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Old 15-08-2022, 02:37   #1
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Acrylic portlight thickness.

Hi all,

Can anyone point me to guidelines on determining an appropriate acrylic thickness for home made portlights?

I feel like there should be some kind of formula involving area or maximum span.

Matt
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Old 15-08-2022, 03:44   #2
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Re: Acrylic portlight thickness.

Like most things boating... it depends, and maybe the CF member with that name can contribute as well.
I do not have a formula for that, but if there is one, the following variables should be in such formula:
1a. Strength of material. Note there are a lot of non-brand stuff around, ask for the specs! like UV protection etc.
1b. Polycarbonate or acrylic (the former is much stronger and is flexible)
2. Where is the window? in the hull,? needs to be as strong as the hull. In the cockpit? can be much thinner
3. Where on the boat is the window? Facing forward and close to the bow needs to be much stronger than on the stern facing aft
4. How big is the window? large unsupported surfaces needs to be much stronger then small little portholes
5. where does one sail? ie add in the worst possible weather /wave conditions you encounter. When day sailing on a small inland lake, windows can be fairly thin, when sailing in the roaring forties....... just a but more thickness is required

I do not think size of boat comes into it, except for when having portholes in the hull.
I do not think how it is fastened comes into it, but note that a larger window need a larger supporting surface!!!! I use 25 to 50 mm supporting surface.

You may want to check with other similar boats what they use. I mainly use acrylic for smallish windows 6 mm, and 10 or 12 mm for larger windows or facing forward. When using polycarbonate these sizes can be each 1 to 4 mm less, ie 4 or 5 mm for general use, and 6 to 10 mm for larger windows.

Buying it by the sheet makes it a lot cheaper!!! Or buy a sheet with an other boatie.... if you can both agree on thickness, colour and quality.
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Old 15-08-2022, 04:57   #3
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Re: Acrylic portlight thickness.

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Originally Posted by GILow View Post
Hi all,

Can anyone point me to guidelines on determining an appropriate acrylic thickness for home made portlights?

I feel like there should be some kind of formula involving area or maximum span.

Matt

Why are you considering acrylic? Surely tempered (toughened) glass is the way to go?

Quality yachts like Hallberg Rassy, Amel, Hylas, Bavaria have glass ports.
(Of course if you are working to a strict budget.......)

Glass portlights don't need frames but can be glued into position just as a car windscreen is.


DIY: Replacing Fixed Portlights
https://www.sailmagazine.com/diy/rep...xed-portlights

https://www.cruisersforum.com/forums...ic-147003.html
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Old 15-08-2022, 06:45   #4
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Re: Acrylic portlight thickness.

QUOTE: "Glass portlights don't need frames but can be glued into position just as a car windscreen is."

Whoa. I used to run a 25 ft. work boat with a tempered glass windshield mounted that way. Another user crossed a wake, took it on the bow, and spread the windshield all over the floor of the wheelhouse. I think clamping strips or an overlap are called for.
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Old 15-08-2022, 12:15   #5
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Re: Acrylic portlight thickness.

It has to be acrylic. That’s why I asked about acrylic, not glass.
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Old 15-08-2022, 12:47   #6
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Re: Acrylic portlight thickness.

If you’d like an aircraft style port no worries. I have the patern for a double glazed Marine port. It’s designed to adjust to size and then be 3D printed.
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Old 15-08-2022, 12:58   #7
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Re: Acrylic portlight thickness.

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If you’d like an aircraft style port no worries. I have the patern for a double glazed Marine port. It’s designed to adjust to size and then be 3D printed.


Thank you, but I’m going with the modern style external bolt on sort. It’s all about a quick fix at this stage.
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Old 15-08-2022, 13:07   #8
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Re: Acrylic portlight thickness.

When I made my port lights I bought the Acrylic sheet first 16mm and made SS frames to fit I don't know if there is a formula but I go for overkill. I also replaced some existing toughened glass ones with the same sheet but a few mods were required.
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Old 15-08-2022, 13:10   #9
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Re: Acrylic portlight thickness.

For an offshore boat 12mm is always safe, unless the glazed area is very large ( not likely to be seen on cruising yachts). 10mm is OK for mid sized openings (most offshore hatches are 10mm). 8mm is often used especially for smaller openings but this makes me nervous offshore.

The required thickness is much the same for hardened/laminated glass, acrylic and polycarbonate, but when choosing plastic material (acrylic, or polycarbonate) the deterioration in strength due to age and UV needs to be taken into account so with these materials err on the generous side unless you are willing to replace frequently.
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Old 15-08-2022, 13:29   #10
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Re: Acrylic portlight thickness.

With the caveat that "nothing too strong ever broke", this is the only calculation for this I have seen. It is specifically for wheelhouse windows, and should work for smaller openings as well:

From Lloyd's Special Service Craft P3, Ch 4, sec 7.8

Thickness calculations for toughened safety glass, not less then 6 mm.

Calculate thickness with:

t = .005b*sqrt(B*p) [mm]

b = length of shorter side of window, in mm
p = design pressure (take 70 kN/m^2).
B = -.17 + .54*A-.078*A^2 for A<=3
= .75 for A > 3

A = aspect ratio of window, a/b
a = length longer side of window

Multiply the calculated thickness by 1.3 for polycarbonate and by 1.5 for acrylic.

Llloyds does not allow Plastic as wheelhouse windows, but you probably aren't building to their standards.

The design standards a a gold mine of design information, and if you are building boats, you should have them. They are free.
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Old 15-08-2022, 13:35   #11
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Re: Acrylic portlight thickness.

5mm is the minimum and 8mm kinda the max. You would likely want to order a smoke to reduce UV.
The 3D print ones are through bolt. Designed for open water very similar to production boat frames. The double glazing adds some great benefits, should you ever need them. Way way stronger than aluminum frame.
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Old 15-08-2022, 13:53   #12
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Re: Acrylic portlight thickness.

Quote:
Originally Posted by ItDepends View Post
With the caveat that "nothing too strong ever broke", this is the only calculation for this I have seen. It is specifically for wheelhouse windows, and should work for smaller openings as well:

From Lloyd's Special Service Craft P3, Ch 4, sec 7.8

Thickness calculations for toughened safety glass, not less then 6 mm.

Calculate thickness with:

t = .005b*sqrt(B*p) [mm]

b = length of shorter side of window, in mm
p = design pressure (take 70 kN/m^2).
B = -.17 + .54*A-.078*A^2 for A&lt;=3
= .75 for A &gt; 3

A = aspect ratio of window, a/b
a = length longer side of window

Multiply the calculated thickness by 1.3 for polycarbonate and by 1.5 for acrylic.

Llloyds does not allow Plastic as wheelhouse windows, but you probably aren't building to their standards.

The design standards a a gold mine of design information, and if you are building boats, you should have them. They are free.


Ah, EXACTLY the thing I was looking for, thank you.
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Old 15-08-2022, 13:54   #13
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Re: Acrylic portlight thickness.

Itdepends is so right. You almost can’t beat glass. On several shipwrecks at some serious depths several wreck went down glass intact. They become a death trap for amateur divers thinking the trapped exhaled air is safe. We took up a practice to kick out at least one glass as not to trap air.
ON the Wolfe islander we removed all the glass to protect divers but didn’t not weld access to the lower hull. Diver got lost in it sadly. It’s now welded. Glass really stands up and laminated.
My power boat has no windscreen for safety sake.
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Old 15-08-2022, 14:01   #14
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Re: Acrylic portlight thickness.

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You would likely want to order a smoke to reduce UV.
The problem with this is thermal expansion.

Clear acrylic expands around 8x more than glass and this creates stress on joints and sealants. This can produce leaks and even complete failure, especially when mechanical fixing is eliminated, as is common on modern yachts.

Tinted acrylic will heat up more, exacerbating the thermal expansion problem.
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Old 15-08-2022, 14:11   #15
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Re: Acrylic portlight thickness.

The newer castings are more stable. Kinda depends on where the plastic comes from.
I print in ASA TPU PEEK PA11-CF. They all shrink curing then never again.
The old smoke Dark was notorious for warping and cracking
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