Catamaran Windows

[ausnp84]

Hey guys,

The time has come to replace the windows.... managed to get them all out today and off to the perspex place, and now I’m trying to work out how to attach the new ones.

1.) Mastic (eg. Sika, bostik, arbo) the windows between perspex and fibreglass inset (using the appropriate primer), then once that’s cured, seal around the edge of each window with more mastic

2.) Butyl tape the windows, then again use a mastic around the edges.

I’m through bolting the windows as well (approx 10 M4 bolts per window) and based on conversations with umpteen contractors at the boatyard, will be painting over the mastic’d area on the outside of the windows (approx 2.5cm / 1” all round the window) with 2 pack poly as that’ll help limit the UV issues perspex windows often suffer from.

SO. Who’s recently done perspex windows? What worked, what didn’t, what brand mastic, butyl etc did you use? I know there’s tonnes of threads already, but products seem to be changing regularly and I’d like to hear real-world experience.

Ta in advance!

N

[44'cruisingcat]

I used 3M VHB double sided tape and Dow Corning 791 or 795 silicon.

Polyurethane sealers dont adhere well to acrylic.

I haven't used any screws.

When cutting the acrylic and in selecting the adhesives and sealant you use, you have to allow for a considerable amount of expansion. Similarly with through screws, which might be difficult without causing leaks.

From memory the double sided tape I used is 5mm thick, which allows for expansion without sheering.

Your windows look nice and flat, IMO you shouldn't need screws.

[GRIT]

I used 3m vhb tape as well, (4991, which is 2.3mm thick) and Dow 795 around the edges.

I'm at the limits of the tape, in window mass, and expansion. Very large pieces of 12mm Plexiglass.

Scuff the plexiglass with 3m red abrasive pads, on the bonding surfaces, before installing the tape.

No bolts/screws here either.

Our windows have only been installed for a year now, but we're pleased with the results. This winter the temps went down to -22C for several weeks, and the windows are still there.

Cheers.
Paul.

[ausnp84]

Thanks guys, this is the exact sort of real-world experience I’m after.

The windows are 10mm and quite large overall, but sloped backwards so the structure takes the weight (rather than say a Lagoon, where they’re vertical). The reason for the screws is a four of the windows have a slight twist in them, and need to be nipped in a bit.

Did you fit the windows with the tape first, then just go around the seams with Dow post-installation as a final seal?

N

[ausnp84]

Originally Posted by 44'cruisingcat

I used 3M VHB double sided tape and Dow Corning 791 or 795 silicon.

Polyurethane sealers dont adhere well to acrylic.

I haven't used any screws.

When cutting the acrylic and in selecting the adhesives and sealant you use, you have to allow for a considerable amount of expansion. Similarly with through screws, which might be difficult without causing leaks.

From memory the double sided tape I used is 5mm thick, which allows for expansion without sheering.

Your windows look nice and flat, IMO you shouldn't need screws.

Do you remember the thickness of tape you used as well? I’ve just checked the 3M site and there’s a tonne of options...

N

[44'cruisingcat]

Originally Posted by ausnp84

Do you remember the thickness of tape you used as well? I’ve just checked the 3M site and there’s a tonne of options...

N

It's either 4 or 5 mm thick, 20mm wide.

The VHB tape is very strong.

I ran the tape around the inner edge of the window flange, cut away some of the foam so the cover film had a "tail". Positioned the perspex, then pulled the cover film off using the tail.

Then from the outside, squirted the silicone under the perspex and around the edges to fill the remaining space. I actually continued the silicone up and onto the outside of the perspex. I used a firm cheap foam tape as a "dam" to put an edge on the sealant. Once the sealant was cured that foam tape came off.

[Chotu]

Did the same here as Grit and 44 cruising cat after a lot of trial and error. Very large window sections and exposed to temperatures from 50F to 100F and blazing tropical sun.

No failure over 2 years now.

Get the thickest 3m VHB they make. The thicker it is, the more it can handle the movement of the window material, which is significant due to thermal expansion.

The rest is just sealing and cosmetic with the Dow 795.

[GRIT]

I agree with getting the thickest tape available. The 4959 was the thickest I could find, here in Canada, at 3mm, but 3m said it was too "hard" to be used on my windows. They suggested 4991, which is a medium density tape. It's 2.3mm thick, and I got the 25mm width.

Cheers.
Paul.

[ausnp84]

Ok cool, thanks guys. I’m going to struggle to get Dow in the UK but can get Sika 295UV, which should do as a flexible gap sealer.

Update to follow!

N

[GRIT]

If you're using 295uv, be sure to get the primer too.

We couldn't find Dow in Canada either, and imported it from the USA.

Good luck.
Paul.

[OldGreyB]

Successfully executed Catalina ports replacement late last year. As per our write up in

Cruisers & Sailing Forums > Engineering & Systems > Construction, Maintenance & Refit : New window sealing question.

We:
Surfaced abraded the acrylic sealing areas and then etched with Wattyl Super Etch© Primer (black). Installed using 3M VHB tape (black), holding the new ports under tension for 12 hours. Final external edge sealing with Sikaflex 295 (black).

One trick we used was taping right angle aluminium blocks onto the internal side of the ports, and wooden blocks taped to the interior edge of the structure during a trail fit. This ensured we could check new port placement and re- router when required so our external gaps were consistent. Then when finally placing the port, we had correct placement the first time onto the exposed 3M VHB tape.