Acrylic portlight thickness.

[cndcruiser]

Not all acrylics are the same. Do your research. Compare specifications for genuine Plexiglas (R) and Perspex (R) with others. Genuine Plexiglas has a remarkable30 year warranty against weathering degradation.

[GILow]

Originally Posted by Cpt Mark

Borrowing this thread for my own selfish purposes

….

Selfish or not, you’d be a lot better off starting a new thread.

[GILow]

Originally Posted by cndcruiser

Not all acrylics are the same. Do your research. Compare specifications for genuine Plexiglas (R) and Perspex (R) with others. Genuine Plexiglas has a remarkable30 year warranty against weathering degradation.

A valuable tip, thank you.

[noelex 77]

Originally Posted by skenn_ie

Since glueing them in, several of then have developed substantial cracks starting at the edges. How or why, I haven't yet worked out.

Some glues affect plastics, especially acrylic, and may cause this problem.

[GILow]

Originally Posted by noelex 77

Some glues affect plastics, especially acrylic, and may cause this problem.

Any guidance on how to figure out which glues might be a problem?

[noelex 77]

Originally Posted by GILow

Any guidance on how to figure out which glues might be a problem?

Any of the glues specifically designed for glazing are formulated not to cause or to at least minimise this problem.For example in the Sikaflex range 295 would be acceptable, but the similar 291i is not. This is from the manufacturer.

“Sikaflex®-291i must not be used to seal plastics that are prone to stress cracking (e.g. PMMA, PC, etc.). “

295 instructions in part say:
“Suitable substrates are aluminium (bright or anodized), GRP (polyester resin), stainless steel, timber, 2-component coatings and plastic glazing materials (PC, PMMA). Seek manufacturer’s advice and perform tests on original substrates before using Sikaflex®-295 UV on materials prone to stress cracking.”


Acrylic and Plexiglass are PMMA. Polycarbonate is PC. So 295 (manufactured specifically for glazing) is a better choice. It is also more UV resistant. It is suitable for acrylic or polycarbonate although with these materials the manufacturer still suggests the glue/sealant can cause cracking in some cases and advices a primer first.

If you have acrylic windows you also need to be careful of some household chemicals, as these can also react with the plastic and cause it to become more brittle. The damage and change in properties may not be apparent for some time. Acrylic both scratches easily and is sensitive to chemicals, treat it with great care especially on some modern boats where windows are installed frightening close to the waterline.

[noelex 77]

When plastic windows crack, boat owners generally blame this on stress or UV. These are important factors, but do not ignore chemical attack (from glues, cleaning products etc) contributing to the problem, or even as the primary cause in some cases.

The quote below (from an article on Polycarbonate) explains the effect well.

https://www.bayplastics.co.uk/downlo...nformation.pdf

“The most critical effect of chemical attack is stress cracking or crazing, which may range in size from being visible to the naked eye to being only observable under a microscope. Stress cracks will always result in sheet failure, which will develop from areas of greatest stress (screws, fixings, bends, etc.) “

[HankOnthewater]

Originally Posted by GILow

Any guidance on how to figure out which glues might be a problem?

I have always used sikaflex 295, avaible in black and white, and who knows other colours as well. But...... Sikaflex recommends to use of primers, one for the fibreglass and the other one for acrylic. It always works well for me, but price of primers.......start saving $$

[mitiempo]

One of the best choices of sealant/adhesive is Dow 795

[GILow]

Thanks Hank and Noelex.

I’ve yet to decide how I’ll attach these things. I’ve had great success with EPDM double sided tape when I made my deck hatches. I’ll try it first, but if I run into problems I know which sealants to try next.

[Boatwright]

Replaced all the hatches and portlights a few years ago because of crazing. 1/2" acrylic (Plexiglas) per the original. This is in a very well built 35' sloop. The largest piece in an aluminum framed deck hatch is 30" x 30". Portlights are also framed and are 8" tall x 16" wide.

Very strong. This stuff is expensive -- I was able to find a guy on eBay selling offcuts from his plastic fab shop and saved some money there. I can recommend the smoke grey tint to keep solar gains down.

The Dow 795 is the best sealant. Mask carefully. Watch out for other sealants as they may contain solvents which can damage the acrylic.

[chris mac]

Originally Posted by mitiempo

One of the best choices of sealant/adhesive is Dow 795

Agree wholeheartedly! I replaced our large curved acrylic windows and they are solid. Read the literature on it and follow the recommendations.
Dow 795 holds very strong while still allowing expansion.

[BAD ORCA]

Originally Posted by GILow

So crunching the numbers for an opening 340mm x 170mm gives me 7.1mm acrylic. In practice this means 8 mm acrylic.

Bugger.

Real trouble finding 8 mm acrylic in anything with a tint. Some ok choices in 6 mm but I think I'd rather trust Loyds on this one.

10 mm has some tints available but I suspect it will look a bit odd.

Hmmmm.....

Look for 3/8" acrylic. Tints are readily available in that thickness, at least here in the U.S., but i know they are manufactured. We order them all the time. I know you are not in the U.S. but they do exist. There are at least three shades of bronze and three of grey readily available. 3/8" acrylic is actually nominal and mics out at around 0.343" This will vary a tad from manufacturer to manufacturer, but look for a similar decimal thickness if shopping in mm's

I own/operate an acrylic fabrication business and agree that would be the appropriate thickness for an offshore boat. Im re-doing all my ports in 3/8 thickness and they will get 1/2" polycarbonate storm shields over that.

My Bomar deck hatches (20 x 20" approx) that have the reinforcing cross beams, are getting re-done with 1/2" thick plexi which mics out at about 0.475". That will allow enough strength to walk on without worry.

If the thickness of the acrylic sticks out above the the thickness of the frame, just put a slight bevel on the edge to remove that sharp 90deg corner. It sounds like that wont be an issue for your istallation though.

[Rumrace]

I’ve used Lexel for years on boat windows and hatches. It’s just great stuff. I carry an emergency tube.
As for surface mounted framed ports most seem to weaken the structure while a port designed to sandwich opening adds to the boats integrity.
Its why I showed you that design with duel lens with frame which sandwich the opening. Double lens reduce fogging, insulate as well adding a stabilizing structure to the full frame.

[Bill Seal]

If you're going to use glass, use laminated, not tempered. Tempered is stupid strong, but a small sharp object will cause it to crumble, which is why car windshields are made from laminated.