Hi Mike,
On my last boat, the '77 Californian, the floor was plain plywood and the hatches seemed to be cut with a circular saw. The fit was "sloppy". My best guess is that, sometime after the boat was built, someone needed to do some real work and made them much larger than the originals using tools and skills readily available. It didn't even have hatches -- just finger holes for lifting.
So, when I did that boat, I laid down a square, and if it needed cutting, just cut it with a utility knife and a straight edge as needed, and moved on to the next one. This always gave me a place to easily see where to start a cut with the knife. I normally accessed the engine room from the door forward, so I didn't bother to add latches and didn't want the finger holes. When I sold the boat, the seams were so seamless that the new owner actually called me up a month later and asked me if there was a way to take the floor apart without damaging it, because he was tired of being a hunch back down there. I explained that the hatches were already there, and the carpeting was already cut, it was just hard to see, and I pushed it up from the inside for major work. He was pleased -- and added latches to lift from the top without delay!
But, as he learned, one challenge in this particular installation is that, because the plywood was a "sloppy" fit The carpeting had an overhand of about 1/4"-3/8" on some sides. If not very carefully when putting the hatch back down, this liked to get caught in the "crack" and pinched. It still worked nicely. And, I was back on the boat a couple weeks ago and it still looks good.
In that boat I went with the dark brown color and, in the salon, threw down a light colored white wal-mart or target throw rug to brighten it up. When I was back on the boat, they had changed to a more colorful throw rug. I didn't ask, but my guess was either that white wasn't a great color on a boat, that they felt it too much of a contrast with the dark floor, or just wanted something more colorful.
In the new boat, half of the hatches were original and fine carpentry. The other half of the hatches had been replaced when the engines had been removed for rebuild. They basically cut out the floor by the galley and threw three large pieces of plywood over it. It is too bad about the nice teak sole that was once there (I have the pieces, but they no longer fit as they also had to move the bar a foot aft). I was originally sad. But, then when I worked on the engines for the first time I realize that it is for the best.
Regardless, when doing the new boat, I had a different problem. The gap around the hatches, especially the originals, was approximately 0". I couldn't fit a knife blade into it. So, when I tried my original trick of laying the carpeting down first and then cutting it -- no go.
For this boat, I didn't have as consistent a way of doing it. The easiest pieces were where I had a line on both sides, e.g. middle pieces with a cut on one adjacent neighbor and the other side exposed. In these cases, I just put the piece down and cut with a straight edge from the adjoining cut to the visible interface. It somehow seemed that, more often, I was covering a corner, leaving me no reference on one side. In these cases I pulled the hatch out, put the piece down, cut it, put the floor back, and put then put cut piece back down, and moved on. In these cases I did make the cut using both feel and a straight edge. I'd start the cut by "plunging" in the hidden corner, and then go an inch or two to get a nice short line, and then finish using a straight edge so I didn't accidentally get wiggly or cut into the wood.
The more pain in the neck things were actually cutting regular room corners, bump outs, and things like that. When measuring, I sometimes assumed squareness -- when there wasn't. In these cases, I often resorted to using a roll of blue masking tape to make a template on the floor, using as much and as many tiny pieces as needed to get an exact fit. Then, I peeled it up, stuck it down on top of the carpeting, and cut away. For big pieces that just had a complex edge, I "outlined" the space, first, getting straight tape lines on the normal corners and edges and a really approximate line on the complicated one and made a big "X" across it with 1.5" or 2" tape. Then I used all the tape I needed to get a couple of inches worth of solid template at the complex edge. Then I peeled it up and off to the races. One trick I learned it to make sure that one edge is "underneath". That way, when pulling up the template, one can start there and the whole template peels up. Sometimes early on I out down a piece of tape here and a piece of tape there was no place that was a good starting place to pull.
In each installation I probably ended up templating 2-3 pieces like this. It didn't take as long as it sounds, maybe 2-3 minutes all in. And, it made for a true perfect fit.
In the new boat when it came to latches, some were real lift-up-ring recessed boat latches (the originals). And some were bolts with dome-type heads with lock nuts and washers on the bottom. Surprisingly, that nickle hack works really nicely. Grab the bolt head and the bolt pulls through and gives you a pull, holding on the inside by the washers and lock nut. Drop it and the bolt falls down leaving only a 5/8" or so low-profile dome-type bolt head visible.
In the case of the bolt heads, I took them off before I started, laid the carpeting, drilled the holes through the carpeting from the bottom right through the original holes in the wood, and put the bolt pulls back on.
In the case of the traditional pulls, which had a rectangular frame and were recessed, I really had no choice but to carpet over them and then cut them out. In the best cases, a single piece of carpeting didn't complete cover the latch. In these cases, using what I could see, I would just cut out a bunch of the middle of the latch to debulk it. Then, I carefully cut the straight edges cut out pieces slowly until I found the corners, and then cleaning up. It took a minute or two of patience. Not bad.
When I came up to the point where the original teak sole bull-nosed over the first step, just stopped the carpeting so it didn't cover the bull-nose. I was originally going to put trim over the edge -- but it wasn't needed. The edges don't unravel, even where cut. The same is true of the step below that one. I just put it there covering everything but the bull-nose. When my neighbor did the steps leading to his 2nd level, he did the same (that's where I got the idea).
I already mentioned the only case that really annoyed me, which was where I had narrow strips or L-shapes of carpeting, such as along or between the bar and the hatches. Where this happened along a wall, it was fine. The little bit of stickiness held the carpeting okay. But, where this happened near a hatch, the movement of the hatch shook it loose within an opening or two. I initially fixed this with double sided gummy carpet tape, which worked great -- and instantly. But, I noticed that if you really looked, you could see the carpeting was a little higher where I did that. So, the next couple of these that came loose, I just glued and theft things alone long enough for the glue to dry. If I'd ever need to take those up, I'd need to scrape that glue off. But, there just aren't very many of those cases.
I saw some pictures of my boat two owners ago with what looked like custom installed dense, high-pile carpet with all of the edges trimmed for the hatches, all glued down. It looked really great. But, I think this looks 90% as good, is probably more cleanable, and I am 100% sure cost a lot less and took a lot less of my time (to track down and manage installers), never mind installer time.
I'll be back on the boat in about a week. I can take some pictures of how it looks overall and of some of the edges, pulls, and steps, if you'd like.
Attached here, because it is all I have, is a similar carpet tile installed in an academic building office and hallway.
Hope this helps.
Cheers!
-Greg