Birdwatcher-2 Polcarbonate (Lexan) Window Expansion Notes
Birdwatcher-2 Sailboat, designed by Phil Bolger
Jack Wyman is building a Birdwatcher-2 and was generous in sharing his thoughts on the mounting of the polycarbonate plastic. The follow are Jack's notes on calculations of the hole sizes required to allow for the expansion and contraction of the plastic.
We want this panel to be 18.000 x 78.000 inches at the midpoint (50F) of the expected temperature range. Therefore in our 70 F shop, we cut a panel 18.014 x 78.059 inches (per the coefficient of expansion). Now draw lines 0.750 inches in from each edge on the protective paper (ignore very small error here due to 70 F shop over only 0.750 inch distance). The closest we can get to the desired 5-inch hole spacing on the long side of the window is 5.100 inches. (16 holes spaced 5.100 inches apart take up 76.500 inches, plus 0.750 inches in from the ends totals our 78.000 inch window length at 50 F). The closest we can get to a desired 5-inch hole spacing on the short side of the window is 5.500 inches. (4 holes spaced 5.500 inches apart takes up 16.500 inches, plus 0.750 inches in from the ends totals our 18.000 inch window height at 50 F).
In determining the actual exact window lengths (in the 70 F shop), leave a theoretical minimum clearance between windows of 0.119 inches to accomodate for expansion up to 110 F. (Think of two adjacent Ground Zeroes going nowhere and of the adjacent window halves between these GZs expanding toward each other).
Now consider the upper right quadrant of the panel. There are eight holes along the top edge, and one more along the right edge. The hole in the upper right corner is part of both hole rows. Ground Zero is in the center of the panel. #1 hole is about 8-1/4 inches above and about 2.55 inches to the right of Ground Zero. #2 hole is about 5.1 inches to the right of #1, and so forth. #8 hole is in the upper right corner of the panel. #9 hole is about 5-1/2 inches below #8. The three other quadrants of the panel are mirror images of this quadrant.
X-coordinates (inches) of holes at various temperatures:
Y-coordinates (inches) of holes at various temperatures:
|Temp||#1 to #8||#9|
In the 70F workshop, layout all 36 holes in the panel per above. On a drill press, use a tap drill bit for a #10 screw-into-wod and drill all 36 holes in the panel. Now center the panel on the boat frame and clamp it in position. Use a hand drill and the same tapdrill bit and drill into the boat frames the appropriate depth.
Remove panel from boat. Enlarge each panel hole to accomodate expansion/contraction as follows. This is for the upper right quadrant. Other quadrants are mirror images.
Vertical elongation (inches) at 70F:
|#1 to #8||#9|
Horizontal elongation (inches) at 70F:
Minimum slotting would be on diagonals radiating from center-of-panel Ground Zero, but it's too much work, I think!
It wouldn't hurt to check all this. The idea is to get the ideal mid-range temperature (50F) dimension, then calculate the offsets for min temp (-10F), workshop (70F) and max temp (110F).
Then at the very end, take a coarse rat tail file and go to town!
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This was last revised 8 May 2006. Bob Larkin, Corvallis, Oregon