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Old 15-03-2008, 15:58   #31
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Originally Posted by delmarrey View Post
Quote: MidlandOne
Lexan is no stronger than acrylic but withstands shock much better - but it gets its ability to withstand shock from its elasticity and deformation (look at your own figures, by the time it fails it will have deformed by around 130%!!!, whereas acrylic will hardly have deformed at all. Your figures do not give elongation at failure for acrylic but is typically around 5%).
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Originally Posted by delmarrey View Post
Obviously, you didn't read the lines in orange! And these are not MY figures but of the plastics industries! Izod Impact Strength Testing of Plastics
There in your response lies the general problem when people promote Lexan as you have. You need to understand what "stronger" means.

If you look at the strength properties of the two materials ie their ability to withstand applied forces without breaking such as their tensile strength then you will find they are quite similar (but if you compare the strength properties of the two materials by their ability to withstand applied forces without permanently deforming then acrylic is superior - see below).

What you are saying is that because Lexan is more resistant to sharp impacts (you quote the Izod test, those being the figures in orange that you rely on) it is stronger. That is incorrect in that you are confusing resistance to shock loads with strength - resistance to shock loads comes from the ability to absorb and disperse the energy if applied in a shock manner whereas strength comes from its ability to resist forces without strain (ie without deformation). For example, rubber is very resistant to shock loads but is not very strong, or a high carbon steel may be very strong compared to mild steel but may have a low resistance to shock loads.

A problem with Lexan is it gets much of its resistance to shock loads from its ability to deform ie it is easily strained, and as the figures you gave show by the time it breaks it will have elongated 130% (and that permanently), and you will find that acrylic will, typically, have only elongated by around 5% under the same load. Another difference is that acrylic only starts elongating (deforming) when loaded to very close to its point of breaking whereas Lexan starts elongating much earlier. If you cannot see the importance of these things to windows and hatches then I am afraid that I can't help you.

Getting back to the Izod test. It tests a small notched samples resistance to failure when hit with a hammer type load. What people lose sight of is that the results speak only for the performance of a small notched sample, not for a real life situation. So, for example, I think most of us would see that a flat panel of some material is much more resistant to shock loads than a small notched sample is. That is because they can absorb shock energy by flexing and also transferring energy outwards into the surrounding supporting structure. It turns out that windows and hatches are mostly flat panels in surrounding supporting structures so their resistance to impact loads is very much improved over what an Izod test piece might imply.

But Lexan, even so, is much more resistant to shock loads but it suffers in that it is about twice as elastic as acrylic so for a given force it will deflect about twice as much (that depending very much on the panel size, shape and how it is fixed into its frame). I assume this is why the few Lexan hatches around mostly (all?) seem to have support bars under the Lexan. This defection results in more strain on the sealing around the panel - if one pushes the centre of a panel down the edges move inwards and then outwards again when the load is reduced.

So one must be careful not to quote figures for ones case without knowing what they mean and how they may not be entirely or even remotely relevant to the case.

I will not bore everyone with going through the other points you raise as they are similarly flawed, but they are worrying as they imply you have taken a firm immovable stance as to the superiority of Lexan for the purpose without actually understanding why.

I really don't want to go through all this again so won't respond again to the anti's. All I can say is that people are free to use whatever they want (mostly) in pleasure vessels but hope that some with open minds will consider carefully the properties of what they do use.
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Old 15-03-2008, 17:43   #32
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Midlandone,

Is there a formula that can be used to calculate the thickness of acrylic necesary for a given surface area in cabin windows that will be exposed to breaking waves?

Thanks, Mike
Sorry but I cannot point you to one, but that mainly because all the vessels I work with acrylic (and Lexan) is prohibited for windows or else just not considered as being an appropriate material for them so never used. Note, by windows I mean large paned items bigger in smallest dimension than portlights are.

I think you will find that for pleasure vessels the design of hatches and portlights is done purely from experience of what has survived before so can only suggest that you follow the example of other vessels. In that I suspect that is how the manufacturers of hatches and portlights do it, ie by their own and collective experience - and I think that is what Benjamaphone has inferred.

If plastic was to be used for windows then I would think that again just what has been seen before is used as far as thickness is concerned - but I would be very hard pressed to find a sailboat around here that has plastic windows to use as a guide.

However, I would say it is very unwise to use acrylic (or Lexan) for windows in a sailboat intended for other than sailing in sheltered waters (windows being of large panes, so strip "windows" as are common on smaller sailboats are really no more than long fixed portlights and can be considered so for design). But one needs for normal boat profiles to have a very large sailboat to have windows in it at all and that would normally infer other than sheltered waters sailing and so against acrylic (I recognise that the trend towards motor sailors or pilothouse types is bring windows more commonly into smaller boats though). Unless in a hard dodger of course, but even there, though not required to be watertight, acrylic (or Lexan) is a genuinely cheap and poor solution in my view. I know plastics are used, perhaps mostly in amateur builds but all the pleasure sailboats that my friends own use toughened glass for hard dodgers even though some of those are amateur builds (and built before they knew me :-) ).

You may find something for plastic window design in some country's construction codes for small commercial vessels but I have not come across such - normally for such vessels if hatches and portlights are of acrylic they are bought in and not fabricated by the builder. And for windows plastics are either severely frowned upon or prohibited (toughened glass being the normal requirement) unless very small.

I do not recall anything in the rules of the main classification societies for design of plastic windows as all vessels I have been involved with in class have used toughened glass as a matter of course - I suspect plastics would not be accepted for windows required to be watertight unless a very convincing first principles design case was prepared. I base that, in part, on my having been involved with managing the design and construction of fast ferries where weight saving is very important and a first principles design case has been done for the windows of those in order to go for the absolute minimum pane thickness and weight, but even with the driver of minimising weight plastics were never considered as a sensible possibilty.
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Old 16-03-2008, 04:07   #33
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Guideline for Design of Thin Windows for Vacuum Vessels
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AAMA* 101 & ASTM D 4099* (& I’m ceretain ISO, et al) each specify a minimum Performance Class for each (window) Grade, depending upon specific design pressures
* American Architectural Manufacturers Association
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Old 17-03-2008, 22:40   #34
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Midlandone,

Thanks for the very informative reply. This is a very confusing subject for one who has no training or experience in the relative strengths of glass vs polycarbonate vs acrylic. I am in the process of designing a cabin for a sailing cat that will have approximately 60cm x 60cm windows. For the sake of aesthetics and strength I had hoped to design a radius into the cabin sides but this would mean that the windows would have a radius as well (approx 340cm). My own preference is for glass simply because of the fact that I have never seen a window or a hatch made of plastic that was not woefully cloudy or crazed after a few years. However, the glass suppliers I spoke with told me that bent glass would be very expensive. I learned of cast acrylic on this thread and hoped that might provide a solution but in light of your comments I am not so sure. My feeling now is that it would probably be best to design a flat-sided cabin that could have glass windows. Apologies in advance for the next question, but is there a formula for glass thickness for a given window size?

Thanks, Mike
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Old 18-03-2008, 02:46   #35
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...but is there a formula for glass thickness for a given window size?
All the main classification societies give requirements for glass thickness in their rules, but the problem is accessing them if the vessel is not going into class. However, most countries have standards for the manufacture of marine glass (not giving the thickness though) and major glass suppliers will be familiar with those and should be able to advise on thickness (in the same manner as they give advice on architectural glass).

For example, Pilkington's here in NZ make marine glass and their technical people have been helpful - I would expect that Pilkington North America do the same. There is alot of marine glass made so should be quite easy to get a lead to good advice in your home country. Such glassmakers should be able to both advise on the thickness and also provide the glass which will be cut (and curved, if required) to the size you give them. There are also non manufacturing suppliers of marine glass but I cannot give any leads to them for the USA (maybe GordMay can supply some of his magical links ).

If none of that freeby approach works for getting the thickness then any naval architect should be able to make a recommendation for very little cost.

But, the glass will not be very thick - big 40 knot powerboats built to class I have been involved with have had 6mm glass in much bigger panes than yours in side windows (but forward facing glass thicker).

If you go to glass it should be toughened glass not laminated glass (but a marine glass supplier will not mention laminated) and will come from the glassmaker with your county's marine glass standard compliance mark fused into one corner (similar to how it is on automobile glass). I suggest that you do not allow anyone to advise you to use laminated glass as laminated glass is no stronger than ordinary glass. In fact it is weaker compared to a pane of non laminated as the actual glass thickness is thinner (as part of the thickness is taken up by the plastic interlayer). There are laminates of toughened, and laminates incorporating plastics, etc which are fine but they are expensive and in my experience not used on pleasure vessels.

Most (all?) fixed marine glass is now glued in place with no other fixings, just as it is in automobiles. In my own boat's case we only have windows in the hard dodger and the glass is glued into a rebate around the circumference of the cutout so that the outer surface of the glass is flush with the dodger panels - the rebate is wider than the glass sitting in it so that there is a gap around the outside edge of the glass that can be filled with Sika UV sealant to cover the exposed edge of the adhesive and glass.

If surface mounted, it is just glued onto the face of the structure, sitting on a narrow foam strip to lift the glass from the panel under to give the required thickness for the adhesive (there is a minimum thickness and width for the adhesive). Sika USA should advise you which adhesive product is best (probably Sika 296) and the recomended mounting, preparation, primers and UV protection to use for your particular application - look at Sika Corporation | USA to get started, if not already aware of these things.

If you are considering glass for hatches be aware that they are heavier to lift and strut. Personally for other than very large hatches I would go with acrylic and it should give very good life. We have Lewmar hatches which are over 11 years old now and there is no discolouration or UV crazing in any of them - and we live in a very high UV exposure region of the world. The surface of a small hatch directly over the cooker in the galley has very slight surface crazing (easily polished out should I one day get around to it ) which I am assuming is due to cooking fats venting through it.
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Old 18-03-2008, 04:09   #36
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Old 18-03-2008, 23:28   #37
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Midlandone,

Thanks again for a very informative reply. This is invaluable information at this stage of my rebuild.

Gord,

Thanks for the contacts (how do you do that?). A couple are within a few hours drive from me and I will definitely get in touch with them.

Thanks again to both,
Mike
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Old 25-03-2008, 08:03   #38
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Originally Posted by MidLandOne View Post
All I can say is that people are free to use whatever they want (mostly) in pleasure vessels but hope that some with open minds will consider carefully the properties of what they do use.
The bottom line for this discussion is that true quality hatch and port manufacturers take every factor into consideration. The mere fact that bars are placed under large Lexan hatches should prove its inherent weaknesses. If you want to replace the glass on hatches and ports, use what sailors have been using for decades with great success. Trust the people who have the experience and knowledge to help you make the right decision for your boat. Better yet, let the professionals install and repair your old hatches. Often marine professionals will put a warranty on the job, ensuring that you get the best service and value for your money. If you do it right the first time, you won't have to worry about malfunctions/leaks for 10-15 years.
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Old 10-04-2008, 18:33   #39
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Hi Catmando
I am from Hobart
where are you getting this shinkolite from locally
wanting to do my cabin windows and hatches also
how is the weather up there
take care
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Old 10-04-2008, 21:54   #40
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The bottom line for this discussion is that true quality hatch and port manufacturers take every factor into consideration. The mere fact that bars are placed under large Lexan hatches should prove its inherent weaknesses. If you want to replace the glass on hatches and ports, use what sailors have been using for decades with great success. Trust the people who have the experience and knowledge to help you make the right decision for your boat. Better yet, let the professionals install and repair your old hatches. Often marine professionals will put a warranty on the job, ensuring that you get the best service and value for your money. If you do it right the first time, you won't have to worry about malfunctions/leaks for 10-15 years.
That is misrepresentation of the facts. Bars are put under all large panes so that thinner/lighter materials can be used so the hatches don't weigh so much as with thicker panes. I can imagine what my 32" sq. hatch would weigh if it had material thick enough to withstand my weight or a wave coming over the bow.

As for experience, it's getting harder and harder to find any business out there that knows what they're doing. A lot of the places one calls on the phone these days is like calling McDonalds. You ask a question and the response is; Huh?? I didn't even know that stuff existed. Or, I never heard of that. And everything is a whatcha malcallit or a thinga majigger.
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Old 10-04-2008, 23:32   #41
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That is misrepresentation of the facts. Bars are put under all large panes so that thinner/lighter materials can be used so the hatches don't weigh so much as with thicker panes. I can imagine what my 32" sq. hatch would weigh if it had material thick enough to withstand my weight or a wave coming over the bow.
I sometimes wonder what some really know and would question who is misrepresenting the facts in this case because it is usual for properly constructed hatches of the size you mention to not have bars. In fact if one did have bars I would not specify it into any boat for any client of mine unless for sheltered waters service (and then there would be better choices).

For just one example out of just a mid priced but respected range look at Lewmar's Size 77 Ocean Hatch about the same size as your hatch. No bars, acrylic 12mm thick and Lloyds and CE approved for ocean voyaging for all deck and superstructure locations - I would have complete faith in it as have the majority of builders and cruising sailors around the world. If you do the sums you will find that the weight of acrylic in the hatch is only approximately 7.5kg which hardly counts as heavy .
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Old 11-04-2008, 14:34   #42
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What ever.... I happen to like Lexan®

My hatch cover is only around 3 kg, frame and all (6 mm lens), and I can walk on it. And the last lens lasted 25 years and I can still bend it as below. I don't have the weight or strength to get it much farther.



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Old 11-04-2008, 14:42   #43
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So...to repeat an unanswered old question...

Just how or what does anyone figure IS the load caused when, say, a 40' tall breaking green wave drops onto your deck and hatches?

A cubic foot of seawater at 64 pounds, accelerating from a 40' drop, and treated as a solid object falling on it? Or?
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Old 16-04-2008, 20:52   #44
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Speaking of PLEXIGLASS..........

What kind of cleaner should be used??
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Old 16-04-2008, 23:11   #45
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Just how or what does anyone figure IS the load caused when, say, a 40' tall breaking green wave drops onto your deck and hatches?
I know Ocean going vessels had a designed load rating of 15Tons/per square yard I think it is.
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What kind of cleaner should be used??
There are many specialized cleaners and polishes available. I simply use a plain old every day window/glass cleaner. Acrylic is quite tolerant of many cleaners and chemicals. It is Lexan that gets damaged by strong cleaners and solvents.
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