Wednesday, March 13, 2013

Naish Designer on Twin Tip Design

I've also posted this on the Forum but thought it was well worth reposting here for those who are regular visitors to the forum.

This is a good discussion by someone with a engineering background on some of the features of boards including multiple concave.

He also mentioned the new description of the rocker line which I had mistakenly thought was the amount of rocker in each of 3 sections in the board. It seems that its the radius of curvature for each section and not the amount of rocker. However it suggests that each section is an arc which seems too limiting me and of course when you intersect arcs of different radius the are tangential at the intersections so there is more to the rocker line than the piecewise arc design.

Is there some magic about arcs in the design? From my reading the circle has a slightly lift drag ratio of less than one meaning that it generally sucks in fluid flow situations. I'm sure there is an interesting things to find out.


I re-read the article and found the error of my ways. Larse used the term

' tangent radii creating a continuous curvature'. The tangent radii I interpreted as the slope of the tangent at pre-set points but now I think that tangent might be a red herring and what is meant is the curvature (defined as the second derivative of the rocker line) at the pre-set points. This also makes sense in the context of using splines to design the rocker because matching second derivatives is a common, strong, continuity condition applied to splines. However, the curvature of a spline at a discrete number of points does not fully characterise the spline unless x squared is it highest power but this is unlikely to be being used as this would restrict the possible curves available to much. So there are some unspoken constraints that are being applied. 

I made the changes in BoardOff to display the curvature ( which was already calculated to find 'kinks' in the rocker profile and it does give a good description that is not restricted to the continuously increasing issue.

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