Best Material for Windows

[David Broun]

Hi all

I need to replace the windows on my Tiki 38 and am researching the best material and technique for this. I will be cruising in a hot tropical climate. There has been reference to various types of plexiglass, acrylic etc as well as the merits of bolts or just gluing. Any advice would be appreciated, especially including the specific names of glues, plastics etc.

Cheers

Dave

[boatsmith]

Plexiglas is a brand name for acrylic. You want to use cast acrylic as opposed to extruded acrylic. We use Sikaflex 291 and the associated primers. We have installed many windows with this adhesive,acrylic and mineral glass, up to 3' by 16', and have had only good results. I have heard that Dow 795 is a good product and much less expensive without the need for primers.
Scott Williams has a step by step description with photos of the installation technique that I use on his building blog http://tiki26element2.blogspot.com/ David

[David Broun]

Wow great advice and link- thanks David. The framed windows currently on the boat have trapped water and led to rot in one case so the flush mounted ones described look like they will solve this problem as well as looking good.
Cheers
Dave

[Robert Sheridan]

We used lexan to replace our windows. It is what is used for riot shields so we reckon it should stand up to some pretty bad weather! You have to drill the holes for the bolts over size and use a sealant designed for Lexan (vapourless?).

[Hans Hammig]

Acrylic seems to be the best material. On the hulls (Tangaroa MK IV) my windows are mounted on top of the skin and so I use bolts (6 mm stainles) in addition to the silicon. The area covering the silicon should be painted, not as a primer, but as a shield against UV, which is harming the silicon. It also looks better.

On the cabin tops and deck salon, the windows are mounted flush, so no bolts are needed. However, the acrylic should still be painted to protect the silicon.

Sikaflex give very good advice.

[stephen c veale]

I rebeded some windows on my Heavenly Twins last summer using GE Silicon II as well, with good results.

[David Broun]

Thanks so much for all of the advice everyone- one question- is it possible to glue the material (acrylic or lexan) to the outside of the skin without rebating or fixing a frame inside the hull material? Would this be strong enough without bolts/screws? Might not look as nice but would be much easier!

Dave

[Scott Williams]

Dave, that's my blog that Boatsmith referred you to.  I considered simply bonding the windows to the outside of the skin as you are suggesting, and yes, it would be strong enough as long as you have sufficient overlap of the plastic over the edge of the opening.  In the end I went with the rebated lips to reduce the overall size of the windows, which would look far out of proportion to the rest of the cabin sides if they were big enough to overlap properly.  It would be easier and faster, but as it turned out, making the interior lips was not too much trouble, and I like the fact that the windows are now flush with the outside of the cabin sides.  I would never use screws again to fasten windows after discovering this technique. 

Scott

[Hans Hammig]

We are now at the heart of the question. All seem to agree that there are no problems as long as the windows are in a suitable recess. Now to the hull port holes. I was thinking of gluing in an internal 'lip', but that makes the window smaller. I also did not want to add an external frame. So I thought I would use the bolts. Sure enough, I would put in 5 mm shims until the silicon has settled. The question: Would you rely on a simply 'glued on top of the skin' acrylic sheet, one foot above the water line? What if another boat scrapes along your hull? Would it be strong enough even if it hooked up somehow?

So I will still add the bolts, but, as pointed out, not to tighten the acrylic close to the hull, just as added strength. Shims of 3 - 5 mm must be used until the silicon has settled.

Any other fire-sure, space technology?

[Scott Williams]

The internal lip that recesses the windows is not required for strength, just to make them flush with the outside of the cabin surface, which looks better.  Structurally, there would be no difference in bonding the plastic to the surface of the lip or the outside of the opening, as an overlay (as long as there is a sufficient amount of overlay to make a proper bond). The Dow 795 adhesive that I used is the same material that is used to bond heavy plate glass windows to their frames in modern skyscrapers.  I too had some reservations at first  about not using screws as a back up, but then after researching the adhesive my worries went away.  If you can't trust this structural adhesive designed for this purpose to hold the plastic windows, what is the difference in trusting epoxy to hold the hull panels and all the other parts together without fasteners to back up every joint?  Just as new epoxy users should make some test joints to learn the process, I suggest getting some of the plastic and the adhesive and bonding some test samples.  Let it fully cure and then try a destructive test to see for yourself.

[Brian]

If it makes anyone feel better we also use DOW 795 to bond aquarium windows to concrete tanks...it holds.

[Hans Hammig]

Well, I admit that all the arguments supplied have made me think of a compromise solution. Let us cut the acrylic glass at an angle of about 45º so that it cannot be hooked by an object scraping along the hull. Glue only (silicon). Will it pass our marine inspection, surveyors, South African Marine Safety Agency (people who don' t know the difference between a belay pin and a nail)?

It means that we (I) will bevel the protruding windows in a way that an object scraping along the hull will simply be deflected, but will not hook onto the window, and hence will not exert an undue force onto it. Thank you very much for saving me all this work!

[David Broun]

Thanks Scott- do you bond the adhesive direct to cured epoxy on the lip and then paint over, or paint the surface first then apply the adhesive to glue the sheeting?

Cheers
Dave