Lexan

[Capn_Tom]

I am preparing to replace the old glass in my deadlights with lexan. The existing glass is 1/4" thick and 5 3/8 in diameter. There is aprox. 1/64 of free space arounf the glass. I know that the lexan needs room to expand. Is there a rule of thumb or rule of law on how much expansion room is required?

[Hirilondë]

First off, Lexan is a trade name. The product you are probably referring to is polycarbonate. There are several brands, Markalon is the one we get at the boatyard, but all of them about equal. It is the most impact resistant material you can get short of the laminate known as bullet proof glass (acrylic laminated to either side of polycarbonate). The downfall of it is that it etches and scratches quite easily. As well as being very impact resistant it is extremely dimensionally stable throughout a wide temperature range. Even a slight space (1/16th in.) should be ample for all but the largest fixed lights. And then 1/8th in. is plenty. McMaster Carr has a very good description of polycarbonate, acrylic and other plastic sheet goods in their catalog, worth a read.

[Ceasar Choppy]

I had this mistake several years ago when I used polycarbonate with the trade name Hyzod. I remember then that the Hyzod required a minimum of 1/16 for every 12 inches sq. of material. I did not pay attention to that and a few weeks later I was forced to disassemble the windows and stop the leaking by cutting down a high spot on the polycarbonate that was distorting the frame because it wasn't allowed to expand.

Then a few years later, it was crazed beyond all recognition. Read up on it carefully as Dave recommends. There should be better stuff out there that is more craze and scratch resistent...

The names are all over the place, but you've basically got some form of either acrylic or polycarbonate. Acrylic, for example is usually Plexiglass or Perspex (the European name).

[Rachel]

I have not done any of this type of replacement on my own boat, but from watching/talking as jobs take place, I believe you might want to use acrylic. Acrylic is not quite as tough in a "won't break" sense as polycarbonate, but it is much more durable in a "won't scratch or haze" way, as I understand it.

Of course whether or not you need the added "structural" toughness of polycarbonate depends on port size and shape, intended usage, your comfort level, etc.

Rachel

[Tim]

Lexan, or generic polycarbonates, can scratch over time, but I really think the concern about it is overblown. I have found that a tiny modicum of care will prevent anything terrible from happening to polycarbonate ports. The typical scratching that might occur from harsher soaps, for example, isn't really even noticeable. I should add that I've often soaped my polycarb deadlights with Comet with no ill effects.

As always, maintenance makes anything work better, and neglect makes anything worse.

[Tim]

McMaster Carr has a very good description of polycarbonate, acrylic and other plastic sheet goods in their catalog, worth a read.

Here it is. Link: More About Polycarbonate, PETG, Acrylic, and Celluose (PDF File from McMaster-Carr)

[Capn_Tom]

Thanks for all the good information. I noticed that Mcmaster lists abrasion resistant polcarbonate. I think I'll give that a try.

[kabauze]

Yes, there are grades of polycarbonate that are more UV- and abrasion-resistant. For example, GE has its "MR10" grade of Lexan which is intended for outdoor windows in architectural use:

http://www.usplastic.com/catalog/produc ... %5Fid=9948

Of course, it's more than twice the cost of normal Lexan (for a 2'x4'x3/8" sheet, I found $113 versus $259)!

[Capn_Tom]

You sold me. I just bought a 1/4x12x14 piece of MR10 on ebay for $12.00. That'll be enough for my 4 deadlights assuming I don't break it while cutting.