DIY cabin windows
Rupert Holmes shows how to make replacement portlights
Money-saving tips and tricks for making acrylic portlights
If you know the simple tricks of the trade for cutting and drilling acrylic sheet, then it’s quick and easy to make your own windows, and much cheaper than buying ready-made. The same techniques can also be used for replacing washboards and sliding companionway hatches.
After 25 years the original portlights of Zest, my partner Kass Schmitt’s boat, had become crazed. While this is usually just a cosmetic issue that only affects the surface of the plastic, there were also a couple of deeper cracks that were more of a concern. The originals were 10mm acrylic, but given they cover much smaller porthole-style cutouts, rather than bigger rectangular openings, it was feasible to save both weight and money by changing to 8mm sheet.
Researching the easiest way of making a neat cut revealed a Festool circular saw blade designed for use with hard plastics with the same diameter inner boss as my cheap unbranded saw. The blade was surprisingly expensive – at £50 it was more than half the cost of a metre-square acrylic sheet and nearly twice the price of the saw. However, it proved worthwhile in that it provided a quick, easy and very neat way to cut out the basic shape.
Once the acrylic sheet had been marked out it was little more than half an hour’s work to cut out all four windows. It was far quicker – and produced a far neater result – than the usual Diyer’s solution of an acrylic blade in a jigsaw.
The most important tip with the circular saw is to keep the saw moving at a steady speed and to support it underneath to help prevent chipping. We used scrap 12mm plywood for that.
Edge finishing
There are many ways to finish the cut edge to produce a mirror-perfect finish. I considered using a router to chamfer the outer edges, but time pressure led me to try simply cutting at a 22.5° angle on a piece of scrap. The results of this test were impressive – a very smooth finish that looked perfect until you put your eye right next to the panel. This was a good enough finish for it not to be worth spending more time.
Those looking for absolute perfection can sand the cut with progressively finer grades of abrasive paper, working up to around 1200-grit before a final polish. Or a quicker option for those with strong nerves and a steady hand is to flame the edge with a propane or welding torch. To do this the protective film needs to be peeled away from the edge and the flame must be kept moving to avoid scorching.
Corners and screw holes
Given that circular saws only cut straight lines, the corners still needed to be rounded off. For this I reverted to my Bosch jigsaw with acrylic-cutting blade. Although it didn’t give as clean a cut as the Festool circular saw blade, it was still acceptable after a quick sand with abrasive paper. When using the jigsaw it’s particularly important to support the underside of the work to stop the blade chattering.
Zest’s windows are attached by a massive number of countersunk screws – almost 50 for each window. The slowest part of the process, therefore, was cutting these and countersinking the holes using
the old windows as a template – even though we reckoned it would still meet acceptable standards if we reduced the number of screws by almost half.
The parallel sides of a regular drill bit risk splitting the acrylic sheet as the hole is drilled, but the solution is easy. You simply put a slight taper of around 10° on the end of the drill bit using a grinding wheel, a process that takes no more than a couple of minutes. We did this both for the drill bits used for pilot holes and the full-size screw holes.
Lubrication also helps create a clean cut without prematurely blunting the bit or creating so much heat the plastic melts. We used a liberal amount of soapy water applied from a spray bottle. It’s also important to support the acrylic underneath to prevent chipping as the drill penetrates the underside. Again we used scrap 12mm plywood for this, which worked perfectly.
As before, it’s also worth practicing first with some scrap sheet – ideally you’ll get a neat spiral of material coming off the drill bit, with no sign of chattering. The pillar drill made easy work of it, though it would also be possible to do this task with a conventional hand-held drill.
sealant and fitting
I’ve always sealed windows using a PU adhesive sealant, however the originals had been bedded onto butyl tape and remained largely waterproof. This is widely used in the domestic and architectural glazing industries and is supplied in a variety of sizes – we used 15mm x 3mm.
It’s certainly much quicker to apply than more conventional sticky sealants and is a lot cleaner both during the application process and when tidying up afterwards. It’s self-adhesive and needs to be compressed by approximately 50% of the thickness. This makes it easy to judge the tension needed on the screws.
Final outcome
The new windows certainly improve the cosmetic appearance of the boat and, with the right tools and techniques, it was surprisingly easy to produce a very neat job without spending lots of time.