The metrics commonly used to compare surf skis are length, width and weight. 

But it seems to be a commonly accepted fact that stiffness also affects a hull's performance.  A Google search resulted in a number of articles that had a common theme: if the hull flexes as you pull a stroke, energy is absorbed (and wasted) that would otherwise propel the hull forwards.

Epic just introduced a new measurement of stiffness in their skis: flexion.

In order for it to be useful of course, you need the equivalent measurements for all the skis for comparison.  Perhaps this is where Surfski.info could be useful!  We could take a range of skis and measure the flexion of each under the same conditions.

At any rate, Oscar Chalupsky sent us these photographs from the Epic factory in China.

The ski in the photographs is an "Ultra" lay-up example i.e. the lightest in the range and therefore the ski that you'd expect to have the most flexible hull.  The measurements were done with supports "about 5.5m apart" and with a 20kg weight in the cockpit.  The deflection on this ski was 7mm.  (The ski looks like a V10L; perhaps the V10 would flex less given the greater height of the foredeck?)

Oscar says that the Standard lay-up ski flexed 6.3mm and they expect the Economy ski to flex still less.

For the results of any comparison to be meaningful too, one would need to come up with some way of estimating the effect of flexion on speed.

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Making the measurements meaningful
written by SimonH, June 22, 2006

Anybody who has paddle carbon and GRP versions of the same ski will know just how much stiffness affects the characteristics of a ski so stiffness is a meaningful metric. Looking at the Epic tests 2 thoughts spring to mind;

1. The degree of deflection is hugely dependant on the distance between the supports. If we want to make meaningful comparisons there must be a clear and strictly adhered to standard. ?About 5.5m? does not cut it.
2. I note that the ski in the Epic test is supported on 2 fairly narrow trestles so that the bearing areas on the hull are small. I was very surprised when I did my own crude comparative tests some time back just how much the hull surface is deflected inwards by the point load of the support. Given that the skin of skis with a heavier lay-up will deflect less it could well be that the hi spec skis were in fact longitudinally stiffer but showed greater displacement due to a higher skin deflection.

In principle a stiffness standard is a good idea. Maybe Epic could make some suggestions as the measurement parameters.

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