I would use 5/16" or even 3/8" Lexan, and a "nuts N bolts" port
installation. If done correctly, it is stronger than the cabin! IF you have a cored cabin, I would dig out the
core for an inch back all around, prep the surfaces, and fill the space with a much denser
epoxy / microballoons mix. It is waterproof as well. I will cut & paste these previously posted instructions... Ours have never leaked in 15 years, and taken tremendous impacts from breaking waves!
The trick is that Lexan expands at an extraordinary rate compared to a
wood or
fiberglass hatch. If it is in a recession, so it is flush on top, make the plastic at least 1/4" smaller than the recession. (= 1/8" space MIN all around) This is for expansion room.
We called GE "who make Lexan" for bedding recommendations, and they suggested "and sell" SilPruf silicone. Back then... It had to be be ordered by the case of 12, so you need to go in with several others, so you don't get stuck with excess. It comes in colors as well and has about a 2 year lifespan. These are unfortunate, but it is still the way to go! It cleans with Isopropol alcohol, which will not hurt the Lexan.
They glaze huge panels of Lexan in
commercial applications with this stuff, and the most remarkable thing is the 35 or 40 minutes of good work time, VS only 5 minutes with most caulks.
Assuming you will mount it on top, like I did in the photos, VS in a recession...
Leave the paper on both sides, then cut out the plastic and round the corners. (optional)... Then if you radius and fine sand the edges, it it's a nice touch.
If you put in a couple of cross bars underneath, like mine pictured above, then 1/4" thick is fine. If there will be no cross bars, I'd go with 3/8", which will be HEAVY! It is not about strength, it's about limiting flex which will work the joint.
I would try to avoid screws, but instead, use through bolting, as this allows the VERY important fine touch in tensioning. I use SS Phillips, Truss
head, #10 machine screws on the outside, and Phillips, truss
head, barrelnuts on the inside. These look like a screw from the top, but are a cylindrical nut, they come in SS and chromed brass. (I prefer the SS, but either will do) Truss head fasteners have a low profile but large dia. head, so need no washers.
I believe that the hole for the barrel nuts in the
hatch, will be 1/4". You center the Lexan on the hatch, and scribe it. The Lexan should be 1.5" larger (on all sides), than the hole it goes over.
After scribing and
removal, you have a pencil line all around the hatch's hole, 1.5" out. Now make another pencil line down the middle of this space all around. It will be 3/4" out from the edge of the hole. This is the bolt holes line. The spacing I used was about 2.5" apart, but 3" will be fine. (If you use 3/8" Lexan, every 4" is fine). It is not etched in stone. You can fudge a bit to get even spacing all around.
Now use a NEW or very sharp 1/4" brad tip drill bit, to drill all of the holes you just marked. Always have a backing block of 2X4 or something underneith where you are drilling, so when it goes through, it wont blow out the back of the wood hatch.
Next, reposition the Lexan on the outside, center it, and duct tape it down.
Now flip it over, and using the brad of the brad tip bit, mark the perfect center of each hole that will be in the plastic. (Have the front of the Lexan marked, so it will go on the same way later).
After marking, remove the Lexan, and drill the holes 1/4" with the same bit, being careful to use the back up block like before. (Go slow, or it will melt) You can also drill a VERY small pilot hole, and then drill the big hole half way through from each side. Now take a countersink bit, and just barely ease the edges of the wood and plastic holes.
Your barrel nuts underneath the hatch, should have a barrel shorter than the hatch thickness, and will be pushed in for a snug fit. The outside fasteners are #10 machine screws, which will be HALF the size of the holes in the Lexan that they are fastening down! This is expansion room from "0" F to over "100" F.
The way you will hold it centered in final assy. Is with two 1/4" machine screws, one on each side. They are for assembly only, will be snug on the hatch as well as the Lexan 1/4" holes, and will make it where, when you screw in the Truss head #10s, the screws will go straight down the center of the greatly oversized holes in the Lexan. After fastening is complete, remove the two full sized marker bolts, and replace them with the barrel nut fasteners like the rest.
This is the premise. Before fastening down, underneath the Lexan, scribe the Lexan's paper to mark the caulk down line, which must be removed. Now remove a bit more paper than that with scissors. Next, re-scribe, and tape off the scribed line with 3-M 471 blue plastic flexable tape. (NAPPA) This exposes only the caulk zone. Now, Scothbrite the caulk line until dull.
Mask the hatch also... for 3" beyond the caulk zone, inside and out!
The less clean up later, the better. On the outside, cut back enough paper to expose all of the fastener zone, then re-attach the paper with the same tape. Now you will have very little clean up underneath, and on the outside, just the fastener zone.
This way, the scratches in the Lexan from cleanup, will not be in the area that you look through, it will be in the hatch overlaps.
Try a dry run fastener or two for practice... Now press in all barrel nuts. They may stay on their own. Otherwise hold the hatch vertically.
Quickly goop up the hatch holes and caulk zone on the Lexan and the hatch, in as thin and consistent a way as possible. (Putty knife, foam brush, squeegee, whatever...)
Mount the hatch lens with the two predetermined, 1/4" Marker bolts. Do not snug up!
Now go all around with one person inside, and one outside. The inside person holds the barrel nut still, while the outside person inserts the truss head machine screws and threads them home. DO NOT SNUG YET!
Caulk should have oozed out everywhere by now, even the outer screw holes in the Lexan. Now in opposing fashion, just like torquing down the head of an
engine, snug the screws down, bit by bit. Don't do one completely, then the other, little at the time!
This is hard to get, but you don't want the two
parts to actually touch! It takes pressure like picking up a butterfly. If you use colored caulk, the second you can see through the caulk, STOP. You don't want to squeeze it out. When it is even all around, BINGO!
Now, use a screwdriver flat blade first to remove excess, then after scooping is done, Use Iso. Alcohol wet paper towels to get the rest. Then remove the paper and get what you missed. Remember, the stuff scratches VERY easily!
You're done! You can walk on it in a few days. If you want privacy, like ours above, sand it with a palm sander and 220 grit.
I have seen hundreds of Nuts N Bolts ports, with either
leaks, or cracks at the holes. It is because they weren't done right! The bolts must be half the size of the hole, perfectly centered, and not overtightened. Done correctly, they are as tough as they come!
Mark
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