Side hull window thickness?

[Kris]

The side windows on my Pahi 42 are not in the best shape... I guess no need for immediate replacement, but sooner or later it will be necessary.

They appear to have been made from 10mm (cast?) acrylic or so, which is also what I found recommended on some general boat forums.

However, I found some pretty good 5mm extruded acrylic sheets here in the local hardware store for a good price and to me at least they seem pretty strong.

Anyone has recommendations or experience with such thinner acrylic windows? 5mm is probably the minimum that can be used?

I also noticed that the current cockpit pod windows are already made with a thinner acrylic...8mm probably. Is the general idea that these windows need to be less strong than those on the main hulls?

[Ed Thomas]

Acrylic or Polycarbonate (Plexiglas or Lexan)

[Kris]

Yes, that I know already, but which thickness is good?

[Brad Ingram]

I used 6mm on my 21, and they seem more than adequate. 

[Kris]

I assume it largely boils down to thickness versus total size or rather maximum unsupported span.

The eight side hull windows on my Pahi 42 are only like 40 x 20 cm each, making them rather small compared to other boat windows.

But how strong is strong enough?

I also read that extruded acrylic is more brittle than cast one, which might be another important criteria.

[Kris]

Ah, found a rather complicated formula to calculate it here:

https://www.cruisersforum.com/forums/f55/acrylic-portlight-thicknes...

I have to still crunch the numbers for my specific window size, but from a response by another person on the same thread it seems like 6-8mm would recommended by this formula for the rather small windows.

Need to think about it, but maybe 5mm will do ok with a bit less safty-margin 🤔

[Kris]

I crunched the numbers for hypothetical 200mm x 400mm windows which is pretty close to the actual size I have and the formula gives out 9.7mm acrylic thickness (6.47mm * 1.5 for acrylic).

But the design pressure of 70 kN/m^2 that is part of the formula seems rather high. According to this online calculator, this corresponds to about 1230 km/h wind-speed. Of course waves crashing on the window is not quite the same as wind, but I am doubtful the plywood around the window is as strong as that.

[Kris]

This design pressure is a bit mysterious. According to this website, a design pressure of 50 psf should be able to withstand a category 5 hurricane. But 50 psf is just 2.4 kN/m² not 70, like the formula uses.

Edit: so using that approximate 2.5 kN/m², this acrylic window would only need to be 2mm thick according to the formula? That seems a bit thin, but on the other hand, the window is pretty small, so I doubt a 2mm window like that would break in a cat 5 hurricane.