Lexan Question

[mikereed100]

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

[MidLandOne]

Originally Posted by mikereed100

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.

[GordMay]

Guideline for Design of Thin Windows for Vacuum Vessels
by Jeffrey L. Western (Fermi National Accelerator Laboratory)
Goto:
http://trshare.triumf.ca/~buckley/wt...ab-TM-1380.pdf

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
* American Society for Testing and Materials

[mikereed100]

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

[MidLandOne]

Originally Posted by mikereed100

...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.

[GordMay]

MARINE GLAZING:
As listed in the MPC Boater’s Directory:
MPC Boaters Directory. Marine Industry Boating Guide. Yellow Pages for Boat Owners & Businesses

A GLASS ACT
820 W. Esther St.
Long Beach, CA 90813
(888) 432-4312

GO WINDOWS - MARK PLASTICS
369 E. Harrison St., Unit G
Corona, CA 92879
(951) 735-7705

MARK PLASTICS - GO WINDOWS
369 E. Harrison, #G
Corona, CA 92879
(951) 735-7705

THE GLASS MAN (Mobile)
(562) 924-5914

BIFF'S BOAT & R.V. GLASS
Window Repair & Installation
San Diego, CA 92106
(619) 226-4501

DICKERSON GLASS CO.
125 Rochester St.
Costa Mesa, CA 92627
(800) 799-9121

INNOVATIVE POLISHING SYSTEMS
2822 SE Monroe St., Unit C3
Stuart, FL 34997
(877) 777-6858

NEW VIEW AUTO GLASS
7555 Industrial
Stanton, CA 90680
(714) 890-1347

TAYLOR MADE SYSTEMS
93 South Boulevard
Gloversville, NY 12078
(518) 773-0636

[mikereed100]

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

[hooked on water]

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

[hellosailor]

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?

[High Cotton]

Speaking of PLEXIGLASS..........

What kind of cleaner should be used??

[Alan Wheeler]

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.

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.

[GordMay]

Do not use any alcohol or ammonia-based cleaners on plastics such as either acrylic or polycarbonate.

[hellosailor]

Wheels-
"Windex" brand is probably the premium glass cleaner in the US, and while they make a dozen flavors, they are quite specific that using "Windex" on plexi or acrylics in general is specifically not recommened. Apparently there is enough ammonia in the product (at least, in the standard blue flavor) so that there is a potential problem with the plastic, which is never supposed to meet ammonia or petrochemicals.

Personally--I've used "glass" cleaners on all sorts of plastics for years without ever seeing a problem, but the folks who make both products say "no". FWIW.

Maybe I just don't clean things often enough to damage them?[vbg]

15 tons per square yard...1.666 tons per square foot...a paltry 23 pounds per square inch?!

[Alan Wheeler]

.a paltry 23 pounds per square inch?!

21lbs is what brought down the building in the Oklahoma bombing.
I have used glass cleaners for years on acrylic. And i have used solvents to take the glue stuff off from the protective paper when it got stuck on. I have never had a problem either. But OK, if they say no, they are supposed to know about their product.

[delmarrey]

Originally Posted by Alan Wheeler

21lbs is what brought down the building in the Oklahoma bombing.

It's a good thing that boats have a much higher building code!!!