Fitting a boat window is more than just measuring it and setting it into place. A boat's windows, or port lights, as they're properly known, are round, square, rectangular or in the shape of a parallelogram--the two sides, although parallel, are angled slightly. The angles are standard, since the marine industry began standardisation long before it began ashore, and the dimensions are stock. When sizes don't quite match, shims are used to mount the window as near as possible to the opening's centre.
Measure the window opening with a measuring tape. If the opening is round, measure the diameter across the centre. If the opening is a square or rectangular, measure height and width. If a parallelogram, measure the opening's height and width.
Select a window that is close to, but not larger than, the size of the opening. Because window sizes are standardised, boat manufacturers make the openings to fit within 1/2 inch of those standard sizes to allow for gasket materials and variations in the manufacturing process of the windows.
Separate the outer part of the window from the inner part of the window by removing the two screws--there are always only two--at the bottom of the frame and the two screws at the top of the frame, if the frame comes assembled. If the trim ring is attached to the inner part of the window, remove the screws that surround the trim ring and lift the trim ring away from the inner part of the window.
Have an assistant slip the waterproof gasket that comes with the window onto the outside part of the port light, then apply caulk on the back of the lip of the outside of the port light. Tell the assistant to set the window into the opening and press it inward.
Attach the inner part of the port light to the outer portion by screwing the screws supplied with the window through the inner part and into the outer part with a screwdriver. This seals the port light and anchors it in the opening. Set the trim ring in place so that it covers any gaps around the inner part of the window. If the trim ring attaches to the inner part of the window, use a screwdriver to set the screws in place in the pre-drilled holes around the perimeter of the trim ring. If the trim ring attaches to the boat's interior wall, fasten it to the wall with screws.
If the window doesn't quite fit, use the shims supplied with the port light unit, or make shims from scrap wood, shaved and carved to the correct shape and dimension.