using cross-sections based on circular curves, it is also applicable to curves such as parabolas, hyperbolas and ellipses, and curves drawn with splines or by freehand. The waterline curves are not limited to the ellipical curves.

Please find enclosed my explanation of calculating half-breadths from the equation of the ellipse. These figures were produced using a basic electronic calculator. Note that I have included three sections in addition to those listed above. If you want to vary the waterline shape, substitute for example 3600 for the 3400, and plot the results against the 3400 curve to see the difference.

End elevation:

The hull - I designed it nearly 3 years ago to provide a platform for my further experimental investigation of proas. It is symmetrical fore and aft, 6800mm loa - this greater length than Windrigger MK I was selected to provide relatively greater pitch-stiffness and accommodate two fore-and-aft sailrigs. Its hullshape - a solid developed by advancing a circular midship section of constant radii, towards the bow/stern following a constant elliptical waterline curve - as shown in Fig 1.


This round shape was selected to minimise rough-sea hull drag/impacts and windage; and to minimise the lateral resistance produced by the hull relative to that produced by a dagger board and spade rudder. The last 300mm of the end of the hull has relatively greater waterline curvature and it was easier to carve a noseblock from a sandwitch of fibre-board than to strip-plank it.

The design process included calculating hull offsets from equations of the circle and ellipse, loading them into HULLFORM software and obtaining associated hydrostatic parameters.

Back to Yacht Research Homepage | Previous page | Next page

e-mail to:smithvanaalst@bigpond.com