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Thread: Portlight replacement

  1. #1
    Join Date
    Nov 2010
    Location
    Sunnyvale, CA
    Posts
    104

    Portlight replacement

    The portlights on Ariel #75 had deteriorated to the point that they were no longer seaworthy. I say that because the interior frames were cracked and turning to aluminum powder. The integrity of the interior frames are all that retain the acrylic windows. If the vessel takes a solid hit from a swell abeam, or if she gets tossed on her side, the interior frames are what keep the acrylic from imploding into the cabin - followed by lots of water. If the frames are falling apart, there's nothing to keep out the seawater. The portlights are then only good for keeping out wind and insects.

    My inner frames had stress cracks running through several mounting holes, and the frames fell apart as soon as I removed the screws. SUGGESTION: If your Ariel has any signs of stress cracks in the frames, remove and inspect them. Otherwise you could have a failure that admits copious quantities of water into the cabin. I tested one portlight by just giving it a solid shove from outside, and the acrylic ended up on the cabin sole with pieces of shattered inner frame. Had that been a wave instead, things would've gotten kinda wet inside... Not seaworthy! Instead, an accident waiting to happen.

    The design of the whole portlight assembly is of questionable quality: steel screws pass through the aluminum inner frame to threaded holes in the aluminum outer frame -- and threaded aluminum isn't very strong stuff. If the inner frame detaches from the outer frame, then the only substance holding the acrylic in place is whatever sealant was used around the acrylic edges... assuming the whole assembly doesn't just fall off the cabin side. So instead of repeating that configuration, I opted to install oversized acrylic on the outside with replacement bronze frames on the inside, with the acrylic throughbolted to the inner frames. Throughbolting acrylic is tricky because acrylic has a high coefficient of thermal expansion. You have to balance rigidity with allowance for expansion, or else the acrylic cracks and crazes. The mounting holes have to be oversized yet water tight, and the panel needs to be free to expand while still being watertight around its edges.

    Judging by the stress failures in the frames, it's clear that the cabin undergoes a considerable amount of twisting. The cabin side also has a little camber which would make sealing 1/2 inch-thick acrylic sheets directly to the cabin side very difficult without using outer frames and conventional sealants. The condition of the cabin side surface under the outer frames was also severely degraded - chipped and irregular. So instead of using butyl rubber tape or a silicon sealant (like Dow 795), I instead used 3M VHB ("very high bond") tape, formulation 5952. This is a conformal foam double stick tape that is used for jobs like attaching glass windows without fasteners to aluminum frames in skyscrapers. The tape can be easily bent around curves after removing the backer. It isn't degraded by seawater or sunlight, and it's amazingly strong stuff. The acrylic panes actually float in their attachment to the cabin, and the attachment screws are there only to retain the panes if they are subjected to sheering forces. This configuration permits some independent movement of the acrylic panes to the bronze inner frames, reducing stresses on both.

    The attachment screw holes are oversized to permit thermal expansion/contraction of the acrylic (which I calculate to be as much as 1/8th inch over expected temperature extremes). I used nine #10 screws around the acrylic perimeter. Under each screwhead is a finish washer (purely for cosmetics) and under the finish washer is a stainless steel bonded sealing washer. These washers have rubber on one surface and a slightly conical stainless steal surface on the other side. They're basically a rubber-backed Belleville washer. The steel surface deforms under slight pressure, squeezing the rubber surface around the screw for a water tight seal. It also means that only the rubber surface is in contact with the acrylic. So surface contact with the acrylic panels is VHB tape on the inside surface, and the rubber contact area of the bonded washers on the outside surface. This configuration provides a conformal contact on both surfaces of the acrylic, permitting some movement from cabin twisting, and thermal expansion. The screws were torqued down only to the point of being snug. Each screw end is then capped off with a bronze cap nut for cosmetics - and to cover the sharp screw ends. I have no exposed screw or bolt ends in my cabin. Having been once lacerated by being thrown into a sharp bolt end, I've learned not to expose them where they might come in contact with scalps and hands. Scalp wounds bleed mightily, and I hate trying to remove blood stains...

    Added weight? The bronze inner frames weight more than the original aluminum, and the thicker acrylic is heavier. But I'm not a racer - I care more about seaworthiness. And I can make up the difference by going on a diet and losing a few pounds myself.

    Below are two "before" pictures of my crazed acrylic and the fractured frames. In the third photo are the pieces of inner frames I ended up with after removing the attachment screws. Note that most of the fractures are through the more-vulnerable screw holes where deformation forces would concentrate. The aluminum inside each fracture surface was reduced to a grainy powder. The frame reduced to lots of little pieces was the one that failed in my push-against-it-to-simulate-a-wave-strike test. The rest are "after" photos.
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    Last edited by pbryant; 08-19-2013 at 09:15 PM.

  2. #2
    Join Date
    Nov 2010
    Location
    Sunnyvale, CA
    Posts
    104
    Here are a few more photos.
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