Moving right along

Back again. A couple of good things have happened since I last posted. The first is that my dear partner has decided that she will support (tolerate actually) my longboard construction project. It took a little bit of convincing for this to happen (and hence the length of time since the last posting!), but at last we have came to an agreement (painting part of the house probably helped). The other useful thing is that one Malcolm Jones (of Melbourne too would you believe) posted a very nice step-by-step account of a similar construction project (thanks LBWS). Malcolm has, to my mind, solved a number of the engineering problems, which leaves me free to concentrate more on the design and aesthetics of the longboard.

Some encouragement has also come from a three part video released by Witchcraft Sailboards which shows how the experts design and construct a sailboard using state-of-the-art CAD design and shaping equipment. Very nice if you have the equipment. However for me its still gonna be the hand saw, the rasp, some templates and careful use of "the eye".

So back to the design process. In the last posting there was some discussion about what should constitue the correct rocker shape for a longboard, and following the scaling of some longboard photos using the "match new photo" option in Sketchup, a suitable composite profile was developed. However, I'm now having some doubts as to the accuracy of the profile. The problem is that none of the photos give a true side-on view of the boards, and so it is necessary to first correct the photos to remove perspective effects. This process of correction can lead to errors in the measurement of the rocker.

The magnitude of this error was tested using an old Tiga 330 which a friend has loaned to me for the summer. By directly measuring the rocker, and comparing it with the rocker derived from photos, it is possible to determine the magnitude of the error due to perspective effects in the photos. Shown below are three examples of how an image may look together with the an indication of how the rocker is scaled off the photos after correcting for perspective effects.

The rocker is the most difficult to scale and measure in the image on the left because the board does not line up with any straight edges in the image and also because the nose of the board is closer to the camera and so a larger correction has to be made for perspective effects. The middle image is better because the board is lined up with a straight edge and so the perspective correction can be more accurately determined. The right hand image is the best because the board has been photographed from a side on view and so the need to correct for perspective is minimised.

The magnitude of the errors in measuring the rocker is shown in the figure below, where the derived rocker is compared with the measured rocker for the three examples shown above.

You can see that the best match between the measured rocker and the derived rocker occurs for the right hand image where the perspective correction is small. So this means that I should go hunting for some side-on images of longboards in order to be confident that the rocker is correctly measured. Alternatively if any of you kind longboarders out there in the blogosphere are willing to take and email in a good photo, I'll be eternally grateful. In the interim, it's time to check out some plan views of the various longboards out there and to come up with a good overall boardshape.

Bye for now.