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Ahoi to the forum, I have found this website recently and at first put some pics of my Narai MK IV. I have bought her as a nearly finished project, and now I am building her the last 7 years, 3 years on the ground now for 4 years motoring and further on building.


one of the first things I mounted were the windows, made of acrylic glas mounted with SIKA flex glue, the specific sika for acrylic windows, without screws. I wanted to avoid screw holes in the windows. I also wanted to avoid making special window frames. I mounted the windows with distance pieces of 6 mmso the sika glue can "work" independent from the decks and windows.


Now after some years some of the windows are cracked from the outer rim to the center. The sika gleu itself is perfect glueing and the consistence is like it should, not dried out but flexible. I will add some pics soon in an album on by page.


I have repaired the cracks with a specific 2 component acryl glue, it works, it is dry inside again, but the cracks do not stop. Last year a already exchanged one window to a new one, but also this window turned out to get cracks.


Please comment on that since I will sail from next year on and when there is a better solution I will try to fix it before next season. Thanks!

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The short of it is the cracks will never stop. The single greatest enemy to marine glazing is UV. Most windows are glued and screwed for ease of replacment and even modern UV and salt water tolerant plastics for marine glazing may need replacing after 5 years or so. If you are going to mount within a frame or drill for screw holes for mounting remember to leave an approx a 2mm gap all round and drill holes oversized to allow for expansion also do not overtighten any fastenings this helps cut down on cracking as the windows age.
There is cast acrylic and there is extruded acrylic. The cast acrylic is far superior to the extruded. I am not sure but I believe that you have used the extruded. It is much harder to work with as it tends to melt back together as you cut it and also tends to crack much easier. Learned this the hard way. David
Wondering why people don't use safety glass, you can see thru and easy to clean, won't break.
Hello Boatsmith, thanks for that comment, but the glas is alrfeady the casted one. This was one of the hints I followed some 7 years ago. But in any case thanks for your reply.

boatsmith said:
There is cast acrylic and there is extruded acrylic. The cast acrylic is far superior to the extruded. I am not sure but I believe that you have used the extruded. It is much harder to work with as it tends to melt back together as you cut it and also tends to crack much easier. Learned this the hard way. David
Frank, I don't know but, maybe the smoked acrylic has more uv resistance or possibly the edges of the acrylic being exposed to sunlight is accelerating the degradation. Hatches use acrylic and they typically last well over 7 years. I have seen smoked acrylic port lights with the edges covered that are ten years old with no cracks. David
I'm not an expert but as far as I know acrylic-glass has an expansion coefficient of 70-80 (Alpha 10-6/°K) this means up to 2.5 mm expansion for 50 cm length and 60 grade celcius difference. To avoid crack is not only a matter of strenght but also flexibility/durability and how you fix it (flexible).
Not really, that glue is an epoxy base. It really dosen't matter what glue you use you will not stop the sun from shining and plastics, all plastics from taking their natural course because of it. In a marine environment UV can be just as much as, if not more damaging than anything else, that's why most GRP boats are white to reflect the light and minimise the damage, yet still after some years cracking on the gel-coat becomes extensive as the polymers start to break down. Hatch plastics are no different but because they are transparent they don't have any gel-coat protecting them, therefore they break down a lot faster.

Chuck Valley said:
What do they use to glue automobile winshields? Would that stuff work on arylic?
The only real option besides replacement would be to drill a small 1.5-2 mm hole, depending on the width of the crack, at the end of the crack and then seal it with your glue. This should arrest any further crack development along that path.
It will not look real good but should solve your issue until you replace them.
Use Poly carbonate not acrylic it costs more but lasts a lot longer.

Also with new window edges it might be a good idea to run a belt sander over them to create a smooth surface and get rid of the saw cut marks. Perhaps then paint the edges or cover with Sika to slow up UV degradation.

Good luck.
Hello Lee,

thanks for the hints. one thing I already took care when installing the windows, the saw cuts were flamed to close the surface of the acryllic glass again. TZHat also worked fine. But another hint I will try now, after filling the cracks with the 2 component glue, I will paint the rims with normal 2 component paint, so also here a UV shield could be better.

thansk again and fair winds

I think you may be making a mistake in using the black color for sealant. I agree it looks good, but the heat is increased on the dark surface. We have had success using 4000 in white and the nuts and bolts in large holes. Finally we are happy! Good luck! Ann and Nev
I should admit that one of our pod windows has cracked recently. It is 9 years old and we did rebed it many times before we found that the 4000 was the best stickum to use. The process of removing, cleaning, and rebedding is stressful for the acrylic and cracks appeared when we were screwing it down each time. Also we did not know to use pan head screws with it and the flat head machine screws we have been using do put quite a strain on the glass. Also the pod is made of thinner ply than the hulls and likely flexes more. The hull windows are ok still as are the other pod windows unless something happened when I was not looking! Just joking... Ann and Nev

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