Boat Windows Question ?

[S/V Mother Ocean]

I am getting redy to install new lexan for my windows. I was going to use stainless steel screws and silicone to attach them. That is how they are done now from the factory and they leak bad. They only have a 1" lip on them. I am making them oversized (2" over what is ther now, for a 3"' overall width) to allow the 1 3/4" that the lexan manufacture suggest before using a fastner. Also I am going with one big piece of plastic insted of three smaller ones. That get smaller as they go foward. This will give it more strenght with the post to support them from behind.


Note three window config.

There is a local Jenneau dealer that says not to use screws but to just glue them down with the stuff they use for windshields and airplane windows. The cabin sides are curved. I just don't see how to glue them down with out screws. They are not flat and do not have a indentation to fit in.

So what to do?

Dan

[daddle]

There's some evidence that you should not make one continuous piece. The window and hull will expand at different rates. This stresses the sealant. However you can make the joints between panels very small.

Here's some good info.

Jenneau dealer? Now there's an authority. Autos use a completely different system that we cannot. Including that very important black edge.

[SailFastTri]

I have some experience with Silicone and Lexan. Yes that is the stuff to use for Lexan.

I agree with "daddle" that using one larger piece might cause problems from expansion/contraction.

I also suggest you make a snap-on sunbrella cover after you are done with your project, for when the boat is not in use. Lexan is subject to crazing/hazing from UV, and a cover will make it last longer.

[Roy M]

Lexan gets a "blush" or clouding over, after several years of UV exposure. Acrylic "crazes" over the same period, but loses only a certain integrity. Personally, I have chosen acrylic every time I've changed out my own ports rather than have something I can't see through.

Further, each of my fixed ports is individually sealed, but using a different technique requiring "finishing rings" to seal the port.

A deeper discussion of this issue can be found at http://www.cruisersforum.com/forums/...ing-11539.html

[Pblais]

Daddle has the important parts. Difference in the coefficient of expansion is what ports are all about. It's 5200 is a poor choice because it womn't expand and contract well and eventaully the sealant fails. Lexan,. metal and ficerglass all shrink and expand different so you need to take this into account. You want a highly elastic sealant that won't break down quickly. UV and salt combine with heat and cold to really stress the ports, frames and boat.

I use shoulder bolts to hold the frame in place but it really is just to position the frame so it can be set properly while the sealant cures. That is the better way to think about it. I like Silkaflex sealants because they clean up easily and are very elastic. The sealant should gush out all the way around. You let it cure then come back and cut off the excess.

[Pblais]

Lexan gets a "blush" or clouding over, after several years of UV exposure. Acrylic "crazes" over the same period, but loses only a certain integrity.

Cast acrylic is the ticket. Does not craze. Costs is pretty high though.

[S/V Mother Ocean]

This is extruded polycarb. It is susposed to be UV treated.

Dan

[Sailorman Ed]

Dan, ALL polycagonate sheet is extruded. And most acrylic also. Only the cast acrylic is cast - porperties are different fromt he extruded sheet (and smells different). Suggest you find some GE's Silproof SCS 2000 as this will work well. Try to find polycarbonate with the "hard coating" to prevent scratching. Polycarbonate willl scratch much more easily than arcylic. Some good reading here: S/V MOMO: Replacing Boat Windows: Polycarbonate (i.e. Lexan) or Acrylic (i.e. Plexiglas) to compare acrylic, polycarbonate and tempered glass.
Ed

[bstreep]

As noted above, my choice would NOT be Lexan. Use cast acrylic - in the long run, you will be MUCH happier. There are some reports that Lexan can lose ALL of its strength advantages over acrylic as quickly as 1 year in the sun.

And don't use silicone use Silkiflex or similar product instead - that has a lot of flexibility. Many have said that silicone cement has no place on a boat, and I concur.

[bludden]

I see yacht "windows" installed with bolts squeezing out every last bit of sealant all the time...it can't work because of the different rates of expansion...design a rebate allowing even 4 or 5 mm thickness of sealant to allow this movement without stressing adhesion and "she'll be right". I learned this in my carpentry trade when architects started integrating glasshouses with homes...our first attempts sealing 2.4m X 1.2m acrylic roof sheets failed immediately...the sheets also cracked around the screw holes because the expansion was so great...the ideal bond has large surface contact area of the plastic/glass with a narrow waist profile to allow lots of stretch...we use backer rod to achieve this profile...The same system is used with polyurethane sealant to seal the joints in tilt slab concrete panels...cheers

[S/V Mother Ocean]

Any Idea on how to heat mold these plastic panels?

Dan

[Sailorman Ed]

Dan - both materials are hydroscopic (absorb moisture) and if you just heat them, the moisture inside the plastic will expand and create hundreds of white bubbles, rendering it useless. You need to dry the sheet (250F for PC and I think it is 150F for acrylic) for at least an hour and then need a way to heat it. Commercially, we used a large oven, controlled the heat and then pulled a vacuum. But if you are only bending them a little, I would not bother. Clamp the sheets or use some temporary fasteners until the sealant cures. Be sure to use some spacers in the center so the sheet does not push all the sealant out there. Bludden has a good point.

[Cotemar]

No one in the industry uses SS screws any more
Lagoon & Fountaine Pajot which are the two largest catamaran manufactures. They use the material show below. An Automotive window glass company can get these adhesive for you.

Note the outside of the plastic sheets must be painted black for 2” around the periphery of the window so the UV light from the sun will not break down the adhesive.

Install painters tape around the window to make cleanup easyer. This stuff can be messy.

1) ACRYLITE GP acrylic sheet is a cell-cast acrylic sheet 3/8 thickness

2) Dow Automotive BETAMATE7120 Single-component, high-viscosity, atmospheric humidity-curing polyurethane bonding/sealing compound for high-strength, permanently elastic adhesive joints. Apply ½ thick and squeezed out to 3/8 when window is installed

3) Dow Automotive Betaprime 5404 Pinchweld and encapsulation Primer. Used only on the plastic sheet. Lightly scotbright 2” around bounding edge and apply this primer onto the sanded 2” around the outer edge

Mark