A marine acrylic bulkhead window is chosen for three reasons: it does not shatter, it weighs about half of glass at the same thickness, and it can be re-machined. The penalty is that acrylic scratches and it moves more than glass, so the fixing has to allow for both.
A 14 m charter yacht replaced four saloon windows after a delivery trip in the North Sea. The original glass panels survived but two frames had cracked at the bolt line, and the owner wanted the weight off the coachroof.
Acrylic bulkhead window thickness by opening size
- Up to 400 x 300 mm: 8 mm to 10 mm cast acrylic
- Up to 700 x 500 mm: 12 mm to 15 mm
- Over 700 mm, or low on the hull: 15 mm to 20 mm
- Curved or angled openings: cast sheet formed to the mould, not extruded
The rule that matters is span, not area. A long narrow window of 900 x 250 mm deflects more than a 500 mm square of the same thickness, and wave slap finds the middle of the long span. Frame a mullion into the opening and the thickness can drop a grade.
Fixing detail: oversized holes and neutral sealant
Acrylic moves about 0.065 mm per metre per degree C, roughly six times glass. Bolt holes must be 2 mm larger than the bolt, and the frame must clamp through a compliant gasket rather than metal on plastic. A tight hole in cold weather becomes a crack in summer.
Sealant has to be neutral cure. Acid-cure silicone releases acetic acid as it sets and attacks the panel surface, leaving a crazed halo around the frame that cannot be polished out.
UV is the long-term issue on deck. Standard cast acrylic holds up for years; the surface can chalk and lose a little clarity after a decade in tropical sun. A UV-absorbing grade is worth specifying on a vessel that will live in the Med or the Caribbean.
Where the window will be cleaned with marine products, note that some deck cleaners are aggressive. Our cleaning and maintenance guide lists compatible agents, and the acrylic yellowing and UV lifespan guide explains how grades differ outdoors.
Send your opening sizes, hull position and vessel class to oversales@rpplexiglass.com for a marine acrylic quotation.
Frequently Asked Questions
What thickness acrylic for a bulkhead window?
10 mm for windows up to about 500 x 400 mm above the waterline, 15 mm to 20 mm for larger openings or lower positions where wave impact is credible. Thickness is set by the largest unsupported span, not by the area.
Why not use glass for a bulkhead window?
Glass is heavier, brittle against wave and dock impact, and a crack at sea is a flooding risk. Cast acrylic flexes, absorbs impact and can be cut and re-sealed in a workshop, which is why most production yachts specify it.
How is a marine acrylic window sealed?
Neutral-cure silicone between the acrylic and the frame, with a dry joint and no acid-cure silicone anywhere near the panel. Bolt holes are oversized for thermal movement and fitted with a sealing washer on each side.