Acrylic windows are cracking

[john james]

Lots of hairline cracks appearing around the bolt holes in my acrylic the holes are drilled oversize and the acrylic his bedded in 3M 4000 UV I AM PLANNING TO TRY A SOFTER CAULKING MATERIAL SUCH AS SUDBURY ELASTOMERIC MARINE SEALANT AND HOPE FOR BETTER RESULTS. Any comments or suggestions I can Report that 3M product is wickedly hard to Break loose.

[Steve Denton]

Hello John,

By chance, did you use Loctite on the screws and or nuts?  In the past I was building a RV6 aircraft and one of the problems that surfaced with other builders was the screw holes had radial cracks emanating from them.  The manufacture traced it back to Loctite use on the screws.

 

[Éric Bouvéron]

Please try to visit this page:

http://tiki26element2.blogspot.hk/2010/09/window-installation-steps...

That's what I did on my not yet launched Tiki21.

Or some are using tape:

https://www.youtube.com/watch?v=MQyjxVUskd8

I didn't used the same product, but one very similar as permanent sealant. I may give you a name later, I just don't remember right now.

But when I tried to buy this, it was (where I live) more expensive than gold.

I think these two methods are far better than the bolts, future will say...

All the best, 

Éric

[agur paesüld]

Instead of screws I used bolts'n'nuts and washers. On a first attempt I made too small bolt holes - therefore cracks were soon developed, probably catalyzed by the fact that I over-tightened the nuts. With more gentle approach and slightly larger bolt holes I managed to avoid cracks so far. I used plexiglass, specifically Makrolon brand, which is poly carbonate as far as I understand... utmost flexible material, you can bend it almost 180 degrees... For sealant I used marine grade sikaflex – excellent stuff as it creates really strong bond but stays somewhat flexible in same time, like rubber...

[Robert Hughes]

What shape are the heads of the screws?   If they're countersink heads they tend to act like wedges and expand the holes, just like if the holes were too small.  I've used pan heads this time for the windows on my T26.  They're not as neat as countersunk would be, but they don't cause cracking like on the old windows.

[john james]

No I did not use Loctite. I used small Panhead bolts and nuts with washers.

Steve Denton said:

Hello John,

By chance, did you use Loctite on the screws and or nuts?  In the past I was building a RV6 aircraft and one of the problems that surfaced with other builders was the screw holes had radial cracks emanating from them.  The manufacture traced it back to Loctite use on the screws.

 

[agur paesüld]

Anyways, be careful not to over tighten the nuts, as Plexiglass-type of material expands not only laterally but to "height" as well – in Itatae case, those bolts that were really tighten (looking from certain angle I noticed that plexiglass was curving down below those bolt heads) developed more and bigger cracks. For example Sikaflex bonds so well that basically one may install the window without screws/bolts as I learned well when I messed about hour to remove damaged window – I did burn lots of calories indeed before last strip of Sika was scratched off the boat.

(Im not suggesting here to skip screws/bolts, as saltwater and UV will degrade Sika or whatever other sealant by time...)

[boatsmith]

It is important to use cast acrylic as opposed to extruded acrylic. I prefer acrylic to polycarbonate as it stays clear longer. We bed windows with Sikaflex,no screws or bolts. I have not used it but Dow make some tape that I have heard works well.

[Ian R]

I glued my macrolon (ie slightly different) windows on without any bolts. If glues (I chose a damn good one) don't work, the boat will fall apart, so why have the bolts? I can't push my windows out from the inside with my feet and I have footballer's legs.

Your choice of sealant/glue appears to be wrong, but I'm not an expert on this very complicated area and won't advise, you need to explore the options yourself, and maybe contact the techs at 3M or Dow Corning direct as I did.

[john james]

I don't know the difference between cast and extruded acrylic? I think my problem is mainly being over zealous in tightening those bolts. I've never had cracked acrylic on any of my other boats but they were fastened with wood screws which are not likely to get over tightened. On another vein ,what type of rope do you recommend for rope lashings hull to crossbeams pre-stretched or not, diameter?

boatsmith said:

It is important to use cast acrylic as opposed to extruded acrylic. I prefer acrylic to polycarbonate as it stays clear longer. We bed windows with Sikaflex,no screws or bolts. I have not used it but Dow make some tape that I have heard works well.

[Éric Bouvéron]

On my Tiki21, I used a 4mm polyester, one that doesn't streched. 

However I made the lashing in another way that the one recommended on the plans.

I do not turn like making an A, but rather like making a V. That means that my line does not made full turns, but goes back around the wood block everytime it goes in the shroud. At the end, I made a succession of 8 under the shrouds, making the line going inside the previous eight. Easier to get tension, and quite strong.

Check your plan for the diameter needed

[boatsmith]

Your plastic supplier should be able to tell you what he is selling. Extruded acrylic likes to melt back together when cutting with a jig saw. We use stay-set double braid polyester for the lashings and dynex for the frappings.
boatsmith said:

It is important to use cast acrylic as opposed to extruded acrylic. I prefer acrylic to polycarbonate as it stays clear longer. We bed windows with Sikaflex,no screws or bolts. I have not used it but Dow make some tape that I have heard works well.