|Topic Review (Newest First)|
|01-14-2010 07:13 AM|
|CoastalEddie||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.|
|01-03-2010 03:01 AM|
|tager||If you scrub your index finger on your teeth it makes a cool noise.|
|01-03-2010 01:38 AM|
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.
|01-02-2010 12:50 PM|
"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.
|01-01-2010 04:31 PM|
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?
|10-18-2009 08:20 PM|
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.
|06-21-2009 08:18 PM|
|bigsarg1||I know this thread has been done a long time ago, but I am wondering if I can do this project over my flush mounted windows on my 37 Irwin. Double the window and double the protection? Any thoughts|
|06-21-2007 02:28 PM|
There are also two very different types of neoprene foam:
"Gas Blown" from Rubatex and some other makers, and the more common "chemically foamed" neoprene. The Gas Blown type is made by injecting high pressure nitrogen into the rubber "batter" and then allowing it to expand and cool, producing one inert sheet of neoprene foam.
The "chemically foamed" type is made by mixing in chemicals and the "cooking" the batter to expand and set it, like a giant pancake baking. The problem is, chemically foamed neoprene does SHRINK over the years, and sometimes degrades due to chemical actions or bad chemical choices.
But, most foamed neoprene on the market is chemically foamed since it can cost 1/2 as much to make it that way.
If you are using neoprene foam and want it "forever", buy the nitrogen foamed stuff. There will be a tag or other identifier on the roll. With a good eye, you can also see a difference in the texture of the rubber on bare cut edges.
|06-21-2007 02:10 PM|
Further on surface-mounted ports
Greetings to those of you who are contemplating doing something like Steve Stoehr did to his Tartan 30. After this article ran in Good Old Boat in March 2006, our email hummed with interest in Steve's project and it went on for months. After reading Mike Pz's request about the material for the ports, I thought I'd cut and paste the follow-up comments from our magazine and the subscriber newsletter so you'd have the rest of the story. I apologize for the size of what is to follow (1,400 words), but if you're interested in this project, you'll find what goes below to be very interesting. I have never posted to any thread or discussion group but I suppose the size of what follows will make up for any lost time!
Karen Larson, founding editor of Good Old Boat
Good Old Boat Subscriber Newsletter, April 2006
Surface-mounted ports, Part I
In the March 2006 issue, Steve Stoehr wrote about replacing leaky portlights with surface-mounted acrylic. I have an older Catalina 25 and deal with the same issues with water leaks. Unless I misread the article, Steveís instructions were to predrill holes in the cabintop for the self-tapping screws, yet I see in the photo the existing holes that held the original frame. My questions are: did he use these holes, did he fill in the holes, or could you use the existing holes for mechanical screws to anchor the acrylic to the cabintop?
Like so many of your readers, Iíve given up on the slick sailing
magazines. Keep up the good work.
Surface-mounted ports, Part II
Iím not sure he filled the old holes with anything because they would be hidden by the rubber gasket. I am sure he did not reuse the holes. There are ways to reuse the holes, but it is a matter of piloting, by which I mean that one would have to transfer the location
of the old holes to the new windows. It is actually easier to mount the new windows and transfer the hole locations to the boat by drilling through with the tap-sized drill and then later drill out the window to the clearance-size hole.
Jerry Powlas, technical editor
Surface-mounted ports, Part III
Jerry is exactly right. I covered the original holes in the cabin wall with the gasket and drilled new holes for the self-tapping screws. By drilling new holes I could follow the shape of the new port for a neater appearance, and better center the screws out over the sealing area. You could use bolts through the acrylic, the old holes, and the inside frame. However, as Jerry pointed out, that would be difficult to pilot and would be more expensive. Barrel bolts or bolts with cap nuts cost much more than stainless self-tapping screws, are more difficult to seal, and each bolt would need to be cut to length.
Surface-mounted ports, Part IV
Thanks, Steve; I think Iíll go with your approach. I canít get too excited about the idea of rebeddng (once again) old portlights. Plus, I think Iíll like the updated look.
Surface-mounted ports, Part V
Yes, there really is a lot of interest in this subject. Last week I received a call from a fellow sailor in Sacramento, Calif., who read the article. He went to the trouble of finding me using
the Internet (Tartan website and Google) to get his question answered about the gasket material. Leaking ports in his Freedom 32 ruined his favorite book, so he plans to try surface-mounted ports.
Thanks for sending the comments from Jim Donovan of Braintree, Mass. (Mail Buoy, May 2005 of Good Old Boat ). I looked up a Seidelmann 30 (29.9) and can see why Jim may
have experienced some chipping and cracking around the fasteners in his attempt at surface-mounted ports. That boat has a very low curved coachroof so the portlights are at about
a 45-degree angle from horizontal. That makes them very vulnerable to lateral stress when stepped upon or otherwise knocked about.
If I were designing surface-mounted ports for the Seidelmann 30, I would increase the acrylic thickness from 3/ 8-inch to1/2-inch, bevel the edge, and make sure there is at least a 2-inch overlap all around the port. I would not recommend using stronger polycarbonate because polycarbonate turns cloudy from sunshine.
Good Old Boat magazine, July 2006
Ports discussion continues
Having read the surface-mounted ports article in the March 2006 issue, Iíve been trying to find some of the rubber adhesive sheets you used between the fiberglass and the acrylic windows. If you had an exact name for a lead on the stuff, it would be greatly appreciated.
Author Steve Stoehr replies
The rubber gasketing material I used was plain 1⁄8-inch closed-cell foam neoprene rubber sheet stock. It was not adhesive sheet stock. Go to your nearest industrial rubber distributor (there is at least one in every city) and tell the man at the counter what you need. If you must substitute another material, choose one that is UV- and chemical-resistant. Closed-cell means that water can not migrate through it like it can through a sponge.
Ports: what color acrylic?
I read the article about surface-mounted ports by Steve Stoehr and Iím wondering what material was used to make the ports? The article says 3⁄8-inch acrylic stock. Can you be more specific about the color and the series or any specific information? I am trying to do the same with my Catalina and have already wasted $200 on plastic that is the wrong color. I would really like to get it right this time. I have to order the material from 60 miles away and I just canít drive there twice, once to look at samples and then again to pick the material up. I canít believe that it is this difficult to figure out what material to use. Any help you can give me would be greatly appreciated.
Back to Steve again
Just about any town of any size has at least one plastics distributor. Check the Yellow Pages under ďplastics.Ē They usually have clear acrylic sheet (Plexiglas) in stock but may need to order tinted acrylic for you. If so, they should have samples to look at to determine what tint you want.
I bought Acrylite GP acrylic made by Cyro Industries. My choices were clear, smoked, and reflective coated. The smoked tint is optically clear for good visibility yet helps filter out bright sunshine and offers some privacy below. The shiny reflective coated version is even better for blocking sunshine, but it looks tacky to me.
Check also for plastic fabricators in your area. They buy from the distributors and make custom things out of plastic, such as display shelves and cases. With any luck, you may find one with enough material left over from a job to make your portlights.
Here are a few more tips. Do not use polycarbonate (Lexan) for your portlights. While Lexan may stop a bullet, it scratches easily and turns hazy in sunshine over time. If you plan to use silicone to seal the portlights, use a silicone that does not smell like vinegar (acetic acid) when it cures. That kind will craze the acrylic over time.
Good Old Boat Subscriber Newsletter, October 2006
Surface-mounted ports, continued
I know that Steve Stoehrís port replacement article has almost created too much interest, but I just replaced the ports on my 30-year-old Irwin with incredible results. I completed the entire project in one weekend, for less than $100, and without the slightest leak.
One deviation that seems to have worked was to cut short (3/ 8-inch) pieces of gas line hose to use as spacers around the screws. This allowed me to greatly enlarge the holes in the Plexiglas and provide lots of cushion for expansion/contraction. Thanks, Steve!
Iím interested in further information on the gasket material mentioned by Steve Stoehr in his article, ďSurface-mounted ports,Ē in the March 2006 issue. I am unable to find similar material in my town, Muskegon, Michigan. Perhaps Steve might share the name of his supplier.
And Steve responds
The industrial rubber distributor I bought my gasket material from is Fournier Rubber and Supply Co., 1341 Norton Avenue, Columbus, OH 43212; 614-294-6453; fax: 614-294-0644;
This summer I installed four opening stainless-steel ports, again using some 1/8-inch rubber sheet stock as gasketing material. This time I bought EPDM sheet stock from Fournier because it is very UV- and weather-resistant. EPDM rubber is used for pool lining and for seamless roofing and can be very pricey ($4.25 a pound). EPDM stock is not foamed like the neoprene stock so it is harder than the neoprene stock. It would seal two smooth surfaces but may not seal irregular surfaces.
But it should last forever. If I had it to do over I might use EPDM for the surface-mounted portlights. So far the neoprene material is holding up well, and there are no leaks.
|06-21-2007 01:53 PM|
If a polycarbonate such as Lexan is used, then the oversized holes are a must. In addition, when the screws are tightened down, do not use "extra" force when tightening. Only snug the screws in place. Screws that are too tight also lead to cracking of the material.
We use polycarbonate in our operation as machinery guards for many Fortune 500 companies.
|This thread has more than 10 replies. Click here to review the whole thread.|