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Sunday, January 14, 2007

Advanced Tick-Stick Techniques

Click here for all posts about using a Tick-Stick

Up until now I really havent had a need to accurately measure the curvature of the hull. The floor supports were small enough that I could just approximate the piece using doorskin and fit it by repeatedly fitting and trimming it. The pieces were small and the doorskin easy to cut right there in the boat. Once the doorskin fits snugly, use it as a template to cut the actual piece. Upper bulkheads, however, are much bigger, and using the same method would be a pain.

So, how to measure and cut the bulkheads accurately and efficiently? I had searched the web over the past few months for tools that may help. One would be to call someone in here to get a point cloud scan of the interior and take a cross section at the station line and plot a template from that. But, it costs at least two to three thousand dollars a day to get someone in the boat to do that. Plus there is the time in cutting through the resulting points which would require a lot of post-processing in CAD or Rhino. I did come across a device, used for mapping tunnels, that would rotate a mirror and take sights in a 2D plane. That would have been convenient, but that cost a few thousand dollars.

So, final solution, use the tried and true tick-stick method, but with a high-tech twist. This requires the following:

  • a large sketch board with paper clamps

  • a 2x4

  • long thin stick about 7 feet long with a point on the end

  • a yard stick

  • large sheet plotter

  • CAD program

Mount the sketch board to the wide side of the 2x4. Tightly clamp the wide side against the lower part of the bulkhead already installed on the centerline. In the CAD program, plot a 1-inch grid to fill the largest sheet of paper you can print (24x36 in my case). Put the paper on the sketch board. Attach the yard stick to the long stick, such that you can read a measurement at the grid at the widest and narrowest measurement. Put the pointy end of the stick against the hull at the point you want measured while laying the side of the stick against the grid. Rotate the stick up or down until you can match an inch tick on the yardstick with any interesection on the grid square (this is best done with two people, one person holding the pointy end against the hull). Once you have matched, take a fine tip pen and scribe a line on the grid paper, using the tickstick as a ruler, from the intersection towards the hull. Read the distance off the yardstick and write it on the line (dont forget to add the offset from the end of the yardstick to the pointy end of the stick). Move the stick along the hull 5 or 6 inches and repeat until you have traversed the area to be measured. You now have a sheet of "vectors", the ends of which mark the inside of the hull.

Next, place the grid sheet on the dining room table, take a protractor to each of the vectors and measure the angle. Write the angle on the vector.

In the CAD program, call up the original document that generated the grid sheet. Plot the vectors in the CAD document, just as they are on the sheet. Best done by entering the points as polar coordinates in degrees (if your CAD program cant do this, get another CAD program!). You now have transfered the measured vectors to the CAD program. Now connect the ends of the vectors to get the curvature of the hull (multi-point spline curves seem to work the best).

You now have a pretty accurate measure of the hull curve in CAD. At this point, you can print it out full scale to make a cut-out template. You can also export it to your 3-D software to help in 3-D modelling.

For incorporating this into other drawings there are somethings you can do to make things easier and accurate: level the grid paper before taking points (assuming the boat is level, which the westsail is). Mark the centerline on the grid paper.