Fixed ports & windows

Ports: a template and a mistake

In February 2022 I made a jig to cut the port openings. The ports I bought have an angled spigot. That's good for drainage, but it's a lot more work, because the cut through the plywood core has to follow the angle. My router bit is only 2.5 inches long, so the template is made of two layers: it lines up the cut in the outer skin and sets the angle.

In July I took the first old window out, to see how it's done. Pull the trim, cut the side of the frame, fold it inward and pull it out. Then the template, and route the hole. The old opening was 3–5 mm bigger than the new port, so there wasn't much grinding, mostly the slope for the spigots. I added a sloped plate inside for 2 more degrees, and measured the thickness for the screws: #10, 1-1/4 inch.

Then the first port went in, the pilot. Short story: I bought self-draining ports by mistake, instead of rain-draining ones. Not a big difference, except that the Irwin 34 cabin sides are 17° from vertical, and self-draining ports allow 15° at most. So I needed the inside wall closer to vertical. I calculated that 3 degrees more would do it, and put a sloped shim inside, under the plywood. I also added some reflective insulation to cut the heat coming through the cabin sides. The port went in on 3M 5200, as Bomar recommends.

I had planned to through-bolt the ports, and even bought the bolts. Then I changed my mind and used self-tappers, like the original. With the tilted spigot, the bottom bolts came too close to the edge, and the top ones came out under the outer frame. And a 1-inch #10 screw will break the plastic before it pulls out of epoxied plywood anyway. If a storm is strong enough to break the windows, I'll put covers on the outside instead of trusting screws.

Five more ports

With my son's help I took out the old, cracked, leaking plastic windows. Metal ones will come later, or much later. We cut the old ones out, removed the old inner plywood, and added the slope shims, CNC-cut so the angle is exact. Another 1/2-inch plywood strip below each window supports the interior plywood. The old sealant, which looked like 5200, hadn't held to the plastic. I'll ask Bomar what prep they want.

Then I cut the inner plywood panels and epoxied them in, held with wood screws and temporary plates on the outside, ready for the other five ports.

In August came a long day: route five new openings with the CNC template. At the thickest spot I had to cut through about 1/4 inch of outer glass, 3/4 inch of original plywood, about 1/2 inch of angled spacer and 1/4 inch of inner plywood, and none of it was square. Even the 2-inch bit couldn't reach. So I routed with both templates as deep as I could, took the add-on template off, and cut deeper from the top.

Lots of dust. The ports went in on 5200. Maybe not the best choice, but the maker strictly recommends it. I soaked the openings with sealant first, then pushed the port in, so there's solid polyurethane between the port and every layer of plywood and glass.

It doesn't take many words to tell, but it was a hell of a lot of work: cut, clean, mask, seal, and figure out how to clamp the outer trim rings on a curved surface. Sand bags help, as usual.

Fixed windows, attempt one

The fixed windows had been set straight into fiberglass resin, probably gelcoat, with zero flex. So the original acrylic cracked from expanding and shrinking in the sun. When I got the boat, they were cracked and leaking, both through the cracks and around the edges.

In February 2022 I checked my plywood template. Very close.

The dry fit of the new acrylic was off by about 5 mm on the long corner.

April was a full disaster. Taking the old acrylic out, I chipped the gelcoat all around. There was no flexible sealant at all, just acrylic set in gelcoat.

Then I glued the new ones in.

One spot didn't get enough Sikaflex, so I redid it. Lesson: use more than enough, then clean up the extra.

Attempts two, three, four...

Who designed it this way, I don't know. The frame gives only about 10 mm to bond the lens, and the surface is curved, so it's always trying to pull the ends of the lens out.

The first try was 1/4-inch acrylic cut to size, on the famous Sika 295 UV with activator. The sealant cost more than the acrylic. I followed every instruction.

In September 2022 I saw that the starboard lens wasn't sticking. Probably I had forgotten to prime the acrylic. The other side was fine. Half a year later one window dripped: the Sika had let go of the plastic. I thought I'd messed up the prep or the timing, so I cleaned everything, scuffed the acrylic and did both sides again with Sika.

That lasted a year, and then it dripped again, the same way. My theory was that UV going through the acrylic was killing the bond. So in May 2023 I tried the method everybody recommends: VHB tape and 3M 4000 UV. Cleaning off the old polyurethane was a lot of work. It bonds to fiberglass really well, which is exactly what you don't want at that moment.

In August I filled the gap between the windows and the inner plywood. I'd put it off for a while: it's messy work that has to be done carefully, so you don't damage the plywood face. I used epoxy with a bit of chopped glass for strength, 410 microlight so it sands easier, and 404 plus silica to get the right thickness. Filling a gap overhead is tricky. The next step was to sand it without scratching the acrylic.

The VHB didn't survive half a year. I didn't check why, but I'm sure the small bonding area played a part.

Screws, and a test first

I was tired of fighting adhesives. The new plan came with a triple guarantee:

• Screw the lens down, so the sealant never holds the load. A screw every 70 mm, 36 per window. Lots of holes, I know, but no stress on the bond.
• Use a different sealant on the lip: DOW 795. It holds glass on skyscrapers, so it should be good enough for us. DOW says it works with acrylic and Lexan, and lots of people online recommend it.
• Butyl tape around the outside.

Each window is 1,025 mm long, and my CNC only does 740. So I cut each one in two positions, 400 mm apart, left side first, then right side. At each position: drill the holes, mill the slot around, then cut it out of the sheet.

This time I wanted one shot that lasts 5–10 years, so I ran an adhesion test first. I cut 15 pieces of FR4 circuit board and sanded them to bare laminate at the contact spot, about one square inch, all with 220 grit.

For butyl tape I tested acrylic sanded at 220 and 600, Bed-It against a white butyl tape from Amazon, cleaned with 50% IPA. All of them came off at about 6 lb. At 600 grit, neither tape stuck to the acrylic at all; nothing was left on it. At 220 the Amazon tape held a little, not great, and Bed-It nothing.

For DOW 795 I tested glossy acrylic, 220 and 600 grit, cleaned with IPA or TotalBoat dewaxer, bare or epoxy-coated. Glossy acrylic held some, about 10 lb, and the silicone peeled off clean. Every other sample, whatever the prep, held until the silicone rubber itself tore in the middle, at 12–15 lb. We have a winner.

So it was DOW 795 only, around the edge and in the gap. In December 2023 the new windows went in, screwed every 75 mm, with the acrylic and gelcoat sanded at 220 and cleaned with 50/50 IPA.

Finally dry

July 2025: no leaks for a year and a half. Now I could finish the inside. First I made two templates, with 20 mm and 30 mm corners. The 20 mm one fit best.

The screws still worried me. They sit in fiberglass with silicone, and I don't trust that seal much, so I put rubber washers under them for a compression seal. Maybe I'm wrong, but washers are better than nothing. By the way, in later boats Irwin dropped the fixed windows and used regular ports. I understand them now.

Then came new teak trim, each frame four pieces cut on my CNC in the garage. Only the gap between the glass and the trim is left to paint. Two years, four attempts, and the windows are finally dry.