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Go Back   SailNet Community > Contributing Publishers > Good Old Boat
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  #11  
Old 10-18-2009
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It might work but we used to try to make our own double pane glass for houses and had some problems.
The seal between the two panes was never perfect and moisture would get trapped and fog the window.
In real thermal pane windows they replace the air inside with another inert gas.
This was many years ago but I belive we had to vent the inside pane.
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Old 01-01-2010
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A clean drill of a very small hole should serve this purpose and be inconspicuous. I'd say a 1/16" hole top and bottom of the inner pane would do. Airliner windows were mentioned before ... I believe I've noticed this on these applications as well.

I noticed a criticism of this project was in the stress concentrations created around the hardware. A possible solution for this would be to distribute the clamping force equally through use of a exterior "frame". The frame would not be required to provide any sort of sealing. It would simply bear the tension of the hardware and distribute it to flat surface on the perimeter of the glass.

The result would be a port light sealed by sealant, attached by hardware, and held without stress concentrations. Anyone tried this approach?
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Old 01-02-2010
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"Anyone tried this approach?"
Just about every concept you can think of, has already been long tested and detailed by the folks who make the glazing. Often available online complete with technical drawings, on their product web sites.
A frame that distributes stress, is still creating stress. As that cycles the glazing will develop stress cracks (crazing), and the universal solution from all the makers is to emphasize that the mounting needs to *eliminate* stress buildups completely. Not distribute them, but eliminate them.
Lexan, plexi, whatever from whoever, the bottom line always comes down to the same few "right" ways to mount them, which will eliminate stress problems and seal against leaks--if they are properly implemented.
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Old 01-03-2010
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I have to mention that, for my specific solution, distributing the stresses from the fasteners to an evenly distributed stress over the entire area of the lens that is contacting gasket material eliminates shear stress.

Shear stress is caused by a difference in axial stress over a distance. Distributing axial stress eliminates shear stress. Shear stress and bending are the stresses that lead to cracking.

So, there you go, my specific proposal eliminates shear stress by distributing axial stress. You mention the few "right ways". I am responding to this specific article. It does, by any means, suggest a mounting method free from stress concentrations. My modification to it greatly alleviates the issue.

Also, distributing this axial stress allows overall axial stress to be greatly lessened. Simply tightening the hardware into a distributing frame instead of straight into the lens lets you tighten the hardware much less to achieve the same effective gasket pressure.

If someone does attempt my solution, I'd definitely recommend radiusing the edges of the frame in contact with the lens.

Last edited by scraph; 01-03-2010 at 12:41 AM.
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Old 01-03-2010
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Old 01-14-2010
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One thing that can help to prevent cracking in both Plexiglass and Lexan is to anneal each piece after ALL the machining (holes drilled, edges rounded, everything) is completed. Preferably, if the piece is to be bent into place (as it often is when installing these sorts of windows), this would be done with the piece clamped against a "mold" of the approximate curvature as the finished installation. This will do two things: first, it gets rid of most (ideally, ALL) of the micro-cracking induced by machining; and second, it dramatically reduces the stresses (and thus the strains) imposed on the material when it is installed.
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