An acrylic boat window is chosen for weight and curvature long before anyone discusses colour. A 12 m motor yacht came back from its third season with two crazed windows and a leak into the forward cabin. The windows had been replaced once already, in the wrong grade, bedded with a construction silicone that could not tolerate the movement of a flexing hull.
Choosing acrylic boat window thickness by span
Marine glazing is sized like any other panel under load, with a safety factor against wave impact rather than wind alone. The working numbers are 6 mm to a 600 mm span, 8 mm to 900 mm, and 10 to 12 mm beyond that.
Curvature is free stiffness. A window with 300 mm of camber behaves like a stiffer panel than the same flat sheet, which is why formed windows often run one step thinner than flat ones.
The frame detail that decides whether it leaks
Acrylic moves with temperature and load. A rigid frame bolted hard to the cabin will crack the panel at the bolt line within a season. The detail that survives has three features: a rebate deep enough to hold the panel, a bedding compound that stays elastic, and fasteners that locate without clamping.
- Bedding: a neutral-cure marine sealant, never an acetic silicone
- Rebate depth: at least the panel thickness plus 2 mm
- Fasteners: oversize holes by 1 mm to allow movement
- Edge: polished, because a saw edge starts crazing first
The silicone choice is the one that fails most often. Acetic-cure silicones release acid as they set, and that acid attacks the acrylic edge. Neutral-cure marine grades cost more and last.
Forming a curved window
Cast acrylic forms at 150 to 170 degrees Celsius. It must be dried first and heated evenly on both faces, then formed over a male mould and held until it cools below 60 degrees Celsius. Forced cold bending stores stress that shows up later as crazing at the frame line.
A formed window is also cheaper than it looks, because the alternative is a flat panel in a bulky frame.
Salt, sun and the ten-year window
Salt does not attack cast acrylic; UV does, slowly. A UV-stabilised grade holds clarity for eight to twelve years in temperate use, less in the tropics where the sun is steeper. The visible sign is a yellow cast that appears first at the edges.
Cleaning matters as much as the grade. Salt crystals wiped dry act as an abrasive; fresh water rinse, then a soft cloth with a plastic cleaner, then another rinse. That routine once a month is the whole maintenance programme.
Where the window is cosmetic rather than structural, the yellowing and outdoor lifespan note explains the UV chemistry behind the grade choice.
Repair or replace
A crazed window can sometimes be polished back. Fine crazing within 0.2 mm responds to a compound and a machine polish; crazing that has reached the mid-plane cannot be removed without changing the optical thickness, and the window should be replaced.
A cracked window is a different case: stop drilling the crack tip buys time, but only if the crack starts at a fastener and the frame detail is corrected.
For a refit, order the panel formed to a template taken from the old window before it is removed. Templates made from the aperture afterward rarely match the original curvature. Custom formed acrylic covers the template and tolerance convention.
Send window templates, radii and boat type to oversales@rpplexiglass.com for a formed-panel quotation.
Frequently Asked Questions
What thickness acrylic for a boat window?
About 6 mm up to 600 mm span, 8 mm to 900 mm, 10 to 12 mm beyond that or where the window is walked on. Curvature adds stiffness.
Can acrylic boat windows be curved?
Yes, cast acrylic forms well between 150 and 170 degrees Celsius and holds a compound curve. Complex double curvatures should be formed on a male mould, not forced cold.
How long does acrylic last in sunlight?
A UV-stabilised cast grade holds clarity for eight to twelve years in temperate marine use. Tropical exposure with direct sun shortens that; a UV grade is the difference.