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From: Tom MacNaughton
Date: 01 Feb 2008
Time: 14:25:35 -0500
Remote Name: 66.252.35.219
The behavior of water around a yacht hull is a complex subject built on a few simple general principles. It may help if you understand that given reasonable speed the water tends to take the shortest root around both sides of the hull that will allow it come back together after the boat passes. Since the shortest distance in the longitudinal direction will be along a line representing the intersection of a surface with the hull surface at a 90 degree angle, the diagonal will tend to largely reflect the flow of the water. This is because a well constructed diagonal is essentially the intersection of a plane “normal” to (perpendicular to) the hull surface. Diagonals would have nothing to do with waterlines although if you cut waterlines through a vertical sided keel foil they would be the same as the diagonals or foil section curves. I don’t know what you mean by “flattish” looking diagonals. Basically the diagonals should be as long and smooth as possible. The middle portion should have as large a radius as you can get and then this radius should expand toward both ends to be as close to infinite radius by the ends, that is “straight”, as possible. The lighter the boat the larger the general radius and the closer to straight the ends tend to be. Also the higher the prismatic coefficient and the intended speed potential the straighter the aft diagonals especially will be. “Understanding Boat Design” was last revised awhile back and Ted has since somewhat retired and new software come to the fore. I assure you that the program we use in designing and teach to our students, Rhinoceros®, commonly known as Rhino is absolutely fantastic for working with diagonals. Of all the programs we’ve tried the only one which is really top notch for lines work is Rhino. You can do about anything with this program. We have a nice curriculum in the proper use of Rhino in designing boats which we call our “Computer Assisted Design Course” or “CAD Course” for short. There’s a fair amount on the use of diagonals in there, but our main Yacht Design School curriculum which qualifies graduates as naval architects with a specialty in yacht and small craft design has the most information. Several lessons deal with various aspects of producing excellence in hull lines through proper use of diagonals. There is also a fair amount of information in our publication “Living Aboard – Frequently Asked Questions” by Thomas A. & Nannette M. MacNaughton.