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.

Smoothing the outlines in Board Off

I've just made a minor change to Board Off to smooth the outlines. If you used BoardOff you may have seen that where the different sections (mid, mezz, corner, tip) join there is occasionally a small kink in the outline. This is the reason that the 'gradient ants' where introduced into the Draft window as they show the slope of the outline either side of where the sections join. The goal was then to adjust your parameters until the 3 Ants merged into one or close enough.

This was a short coming of the cubic splines used to draw the outline in that the way I'd implemented it it did not enforce and smoothness conditions at these kinks. I've made an adjustment to it so that for the corner-mezz, mezz-mid joins a continuity condition is in force and so there are no longer any kinks there. It wasn't possible to do it for the tip-corner join because the tips are so steep that the spline needed to approximate it has trouble turning the often small radius corners without causing the line to start going wild (oscillating). Also, for inverted tips its desirable to be able to have a sharp corner.

I've also applied this to the rocker and concave lines so that the kinks ( detected in the rocker and concave by the 'curvature' chart being discontinuous. They are now all smooth and much easier to work with.

The update did require some of the outlines to be modified as the continuity conditions to force a change in the shape of the outline in some cases. So you may need to check your own designs to make sure they haven't been affected.

Any questions, just post here and I'll endeavour to get back as soon as possible.


P.S I saved the new version over the old version 16. When you download it will say v17 even though the link says v16.

PSS Added the Naish rocker description I posted about earlier. I used equal length section ( 1/3 of the half length each). As I looked at the numbers it returned it became pretty clear that Naish is not using equal length sections. If they were then all numbers would need to be increasing otherwise the board would have and flat spot in the rocker. They must has different lengths. For my purposes the equal space ones are still useful as a shorthand way to describe it and compare others designed in board off.

Sunday, March 10, 2013

Parameterised Board Description @ Naish

I followed a link on to a Naish Catalogue and was looking at the boards. I was very stoked to see that for flex and rocker they are using a parameterised description instead of the usual meaningless explanations like 'soft flex or aggressive rocker'. Instead they use more details parameters to describe it e.g. r8/4/10 meaning from the center to the tip the sections of the boards contribute 8mm then 4mm then 10mm to the overall rocker.

This was one of the original goals of the Board-Off Design tool. Namely, to be able to express all details of a boards shape through a small number of parameters so that if made a change to the board you would then have very precise way of describing the change (via the change to the parameters) and so attribute the change in performance to the change in parameters.

Similarly they are expressing flex with a parameter. In Board Off, I've tended to you a standard 'theoretic' test of 20kg force applied to the tip with the midpoint of the board or the midpoint of the foot strap help in place and the deflection calculated. I don't know what there standard test is ( would be great to know) so might see if they'll share any details....

Wednesday, March 6, 2013

New version of BoardOff

New version of BoardOff Design tool just posted. Includes a function to let you draw mutants and surfboards. You do this by design the nose templates and the tail template seperately and then using the new 'Composite' design mode option to piece them together.

Also updated the flex models to accommodate the larger board sizes of up to 180cm.

I've put a download tracker on about a month ago and it seems that in the last month there have been about 30 downloads from all over the globe. Its great to see that its out there getting used. If you are using and you have any questions of comments please let me know as I am keen to develop it further based on input from users.