Hard Dodger Window Material

[sailingharry]

I put tempered glass lights in my Sabre 34 to replace the badly crazed plastic ones. I was very happy with Peninsula Glass, and their online order tool is exceptionally well done. They can handle holes and odd shapes online, and if it is really weird, they will take a drawing.
https://www.onedayglass.com/

[s/v Jedi]

Originally Posted by Knotical

Jedi, that is an excellent write up, visualizing the process I imagine it not being that easy to inject silicone under the plastic with the rebate that you are talking about. Do you suggest making a wide rebate, or maybe use a small and curved nozzle?

Thank you

On the backside of the window, around the perimeter but 6mm away from the edge you mount VHB tape. So the tape sits between flange and acrylic and is the structural adhesive. It’s not very thick, maybe 3mm, so the space behind the window to fill is only 6mm wide and 3mm high. Next to the window you have the 6mm wide channel that you stick the nozzle in, straight down. As you squeeze the sealant in, it will go towards the VHB tape by itself. As soon as it reaches it, you move the nozzle a bit along to fill it there, which also fills the channel that the nozzle is in where you just moved away from. When it doesn’t fill all the way to the top, you pull the nozzle halfway up and/or point it a bit in the direction.

By the time you finished all the windows, you get the hang of it

On the outside surface I spray it with a dish soap + water mixture, then use a plastic spatula (yellow from epoxy work) which is also sprayed to smooth it out. Of course there’s masking tape or protective film so I bend the spatula a little hollow so that it looks good after the tape is removed.

[AKA-None]

Didn’t @chothu have an extensive thread on sealing large windows
Might be something interesting there

[s/v Jedi]

Originally Posted by AKA-None

Didn’t @chothu have an extensive thread on sealing large windows
Might be something interesting there

I don’t know why this is discussed here… the largest of Op’s windows is well under 3’ or did I miss something?

[AKA-None]

Originally Posted by s/v Jedi

I don’t know why this is discussed here… the largest of Op’s windows is well under 3’ or did I miss something?

My misunderstanding then as I thought they were larger

[scherzoja]

Good morning

Based on the advice over the last few days, I changed the design of the layup schedule and the windows.

I redesigned the window openings for acrylic windows, largely based on Jedi's design input (Thanks Jedi). Some of you commented on the inadequate scantlings, so I will use a minimum of 2 layers inside and out, with 3 or 4 tapered layers in the corners. The aft corners between sides and roof will have a gusset on each side. In addition to that, I'll have one or two support beams glassed into the overhead to provide "i-beam" support, running athwartships. These beams are just 4" or 6" wide strips of core material, tapered sides and glassed into the overhead.

I've ruled out aluminum framed glass windows because a set of five windows is between $1500 and $2500; that's just too much to spend of dodger windows. I also don't think I'll be able to make the sides stiff enough to prevent the glass from breaking from twisting/flexing.

I calculated the thermal expansion (0.000040) with an 80 degree F temp change and came up with almost 2.7 mm in width and 0.75mm in height of the largest window (the side window @ almost 34"). Because of Thinwater's observations of failures in windows > 30", I am testing design changes to break up those larger side windows.

The opening window might not suffer from that much expansion problems because, if I understood Jedi's description (post #10), that window will be two separate pieces (frame that is VHB'd to the opening rabbet and the opening piece, likely only being about 27 or 29" wide. That is about 2 mm expansion, but that large panel is floating from the hinge and not bound by VHB or sealant.

I based the expansion calculations on an 80 degree F temp change (50 degrees to 130 degrees). Over the summer, I measured the surface temp of a darkish spot of fiberglass on the cockpit sole in that has been in direct sunlight for several hours when the air temp was 100 F and that surface temp was 130 F, so I know that 130 can easily be reached. Of course, the previous night's low was around 80, not 50; however, sometimes we get cool nights, then warm day temps so this range seemed reasonable to cover that possible high surface temp.

It was interesting to note that the difference in surface temp between a dark spot and a white spot in the cockpit on 100 degree day was about 25 degrees. I actually burned my knees when I knelt down on the dark spot, blisters and all.

Jedi, I attached a diagram of the window opening design. Please let me know if this is an accurate representation of your description in post #10.

Thank again for all the insightful advice.

[Bowdrie]

In post #10, s/v Jedi described a method using a tape product.
Others have also described various methods.
I've not used the VHB tape, so can't comment.
Should you go with some kind of stuff-in-a-tube, this link is from Sika and
should make calculations of expansion and installation somewhat easier.
I have an older, but quite comprehensive download but can't find it.
But this one may be sufficient.
https://usa.sika.com/dam/dms/us01/d/...%20Windows.pdf

[scherzoja]

Bowdrie

I reviewed that document and has nice illustration of the joint; however, because of the need for primer and UV degradation that Thinwater alluded to in post #8, I'll stick with silicone based sealants like Dow 795. The document you referenced and the attached document (Sika's "Pre-Treatment Chart") give the details on these issues.

I spoke with 3M today and, after discussing my project, their recommended VHB tape for this application is their RP+ series. RP+ 230GF is their 2.3mm gray RP+ tape. They used to recommend VHB 5952, but have discovered that this tape fails from fatigue from too many expansion/contraction cycles. They recommended sealant model 3M 590, which is a polyurethane sealant. This was a little puzzling.

One part of this that I don't understand is that the VHB tape is relatively stiff compared with Dow 795 (50% elongation vs 450% elongation). So how will the VHB tape flex with the thermal expansion? I guess it might not matter because if I use 1/2" tape, then 50% elongation is 1/4" and my calculated elongation for the widest window is 0.1088" (2.7 mm), then I'm still within the 50% range. Do I understand this correctly?


I'm going to get back to fiber glassing the dodger, then revisit the window design later next week after glassing is done and rabbets have been routed.

Thank y'all again for the help and have a great weekend.

[Matt Johnson]

BTW, my decks and cabintop have the equivalent of a single layer of 1708 covering all exterior surfaces (but in s-glass), core is 3/4", and the interior is 23 oz Triaxial.

[s/v Jedi]

Originally Posted by scherzoja

Good morning

Based on the advice over the last few days, I changed the design of the layup schedule and the windows.

I redesigned the window openings for acrylic windows, largely based on Jedi's design input (Thanks Jedi). Some of you commented on the inadequate scantlings, so I will use a minimum of 2 layers inside and out, with 3 or 4 tapered layers in the corners. The aft corners between sides and roof will have a gusset on each side. In addition to that, I'll have one or two support beams glassed into the overhead to provide "i-beam" support, running athwartships. These beams are just 4" or 6" wide strips of core material, tapered sides and glassed into the overhead.

I've ruled out aluminum framed glass windows because a set of five windows is between $1500 and $2500; that's just too much to spend of dodger windows. I also don't think I'll be able to make the sides stiff enough to prevent the glass from breaking from twisting/flexing.

I calculated the thermal expansion (0.000040) with an 80 degree F temp change and came up with almost 2.7 mm in width and 0.75mm in height of the largest window (the side window @ almost 34"). Because of Thinwater's observations of failures in windows > 30", I am testing design changes to break up those larger side windows.

The opening window might not suffer from that much expansion problems because, if I understood Jedi's description (post #10), that window will be two separate pieces (frame that is VHB'd to the opening rabbet and the opening piece, likely only being about 27 or 29" wide. That is about 2 mm expansion, but that large panel is floating from the hinge and not bound by VHB or sealant.

I based the expansion calculations on an 80 degree F temp change (50 degrees to 130 degrees). Over the summer, I measured the surface temp of a darkish spot of fiberglass on the cockpit sole in that has been in direct sunlight for several hours when the air temp was 100 F and that surface temp was 130 F, so I know that 130 can easily be reached. Of course, the previous night's low was around 80, not 50; however, sometimes we get cool nights, then warm day temps so this range seemed reasonable to cover that possible high surface temp.

It was interesting to note that the difference in surface temp between a dark spot and a white spot in the cockpit on 100 degree day was about 25 degrees. I actually burned my knees when I knelt down on the dark spot, blisters and all.

Jedi, I attached a diagram of the window opening design. Please let me know if this is an accurate representation of your description in post #10.

Thank again for all the insightful advice.

Yes, you got the diagram correct

On the fiberglass: if you can get it somewhere, instead of two layers 1708, do one layer 1708 plus one layer 1808. This provides an isotropic laminate, strong in every direction. 1808 is like 1708 but instead of +/- 45 degrees, the fibers are aligned at 0/90 degrees.

Smaller windows are not as elegant but stronger.

About protecting the acrylic edges against UV: this is crucial. If UV enters the edge, it gets reflected back into the material by the large surfaces over and over so you get maximum UV damage. It’s like an optic fiber.

[s/v Jedi]

I got one more tip on the acrylic: they sell glue for it but I recommend you experiment with solvent welding it with just acetone and a needle dispenser. The trick is to get the capillary action to do the work. Mask everything very well…

[Tupaia]

Couple of things.

I would use 10mm.

If you go for plastic choose a Cast Acrylic, Persex(t) to my knowledge is extruded and will craze quite quickly. Acrylic can be supplied with anti UV and anti scratch coatings. Choose clear not a tint, the dodger will be difficult to see out of at night if it is tinted.

It should be glued in with flexible silicone (GE 4000) using a 2 dimensional fillet and does not require mechanical fixings. Smaller panels limit the expansion problems.

Many windows leak because the glue cannot resist the combined effects of shear and tension (tear) in multiple directions at the same time. This can be overcome by using foam expansion beads in the joints. There are lots of internet references. Construction example but same applies to boats.

https://media.licdn.com/dms/image/C5...KB0oMt1HrE-IGU

Polycarbonate is very difficult if not impossible to glue successfully where there is any structural movement and is much more expensive than Acrylic so i would not use it for this application.

Armoured glass is an option but heavy and expensive.

[s/v Jedi]

Perspex is the correct material. It is cast acrylic, see https://www.display.3acomposites.com/perspex/

We always sail in hot climate and wouldn’t want clear, untinted windows. The heat is bad enough with tint.

At night you can’t see through windows because it is dark outside, not because the windows are tinted. I can’t see anything through clear portholes either. I normally do a visual 360 scan every 15 minutes and use radar and AIS. This year I am experimenting with a camera.

[Tupaia]

Originally Posted by s/v Jedi

Perspex is the correct material. It is cast acrylic, see https://www.display.3acomposites.com/perspex/

It seems we are both wrong (or right) Perspex comes in both forms.
https://www.perspex.co.uk/materials/...ruded-acrylic/

[Tupaia]

Originally Posted by s/v Jedi

We always sail in hot climate and wouldn’t want clear, untinted windows. The heat is bad enough with tint.

At night you can’t see through windows because it is dark outside, not because the windows are tinted. I can’t see anything through clear portholes either.

Got to disagree.

Try wearing sunglasses at night. It's not because it's dark outside.

This a hard dodger, I assume to replace a soft dodger with PVC clears. If you have ever piloted a powerboat from inside a cabin you will understand. They invariably have clear forward windows. Pretty much the same applies to tinted windows on motor vehicles. Most countries strictly regulate of prohibit tinted windscreens because it cuts down night time vision. Even clear glass/plastic reduces clarity over nothing at all.

Originally Posted by s/v Jedi

I normally do a visual 360 scan every 15 minutes and use radar and AIS. This year I am experimenting with a camera.

Sure there are lots of modern aids but they are just that aids.