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.


  • Coefficient of expansion for Lexan is .000038 inches per inch per degree F. For wood it's close enough to zero to ignore.
  • Work shop is 70 F.
  • Boat will experience -10 to 110 F.
  • Mounting screws are #10 (0.190" dia).
  • The largest window is a rectangle 18.000 x 78.000 inches, with mounting holes 0.750 inches in from edges, spaced about 5 inches apart... the 5 inches will be adjusted per below.
  • "Ground Zero" for calculations will be the center of the window

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:

Temp #1 #2 #3 #4 #5 #6 #7 #8, #9
110F 2.556 7.667 12.779 17.891 23.002 28.114 33.226 38.337
70F 2.552 7.656 12.760 17.864 22.967 28.071 33.175 38.279
50F 2.550 7.650 12.750 17.850 22.950 28.050 33.150 38.250
-10F 2.544 7.633 12.721 17.809 22.898 27.986 33.074 38.163

Y-coordinates (inches) of holes at various temperatures:

Temp #1 to #8 #9
110F 8.269 2.756
70F 8.256 2.752
50F 8.250 2.750
-10F 8.231 2.744

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
Up 0.013 0.004
Down 0.025 0.008

Horizontal elongation (inches) at 70F:

  #1 #2 #3 #4 #5 #6 #7 #8, #9
Right 0.004 0.011 0.019 0.027 0.035 0.043 0.051 0.058
Left 0.008 0.023 0.039 0.055 0.069 0.085 0.101 0.116

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