To make matters worse, no one ( apart from myself) ever bothers to measure rates of curvature. Without knowing the rate of curvature it is almost impossible to compare rocker on boards of different length.
As an example of this, one would probably expect that an 8 foot tail section with 6 inches of rocker has 'more' rocker than a four foot tail with 1.5 inches of rocker, when in fact both these tails have an identical rate of curvature. The usual way of measuring rocker is completely unable to show this, and in my opinion this has led to anomalies in board design particularly in very long boards.
In fact we have found that any functional rate of tail curvature can remain constant over a huge range of board lengths, we have used the same rate of tail ( and nose curve) and in fact the same rocker table for boards of similar planshape proportion ranging from 6 feet to 17 feet with excellent results. This is not to say that we always use the same rocker curve, we don't, the point is that it is important to get a handle on what rates of curvature are used.
The typical assumption and tendency has been to reduce the rate of curvature as a board gets bigger, this is in my opinion quite clearly an unintended side effect of the standard rule of thumb rocker measuring method. As shown above, a board with the same rocker curve gets a much larger overall rocker measurement as the board gets longer and the curve is extrapolated, this tends to make people believe that the longer board has too much rocker, they then reduce the rocker so that the number sounds more reasonable. . . . . and so that the board looks proportionally like a shorter board. When they do this they still think that the longer board has 'more' rocker even though it now has a flatter and lower rate of curve. This has lead to a woeful anorexia of board rocker in longer boards, and generations of blissfully ignorant boardshapers, who unsurprisingly find that the very long boards they attempt don't work very well.
In fact, waves respond to the actual bottom curve not the 'look' of the rocker, so to the wave it doesn't matter that a very long board viewed from the nose looks excessively rockered due to the foreshortening effect of the viewpoint. . the wave reacts to the actual rate of curve, the same rate that looks very flat on a shortboard!
I often get asked why my longer boards have so much rocker. . . . .unfortunately the questioners usually have neither the inclination nor the power of concentration to listen to and understand the explanation given above. What I usually say to them is that the rate of curvature is probably less than what they have on a shortboard. . . they don't believe or understand this either, but at least it's short and sweet and saves me from wasting my breath
Let's look at a few boards to see how this looks in parctice.
Below we have a 6 foot Power Fish, now I probably shouldn't tell you this as it has been a secret for many years, but this board has the same rates of curve and comes from the same rocker table as the apparently ultra rockered longboards further down the page ( The X-15 and the D11-9 )
Here's the mighty X-15 showing a full 9 inches of rocker ( and only an inch and a half of thickness !)
And the D 11-9 with 6 inches of rocker:
FAQ: Why only one rocker measurement rather than two ? The answer to that one tomorrow.
