Unwanted flexibility in polycarbonate bimini window

[BigNut]

We're getting ready to lay up our new bimini top for the helm station on our L421.

We want to install a permanent, non-moving polycarbonate window. An ideal size would seem to be around 2 x 4 feet. I was thinking a thickness of polycarbonate of around ¼".

The question is: would the material be able to span a 2 foot distance without sagging? I am hoping to use only adhesive without screws to fasten it down. The bimini has a small camber, about 2" over 7', but only in the direction the 4' side is parallel to. It is flat in the other direction.

Anyone with direct knowledge or perhaps a link to data?

Many thanks

[billknny]

Originally Posted by BigNut

We're getting ready to lay up our new bimini top for the helm station on our L421.

We want to install a permanent, non-moving polycarbonate window. An ideal size would seem to be around 2 x 4 feet. I was thinking a thickness of polycarbonate of around ¼".

The question is: would the material be able to span a 2 foot distance without sagging? I am hoping to use only adhesive without screws to fasten it down. The bimini has a small camber, about 2" over 7', but only in the direction the 4' side is parallel to. It is flat in the other direction.

Anyone with direct knowledge or perhaps a link to data?

Many thanks

I assume this is an upward facing window in a fiberglass top. If supported by a hard bimini along four sides it will not "sag". There are many, many applications for 1/4" polycarbonate where it spans this distance with support on four sides.

If it is a forward or side facing window, it is too light to sustain wave strikes on what I assume is an ocean going boat. 1/2 inch would be a better choice.

[BigNut]

Originally Posted by billknny

I assume this is an upward facing window in a fiberglass top. If supported by a hard bimini along four sides it will not "sag".

Thanks billknny.
Yes, I see I was not clear enough. The window will form part of the horizontal surface of the bimini top.

That sounds reassuring. Do you know if support with adhesive only will accomplish this goal?

Thanks

[tkeithlu]

If I understand, this is a 2x4 sheet of 1/4 inch poly sitting on a rigid horizontal top that has a bit of a curve. Is the new bimini constructed out of plywood and covered with fiberglass? If so, the poly is not far different structurally from the plywood/fiberglass. A 1 1/2 inch overlap all the way around should supply enough surface for adhesion, however, in that situation why not space small ss bolts along it with nylon-locking ss nuts on the bottom? If there is not a core material, which would leave me wondering how you laid it up, then structural questions leave me unsure as how to advise you. Tell us more.

You can "relax" the poly with a heat gun, carefully, so it assumes the same shape as the top.

[BigNut]

Originally Posted by tkeithlu

If I understand, this is a 2x4 sheet of 1/4 inch poly sitting on a rigid horizontal top that has a bit of a curve. Is the new bimini constructed out of plywood and covered with fiberglass? If so, the poly is not far different structurally from the plywood/fiberglass. A 1 1/2 inch overlap all the way around should supply enough surface for adhesion, however, in that situation why not space small ss bolts along it with nylon-locking ss nuts on the bottom? If there is not a core material, which would leave me wondering how you laid it up, then structural questions leave me unsure as how to advise you. Tell us more.

You can "relax" the poly with a heat gun, carefully, so it assumes the same shape as the top.

Thanks tkeithlu

Not sure how polycarbonate is "not far different structurally from plywood/fiberglass"? Unless you are not referring to modulus of elasticity, or perhaps another quality. I am not an engineer of anything other than my own failures. Wood is very different from plastic in this regard.

In fact this difference is exactly the reason for my question.

The issue is whether the polycarbonate, which is lying horizontally, closing a hole in my semi-rigid bimini, will sag when it heats up in the sun. While the slight camber offers increased rigidity in one dimension, there is none in the other.

There is no need to "relax" the polycarbonate, I believe, as the camber is quite modest, and the increased tension provides strength. I am concerned about too much flexibility /sag in high temperatures.

I am not keen to use steel fasteners for mainly 2 reasons:

1. Every hole I drill increases the risk of stress fractures, and hence cracking down the road.
2. I prefer the aesthetics of a fastener-free appearance, if sensible to do.

1½" seems like a lot to me, for a 24" x 48" non-essential structure. I was thinking more along the lines of ⅞" to 1". Food for thought though. Might be changing my thinking.

BTW, not sure about the relevance, but since you asked, the bimini is: Gelcoat / 2 x 1708 / ½" Divinycell / 1 x 1708. Size: 124" x 79" overall, with supporting framework (6 posts, plus framework [Divinycell/glass] to attach clears to and provide an edge for waterproofing) about 3-5" inside the outside edge. With a hole big enough to support a 2' x 4' clear along its edges. )

Thanks for your thoughts.

[BigNut]

Hehe, I had to look it up:

Young's modulus for polycarbonate is: 2.6
Plywood: 7-8.6
GRP polyester matrix: 17

Not sure about the 1708/Divinycell, or even a ply/glass layup of some sort, but it must be higher than these.

[Cadence]

Originally Posted by BigNut

We're getting ready to lay up our new bimini top for the helm station on our L421.

We want to install a permanent, non-moving polycarbonate window. An ideal size would seem to be around 2 x 4 feet. I was thinking a thickness of polycarbonate of around ¼".

The question is: would the material be able to span a 2 foot distance without sagging? I am hoping to use only adhesive without screws to fasten it down. The bimini has a small camber, about 2" over 7', but only in the direction the 4' side is parallel to. It is flat in the other direction.

Anyone with direct knowledge or perhaps a link to data?

Many thanks

I think you can do it with 1/4 inch poly. although it is reasonably ridged. I would think something mechanical to secure it would be in order. It's not light and even that slight bend will cause stress. You might be able to go with less thickness. The stuff is pretty bullet proof.

[BigNut]

Thanks, Cadence. I agree that ¼" is pretty stiff.

We finally removed the old "bimini" today, and could do proper sight lines. Turns out, a window 2' x 2' will be more than adequate for our needs.

So, even more reason to reduce the thickness of the polycarbonate.

There is not much camber over such a short distance, but it will make things easier. Might very well still go without mechanical fasteners. I figure I could always add them in if I have to.
Remember, the question was about heat-induced SAG, not being overly stiff, lol!

[Cadence]

Originally Posted by BigNut

Thanks, Cadence. I agree that ¼" is pretty stiff.

We finally removed the old "bimini" today, and could do proper sight lines. Turns out, a window 2' x 2' will be more than adequate for our needs.

So, even more reason to reduce the thickness of the polycarbonate.

There is not much camber over such a short distance, but it will make things easier. Might very well still go without mechanical fasteners. I figure I could always add them in if I have to.
Remember, the question was about heat-induced SAG, not being overly stiff, lol!

I doubt you would have any heat induced sag.
Had you considered using it in aluminum rails and being openable?
Just a thought.

[BigNut]

Not really. Preventing leaks would be challenging, and being openable would not provide much benefit. I am planning on placing a second, tinted clear on a track below to keep the sun at bay during the day though.

[mitiempo]

Originally Posted by BigNut

Remember, the question was about heat-induced SAG, not being overly stiff, lol!

It will not sag when it heats up. It will expand with a rising temperature. It will expand more than the rest of the bimini so you will have to allow room for the expansion.

[BigNut]

Originally Posted by mitiempo

It will not sag when it heats up. It will expand with a rising temperature. It will expand more than the rest of the bimini so you will have to allow room for the expansion.

Thanks for this. I planned to have relief but mainly in terms of caulking, not expansion. Great point.