While I was in Florida working with David Halladay on his Tiki 30 project, we discussed a lot of details in the fitting out of our catamarans and tossed ideas back and forth. David spends a lot of time working on interior and exterior carpentry projects for modern, high dollar motoryachts, and consequently has some methods and techniques beyond the basics shown in the Wharram plans for these boats.
One area that he feels can stand some improvement is in the installation of the fixed portlights on the outboard sides of the cabins. The plans show Lexan ports cut to overlap the edges of the openings in the plywood, and fixed in position on a bedding of sealant with small machine screws in holes drilled along the edges of the Lexan. This works fine and is seen on many types of vessels, but David felt it is a bit dated and doesn't look as clean as a more modern installation without fasteners. In addition, Lexan and wood expand and contract at different rates in temperature fluctuations, so the holes must be drilled larger than the screws or the panel can crack. A better method that he often uses for installing portlights is to either create a lip in the cabin side if it is thick enough, or add a ring frame on either the inside or outside of the portlight opening so that the Lexan panel can be mounted flush in the opening. This will be easier to visualize later when I have the actual Lexan and the cabin sides are installed, but the photos below show the process for making these frames.
The ring frames are made of 6mm ply, same as the cabin sides and the thickness of the Lexan that will be used. Since I had already mocked-up one cabin side on the port hull and had already cut the openings for the portlights, I was able to use the cut-out parts as templates for the ring frames. The Lexan will be cut slightly smaller than the existing openings in the cabin sides, to allow for some expansion, the frames will overlap the opening and the Lexan portlights by 5/8", making them a total of 1 1/4" wide.
Below is the sequence for making the frames for the smaller portlight at the forward end of the cabin. I started out by marking a line exactly the size of the opening on a piece of plywood to be used as the pattern. In the photo below, you can see this as the heavier centerline in the middle of the pattern frame. Outside and inside perimeter lines were scribed 5/8" on either side of this central line and the pattern was cut out to this line with a circular saw and jigsaw. The actual cuts were made 1/16" from the line and then the excess material was sanded off with a belt sander on the outside and by hand with sanding blocks on the inside. Below you can see the tube of caulking that I used to sand the inside radius. This was just the right size when fitted with a sheet of sticky-back sandpaper.
When the pattern was cleaned up and trued to exactly the lines and the curves were fair, I then temporarily tacked it to the first workpiece with a few dabs of glue from a hot glue gun, to hold it in position for routing.
Making exact duplicates from a pattern like this is easy with a router fitted with a straight bit with a guide bearing, as shown below. Although you can use a regular flush cutting bit with the bearing on the bottom, I prefer the top bearing kind, as you can better see what you're doing. This bit is long enough to cut more than one frame at a time if they are stacked, but I wanted to do just one first to make sure it was correct. There will be a total of four frames for the small portlights and four for the larger, aft ports, since I want to have both an interior and exterior frame for greater security. This is not really necessary with modern sealants, however, and many such ports are put in with just the sealant and an interior frame, resulting in a cleaner, flush exterior surface. My exterior frames will blend in nicely though, as they will be filleted and glassed right along with the rest of the cabin side and painted the same color.
Here's a view from the bottom showing the pattern-cutting router bit doing its job. For small parts of thin material such as this, I like small one-handed trim routers.
Below is the finished result. The piece of wood under the frame is the pattern for the Lexan port, and gives a hint as to how the actual port will look with the ring frame over the window.