Saturday, April 9, 2011

Board Project 2 - The outline and rocker.

Kicking off Board #2

Board Number 2 is underway after some serious time on Google working out what to tweak to improve the flex without compromising strength. A couple of other goals for this project are

  • Make the board better in chop and better pop for freestyling.

  • Use wood in the core construction to have a natural wood grain finish

  • Improve the construction techniques based on what I learned from board 1.
Chop and Pop

More rocker seems to be the key to both of these. More rocker means less of digging in the nose and board diving like a shag. More rocker also, apparently, improves pop because it allows you to hold the edge in longer while loading the board. My theory here is that with more rocker, when you edge hard the water is able to flow around the board rather then with a flatter board where it has no choice but to go under the board and hence lift the board up and out of the water (so you loose your edge). Also a continuous rocker means more drag as you have a thicker cross section your pulling through the water compare to a flat section. More drag means more load on the lines means more pop.

Flex is also an important feature here. A flexible board will bend in the middle as you edge and the mid section will flatten and you are effectively trying to pop with a flat(ter) board. Not what you want. My feeling about the last board with 12mm PVC foam and 3x6 oz glass both sides was that it was just a bit too stiff. This was mostly fine except in chop the board didn't really seem to absorb much and so it was rough on the knees. However, my reason not to change the flex is that the flexible boards I ridden just feel really 'mushy' when you're trying to pop and you get less pop out of it. So I've decided to keep the save glass schedule but to thin the core a touch. From the previous google discovery, flex is proportional to core thickness cubed(!!) so a 10% reduction in core thickness (1.2mm) is almost 30% reduction in the flexural rigidity-very sensitive!! Part of this will be clawed back by the use of balsa laminate on the deck. The higher modulus of elasticity of balsa, I am hoping, will reduce this loss of rigidity a bit.

Balsa in core

I really like the look of the wood grain finish on kiteboards and so I want to include wood in this board. A full wood core or paulownia seems like a pretty common thing in current boards but its proven a bit hard to track down the small quantity I need. I have seen full balsa boards (12mm , 3x6oz e-glass top and bottom) that look great but my reservations about balsa are that its not water proof and I'm not confident that I won't rupture the glass at some point (already have on board #1 when on a 20 knot day I hit a large rock and came to a griding holt... interesting insights in to the nature of the failure of the material... subject for another post) and it is also very expensive to get high quality stuff with reasonably similar density and elastic modulus. Also, I have heard that the moisture content in balsa can lead to delamination of the glass if the board is left in the sun for long periods of time. This appears to be why some all balsa surfboards have vent screws on them.

So the trade off is to use Klegecell left over from my first board and put a 1.5mm ( 1/16 inch) balsa veneer on the top deck. It should look good and will need to be accompanied by a thinning of the core to ensure that the 10% increase does lead to a board 20% stiffer that board #1.

Board #2 Parameters

Here's what I've decided to go with:

  • length 133cm (bit longer than the last one to assist with increasing flex and to compensate for the narrower board)

  • width - 38cm

  • continuous rocker 45mm measured along the centre

  • single concave 7mm

  • 10mm klegecell core with 1.5mm balsa veneer

  • 3x6oz e-glass ( although need to price up s-glass and potential use 3x4oz s-glass and leave the klegecell core at 12mm) top and bottom. 90 degrees, 45 degree, 90 degrees.

  • thin the core from the foot straps to the tip instead of using leaving a step. Thin the tips to about 3mm so tips are a bit more flexible.

  • use either glecoat for bottom surface or lay cotton material or possible ripstop nylon between layer 1 and 2 on bottom. Pure for decorative purposes.

The outline and rocker.

I've revamped the excel spreadsheet I put together for creating the board outline to include the creation of rocker templates for the center line as well as for the jigs that will clamp the edge of the rocker table surface. This time I have calculated the correction needed to account for the rocker table being almost double the width of the board (using the beam bending equation). In board 1 my estimate was out by about 50-100% and so I ended up with 1/2 the concave I wanted. This time it works very nicely.

Here are the parameters and the plots for the rocker jigs. The light blue line shows the concave profile along the length of the board. The outline of the board was chosen to be something like a Cabrinha Calibre. The drawing tool I created in excel uses cubic splines to make smooth curves on each of 4 section of the outline. The short edge, the tip (corner), the mezzanine section and finally the mid-section. On the drawing below the red dots show each of the points being joined by the cubic splines for these section. These points being joined are referred to as 'knots' or control points in the parameters below. In parameters, knot location and width for each section relate to the section in isolation and referred to the distance between the outer two points in the section and the height different between the lowest and highest point respectively and not to the total length or width of the board. This the upper left quadrant of the board that the above parameters produces. In the spreadsheet I just reflected this chart to make a full half of the board, stretched the chart out to be 7 pages long and calibrated it so that it printed out to the correct length. The paper template was then transferred to directly to the core.

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