Possible to use glass instead of plexy for hatch?

[noelex 77]

Originally Posted by Mike Banks

It is theoretically possible to use glass--just not 8 mm. It would have to be much heavier than that, laminated and tempered. Far too expensive and heavy to be practical.

8mm seems reasonable to me for this size hatch. Tempered and laminated have a very similar strength although if broken, tempered will result in a larger hole.

The other clue to the correct thickness is the hatch had 8mm plastic. Glass (tempered or laminated) is normally specified as a similar thickness to plastic for marine windows. Overall it has very similar strength in this application although the properties are different. Glass is stiffer and slightly stronger with impact from large and medium sized objects (such as for example a spinnaker pole ) but it is very weak compared to plastic when impacted with small, but very high speed objects. Thus, for example, car windows needs to be made otherwise thicker than otherwise necessary to be strong enough to withstand a small stone that may hit a window at high speed (mostly due to the car’s velocity). Plastic could be much thinner. Such small, high speed missiles are not a great concern on a boat, but they are not impossible if, for example, a block explodes.

Polycarbonate is different. It is extremely strong, especially for small high speed projectiles, but it is very flexible. In practice it needs to made a similar thickness to glass or acrylic not because of the strength of the material, but if hit with, for example, a spinnaker pole, the polycarbonate unless thick will flex so much that it is likely to pushed in when the adhesive holding the polycarbonate fails.

The above is for material that is new. One big advantage of glass is that it does not lose strength when exposed to UV like plastic does. Crazed plastic hatches become very weak. In fact, it is not unusual to hear of them occasionally spontaneously completely cracking with no apparent force.

Glass is often considered weak because untreated glass is indeed very easy to break, but toughened or laminated glass is actually a strong, albeit heavy material with the weakness that it is vulnerable to impact by very small objects. Hence the escape hammers kept in some cars have a very small hardened tip that can hit the glass with point force. Car side windows are only very thin by marine standards, but are hard to break in an emergency by say kicking the window, hence the need for the escape hammer.

The best solution is to have the material thickness chosen by competent engineers performing a proper analysis of the expected loads, but for the DIY boatbuilder, substituting glass (hardened or laminated) of the same thickness as acrylic or polycarbonate, is likely to be reasonably satisfactory from a strength point of view. If the plastic is not replaced on a regular basis then the glass is overall less likely to fail, although I would urge extreme caution substituting materials in critical areas such as anything on the hull sides or close to the waterline.

[noelex 77]

Originally Posted by Cadence

I always took safety glass to be that which is used in car windshields that have a film between two glass plates, They used temped in read windows. I had one rear become chards with and air tight VW and a barometric change.

One of the features of “safety glass”, whether tempered, laminated or the very old and little used wire glass, is that if it is broken it should not produce sharp splinters or shards at least nothing like ordinary untreated glass.

This is achieved in different ways. Tempered glass has an inbuilt compression force incorporated into the material. This force holds the glass together if it is broken and causes it to break into relatively benign clumps with blunt edges rather than sharp shards of glass. Laminated glass has a plastic film that holds the broken surface together. This also produces a relatively safe break pattern. Importantly for car windscreens laminated glass retains some transparency so if a car windscreen is shattered by a stone, the driver has some hope of seeing well enough to steer the car to safely. So car windscreens are always laminated, but the side windows are tempered. As on boats ordinary untreated glass is not used.

So the rear window in your car should not have broken into multiple sharp shards at least nothing like the dangerous pattern produced when ordinary glass is broken. However, the break pattern is somewhat dependent on what breaks the glass. Once again small high speed missiles are worse and produce a more dangerous pattern. It sounds like your rear window failed in very unsual way (perhaps from an internal fault within the glass rather than an external force) this might explain the poor and dangerous break pattern. Car rear windows are also a little unusual because of the need to be able to heat and demist the glass.

[Cadence]

Originally Posted by noelex 77

One of the features of “safety glass”, whether tempered, laminated or the very old and little used wire glass, is that if it is broken it should not produce sharp splinters or shards at least nothing like ordinary untreated glass.

This is achieved in different ways. Tempered glass has an inbuilt compression force incorporated into the material. This force holds the glass together if it is broken and causes it to break into relatively benign clumps with blunt edges rather than sharp shards of glass. Laminated glass has a plastic film that holds the broken surface together. This also produces a relatively safe break pattern. Importantly for car windscreens laminated glass retains some transparency so if a car windscreen is shattered by a stone, the driver has some hope of seeing well enough to steer the car to safely. So car windscreens are always laminated, but the side windows are tempered. As on boats ordinary untreated glass is not used.

So the rear window in your car should not have broken into multiple sharp shards at least nothing like the dangerous pattern produced when ordinary glass is broken. However, the break pattern is somewhat dependent on what breaks the glass. Once again small high speed missiles are worse and produce a more dangerous pattern. It sounds like your rear window failed in very unsual way (perhaps from an internal fault within the glass rather than an external force) this might explain the poor and dangerous break pattern. Car rear windows are also a little unusual because of the need to be able to heat and demist the glass.

Nothing wrong with the rear window glass. The car was in a hotel parking lot and three others had the same issue. I'm guessing it was a microburst. Came and went in a few seconds. the hotel shook.

[slug]

Originally Posted by riki

Hi,
as title: i am having trouble to find an 8mm plexy sheet to replace my hatches lens, so i was wondering if i can use 8mm glass instead. It's a 32x32cm hatch,
Opinions?
Cheers.

Glass is a bad choice

Can’t believe that you can’t source plastic

Keep looking

[Anders]

www.plexiglas-shop.com has all the colours you could possibly want and was happy to ship to our yard in Greece.

[Jubilee39]

Originally Posted by riki

Thank you for the suggestion.
Unfortunatelly i am in Crete, Greece, and ordering online would be the last option because it will take quite long to arrive and also because it would be difficult to match the colour. If glass is a viable option i can get it fairly quickly instead.

In Rethymno Crete it is possible to get acrylic, Heraklion also. I wintered over there last year

[Cadence]

Can you not get Lexon or another polycarbonate cut to size and shipped?

[riki]

Hi folks, thanks for all the feedbacks.
So far i only found 3 and 5 mm. I cancelled the order that i placed for the glass lens because even if probably would never give a problem i would still always have in the back of my mind. My wife is travelling to Italy by end of the month and she will take with fer one of the lens and a shop in Rome that i already contacted will do the copies.
Here is not always easy to find what you look for and some little limitations...

[pescietoro]

Hi riki! I wanted to replace my old cracked lenses in the Lewmar skylights on our boat for safetyglass in the same thickness. The problem was that the glass shop, which sends all safetyglass jobs after it's been cut to a workshop that bakes it and they just couldn't do it. I was told the problem was the cut outs for the handles in the glass, it cracked each time they tried. Eventually I ended up with Safire Polycarbonate, that is what is required from insurance companies in deck windows for atlantic crossings. Unfortunately it gets scratched quite easily, even though it's extremely strong. I found no other solution. Best wishes with your project!

[Cadence]

Originally Posted by pescietoro

Hi riki! I wanted to replace my old cracked lenses in the Lewmar skylights on our boat for safetyglass in the same thickness. The problem was that the glass shop, which sends all safetyglass jobs after it's been cut to a workshop that bakes it and they just couldn't do it. I was told the problem was the cut outs for the handles in the glass, it cracked each time they tried. Eventually I ended up with Safire Polycarbonate, that is what is required from insurance companies in deck windows for atlantic crossings. Unfortunately it gets scratched quite easily, even though it's extremely strong. I found no other solution. Best wishes with your project!

It still transmits light scratched and is literally bullet proof.

[sailorladd]

I always understood that anything other than glass would eventually craze in direct sunlight due to the ultra violet. I would not hesitate to install plate glass on my hatch, but I would endeavour to put a few frames of hardwood or stainless steel rods to support it.....

[David Mathis]

Use Lexan. It’s available from many glass dealers. It’s incredibly strong and is bullet resistant in many thicknesses. The price is more than plexiglass but not prohibitive. Using glass in a hatch is not so good for several reasons, one being safety. Think about the outcome if someone were to fall on it.

[slug]

Originally Posted by sailorladd

I always understood that anything other than glass would eventually craze in direct sunlight due to the ultra violet. I would not hesitate to install plate glass on my hatch, but I would endeavour to put a few frames of hardwood or stainless steel rods to support it.....

Yah plastic will craze

Lexan turns yellow

Safety glass. Shatters

Nothing is perfect

Nothing lasts forever

The problem with glass is weight and catastrophic failure

[svmatilda]

Before you get too carried away replacing the material in the hatch. If your insured whats the feed back from your insurer?? Could be worth the time it takes to write an email.

[Minggat]

Originally Posted by slug

Yah plastic will craze

Lexan turns yellow

Safety glass. Shatters


The problem with glass is weight and catastrophic failure

The wrong plastic will craze or fog.

Plexiglass is wrong wrong wrong.

True- Lexan will yellow.

Even Makrolon crazes.

Extruded acrylic will craze.

CAST ACRYLIC is the correct plastic, as I think I only read once, maybe twice in the post above.

Reports of 20 years and still looking new sounds pretty close to "lasting forever" for our purposes.

Honestly, as I was reading through all of the comments, I was surprised by so little mention of cast acrylic. When I was going through this some years ago, CAST ACRYLIC was the obvious choice to those contributors.

So far, in my own experience, they got it right.