Perspex / Acrylic window replacement and fitting.

[samfieldhouse]

We know the score; 80s boat with perspex windows down the side of the coachroof - I'm after a sense check on my plan...

Firstly, there is, as always, great info on this site so if you've come here through a Google search read these articles too:

• Acrylic Windows
• Acrylic window film
• Using Scapa tape
• This one...
• And especially this one!

Hadlow marine have some excellent instructions on how to fit windows using Scapa tape.

Current Status
Windows out (8mm). They were previously secured with self tappers and some otherworldly adhesive. Which made the self tappers pointless. Maybe the adhesive was Sikaflex but by golly. It was 2mm thick and 40mm wide all the way around. Pulled the gelcoat off in places. Also broke the windows if I pulled too eagerly. I ended up using a mallet to drive a scraper between the cabin and the window, and then inserting a bread knife to saw through the adhesive.
Amusingly, despite all that adhesive, they still leaked!

My plan
As the self tappers have penetrated the cabin anyway I've opened out all the holes to 5mm. I've also countersunk the holes to prevent further gelcoat damage at the edge.
I like the idea of using Scapa tape. I'll stick it to the GRP and when the nuts tighten up, it forms a watertight seal under compression. Plus no mess and easy future removal. A good idea?
I'll bolt thought with M4 machine screws, with wide flange heads like these from the nice people at Westfield (highly commended). In the 5mm holes that will leave wiggle room for differential expansion - will they? Which should prevent cracking.
I'll secure internally with either nuts with a captive washer or oversize washers and dome nuts. Thoughts on this?
On the Scapa Tape - it requires a minimum 30% compression to make a seal.
I can get 4.5mm which would need to be compressed to 3.1 or 3mm to 2mm. I'm thinking the 4.5?

So in essence - new windows will be 8mm thick, with 5mm holes for fixings, secured by M4 machine screws, sealed with Scapa tape.
Sound sensible?

 

[geem]

Our coachroof windows are recessed into the grp but that is mainly aesthetic to give them a flush finish.
Our acrylic is 10mm, tinted. The acrylic is fitted with counter sunk flathead bolts with dome nuts in the inside. The adhesive is Geocel Works. 5 Atlantic crossing with these windows and they don't leak.
Use Wonder wipes to clean up easily if you use an adhesive. It makes is super easy

 

[UK-WOOZY]

Got my windows remade by Hadlow in 2018. Installed in that winter by my friend and I using that foam and bolts. No leaks. Some slight hairline cracks around the bolts here and there

 

[VicS]

Our coachroof windows are recessed into the grp but that is mainly aesthetic to give them a flush finish.
Our acrylic is 10mm, tinted. The acrylic is fitted with counter sunk flathead bolts with dome nuts in the inside. The adhesive is Geocel Works. 5 Atlantic crossing with these windows and they don't leak.
Use Wonder wipes to clean up easily if you use an adhesive. It makes is super easy

ITYWF countersunk screws are generally not advised as they are likely to cause cracking of the glazing.
.

 

[Neeves]

You need thick adhesive to accept the expansion/contraction difference between the acryilic and the fibre glass. if the adhesive is insufficiently thick, for its tensile strength, the bond will be compromised.

If you use turps to clean up it may wick between the adhesive and the substrate - reducing the bond.

Speak to the Sika rep (they make a specific sealant adhesive for your task (or the makers of CT1, if they make a specific adhesivez). 3M make a specific adhesive = again speak to one of their technical reps.

Whatever you buy as adhesive you pay for them to employ technicians - use them.

Maybe expansion differential is not important in the UK or you windows are very small. But using some form of bolt with a window that might expand, or contract, seems a recipe for cracking at the bolt holes (my post crossed wth VicS).

Jonathan

A fillet knife makes a useful tool to remove acrylic windows - and a good knife has other uses, f you like fish.

To clean the gelcoat make a chisel scraper from the bits of your windows grind a sharp edge. It will not scratch the gelcoat. Bt you need to keep the cutting edge sharp.

 

[Boathook]

I would secure internally using oversize washers and ordinary nuts. Dome head nuts look nice but you have to be sure of the about of thread on the inside and with an 1980's boat, the GRP thickness can be variable.

 

[Supertramp]

I replaced windows on a small sailing boat. They had cosmetic metal frames but the principle was the same. The problems came from flexing breaking the seal.

I used the interscrews below. There are various lengths and diameters to suit The advantage is they allow tightening while the outer fastening remains seated and doesn't break the seal. You have to get the drilled hole diameter just right so they grip.

I used a Sika sealant, applied thick and even, tightened gently, left to set then tightened a touch more (as per Sika instructions or for deck fittings).

If no metal framing which spreads the load, I would use a washer to widen the tightening area. And avoid countersunk. If you're tightening hard enough to crack the acrylic you are probably overtightening.

It was a messy job but the leaks stopped.

 

[geem]

ITYWF countersunk screws are generally not advised as they are likely to cause cracking of the glazing.
.

Its never happened on my 10mm acrylic in 49,000nm. I have the old windows as spares in case one got broken. They are crazed but no cracks in any of them. My windows are all small so thermal expansion is limited. I think if you have long windows, the thermal expansion will cause problems and this may well generate cracks at the fixing points. Its not a problem of countersunk fixings itself.

 

[samfieldhouse]

I had a chat with the yard making the windows.

I would secure internally using oversize washers and ordinary nuts. Dome head nuts look nice but you have to be sure of the about of thread on the inside and with an 1980's boat, the GRP thickness can be variable.

Yes, this - we discussed interscrews but have indeed opted for machine screws. Nice oversized button heads with socket heads. Internally, penny washers. I have both normal nuts and dome nuts. I buy everything from Westfield Fastners so its £3 for 100 A4 nuts.

The windows are indeed large so there will be no adhesive. The SCAPA tape should do the waterproofing under compression, whilst the oversized holes in the fiberglass should allow for movement. Here's hoping!

 

[geem]

I had a chat with the yard making the windows.


Yes, this - we discussed interscrews but have indeed opted for machine screws. Nice oversized button heads with socket heads. Internally, penny washers. I have both normal nuts and dome nuts. I buy everything from Westfield Fastners so its £3 for 100 A4 nuts.

The windows are indeed large so there will be no adhesive. The SCAPA tape should do the waterproofing under compression, whilst the oversized holes in the fiberglass should allow for movement. Here's hoping!

You need to allow for the full expansion and contraction of the acrylic between summer temperatures and winter temperatures. From zero to 30degC. Higher if your acrylic is tinted as it will self heat in the sun more than clear. You will see over 2mm of movement on a 1 metre long piece of acrylic.
Our longest window is 750mm so we can see about 1.6mm of movement. Countersunk screws, adhesive not tape, and oversized holes in the grp work well for us

 

[Cerebus]

We know the score; 80s boat with perspex windows down the side of the coachroof - I'm after a sense check on my plan...

Firstly, there is, as always, great info on this site so if you've come here through a Google search read these articles too:

• Acrylic Windows
• Acrylic window film
• Using Scapa tape
• This one...
• And especially this one!

Hadlow marine have some excellent instructions on how to fit windows using Scapa tape.

Current Status
Windows out (8mm). They were previously secured with self tappers and some otherworldly adhesive. Which made the self tappers pointless. Maybe the adhesive was Sikaflex but by golly. It was 2mm thick and 40mm wide all the way around. Pulled the gelcoat off in places. Also broke the windows if I pulled too eagerly. I ended up using a mallet to drive a scraper between the cabin and the window, and then inserting a bread knife to saw through the adhesive.
Amusingly, despite all that adhesive, they still leaked!

My plan
As the self tappers have penetrated the cabin anyway I've opened out all the holes to 5mm. I've also countersunk the holes to prevent further gelcoat damage at the edge.
I like the idea of using Scapa tape. I'll stick it to the GRP and when the nuts tighten up, it forms a watertight seal under compression. Plus no mess and easy future removal. A good idea?
I'll bolt thought with M4 machine screws, with wide flange heads like these from the nice people at Westfield (highly commended). In the 5mm holes that will leave wiggle room for differential expansion - will they? Which should prevent cracking.
I'll secure internally with either nuts with a captive washer or oversize washers and dome nuts. Thoughts on this?
On the Scapa Tape - it requires a minimum 30% compression to make a seal.
I can get 4.5mm which would need to be compressed to 3.1 or 3mm to 2mm. I'm thinking the 4.5?

So in essence - new windows will be 8mm thick, with 5mm holes for fixings, secured by M4 machine screws, sealed with Scapa tape.
Sound sensible?

Yes