I just replaced this on my 3818...thought I'd post a few pics/etc while it's still fresh in my mind. The master stateroom hatch on mine was old, the plexiglass was cracked (and patched so it wouldn't leak over winter), the rods that hold it open were corroded and didn't slide easily, the rubbers seals were worn and cracked...just time to replace the whole thing. 
The hatch: Bomar/Pompanette N1039-10A. You can order them from WestMarine, suppliers on Ebay, possibly straight from the mfg, etc. (By the way, the hatches in both heads are Bomar N1029-10A. The "N" is apparently for "Nibo", the low-profile hatches.)
I found it easiest to open the hatch and stand on the bed in the master stateroom. Take the trim ring off first from below...there are several screws and those should come out easily, then open the hatch to expose the screws holding the frame down on the topside. The topside screws will likely come out fair easily (be careful not to strip them, they are old stainless steel screws). It's not obvious looking at it, but the two furthest apart screws in the hinges also hold down the aluminum frame...remove those 4 screws in addition to the obvious ones holding the frame down.
You'll need a flat-edged pry bar and a rubber hammer to break the sealant loose and get the frame out. I hammered the pry bar under the edge on one side, pried that loose, and worked my way around the rest of the frame. The sealant came loose pretty easily. The frame will lift right out over your head, assuming you are standing on the bed with the hatch open. Here's the old hatch out:
Next, with a metal scraper, scrape all of the old sealant off the lip that the frame sits on. This may take a while, but be thorough...any old sealant left could cause a gap, which translates to "leak". You should have a clean surface to work with. (Also, this is the type of pry bar you probably want.)
After you have the surface clean, flip the new hatch over, and run a bead of sealant all the way around in a continuous loop...I did a bead that was around 1/4" radius, so not too large, but not tiny either. (One mistake I believe I made...I used 3M 5200 fast-cure sealant...this stuff is also a pretty heavy-duty adhesive, so that probably wasn't the best idea...if this hatch ever has to be replaced again, it probably will not come loose as easily. I probably should have used 3M 4200 instead, and will use that on the other two hatches that I plan to replace. There are solvents that are supposed to "de-bond" 5200...one is "Marine Formula" by DeBond Corp, which comes in a spray can with a straw, kinda like WD-40. I have not tried this product, however.) After you have it cleaned up, it's probably a good idea to fill every screw hole with 3M 4200 sealant so water can't possibly get into the wood core of the foredeck. I'd use a sealant you can break loose...3M 5200 might bond those screw permanently to the deck, insuring that they strip or the heads break off when/if you or someone else tries to remove them years later.
Open the hatch, and drop it into place by lowering it around your head and upper body (again, assuming you are standing on the bed with your head and chest sticking out of the hatch hole). Slide it around just a little to flatten out the bead of sealant. On mine, some of the original holes lined up, and some did not (which is why I filled all the old screw holes with 3M 4200 sealant BEFORE I dropped the new hatch into place). Don't try to force it to align with the old screw holes...just forget those screw holes you've already filled with 3M 4200 are under there, and drill new holes where necessary. Check the spacing inside...you want to get it centered well enough that the trim ring will fit, since fairly ugly fiberglass edge and wood are exposed underneath. I drilled new holes where necessary with a 5/32" drill bit (if I recall correctly). #10 1" stainless flat-head screws fit the hatch nicely. I also dipped each screw in 3M 4200 sealant as I drove them in (that should seal well without sticking...it has much less adhesive than the 3M 5200). Tighten all the screws most of the way tight, then examine the inside to insure the spacing is still right, then tighten them the rest of the way down.
Check that it opens and closes easily, and sits flush (in other words, make sure you didn't tight it down so much that it's warping the shape of the aluminum frame...it should be tight, but not about-to-strip-the-screw tight). And here it is completely installed.
If you managed to get the trim ring off without breaking it, wiggle it back into place. I broke pieces of mine, so I ordered a new one...it's a Bomar NT-2039 trim ring. The trim ring hides the rough edges of fiberglass and wood, but also gives you something to hold a screen (if you use screens).
This took me around 3-4 hours, but I was kinda taking my time. The hatch and trim ring aren't exactly cheap (around $400-500 for the hatch, another $60-80 for the trim ring), so I'd suggest taking your time, thinking it through well, and don't attempt it on a day when there's rain in the forecast for the same day or next day, etc.
Good luck!
Dave
The hatch: Bomar/Pompanette N1039-10A. You can order them from WestMarine, suppliers on Ebay, possibly straight from the mfg, etc. (By the way, the hatches in both heads are Bomar N1029-10A. The "N" is apparently for "Nibo", the low-profile hatches.)
I found it easiest to open the hatch and stand on the bed in the master stateroom. Take the trim ring off first from below...there are several screws and those should come out easily, then open the hatch to expose the screws holding the frame down on the topside. The topside screws will likely come out fair easily (be careful not to strip them, they are old stainless steel screws). It's not obvious looking at it, but the two furthest apart screws in the hinges also hold down the aluminum frame...remove those 4 screws in addition to the obvious ones holding the frame down.
You'll need a flat-edged pry bar and a rubber hammer to break the sealant loose and get the frame out. I hammered the pry bar under the edge on one side, pried that loose, and worked my way around the rest of the frame. The sealant came loose pretty easily. The frame will lift right out over your head, assuming you are standing on the bed with the hatch open. Here's the old hatch out:
Next, with a metal scraper, scrape all of the old sealant off the lip that the frame sits on. This may take a while, but be thorough...any old sealant left could cause a gap, which translates to "leak". You should have a clean surface to work with. (Also, this is the type of pry bar you probably want.)
After you have the surface clean, flip the new hatch over, and run a bead of sealant all the way around in a continuous loop...I did a bead that was around 1/4" radius, so not too large, but not tiny either. (One mistake I believe I made...I used 3M 5200 fast-cure sealant...this stuff is also a pretty heavy-duty adhesive, so that probably wasn't the best idea...if this hatch ever has to be replaced again, it probably will not come loose as easily. I probably should have used 3M 4200 instead, and will use that on the other two hatches that I plan to replace. There are solvents that are supposed to "de-bond" 5200...one is "Marine Formula" by DeBond Corp, which comes in a spray can with a straw, kinda like WD-40. I have not tried this product, however.) After you have it cleaned up, it's probably a good idea to fill every screw hole with 3M 4200 sealant so water can't possibly get into the wood core of the foredeck. I'd use a sealant you can break loose...3M 5200 might bond those screw permanently to the deck, insuring that they strip or the heads break off when/if you or someone else tries to remove them years later.
Open the hatch, and drop it into place by lowering it around your head and upper body (again, assuming you are standing on the bed with your head and chest sticking out of the hatch hole). Slide it around just a little to flatten out the bead of sealant. On mine, some of the original holes lined up, and some did not (which is why I filled all the old screw holes with 3M 4200 sealant BEFORE I dropped the new hatch into place). Don't try to force it to align with the old screw holes...just forget those screw holes you've already filled with 3M 4200 are under there, and drill new holes where necessary. Check the spacing inside...you want to get it centered well enough that the trim ring will fit, since fairly ugly fiberglass edge and wood are exposed underneath. I drilled new holes where necessary with a 5/32" drill bit (if I recall correctly). #10 1" stainless flat-head screws fit the hatch nicely. I also dipped each screw in 3M 4200 sealant as I drove them in (that should seal well without sticking...it has much less adhesive than the 3M 5200). Tighten all the screws most of the way tight, then examine the inside to insure the spacing is still right, then tighten them the rest of the way down.
Check that it opens and closes easily, and sits flush (in other words, make sure you didn't tight it down so much that it's warping the shape of the aluminum frame...it should be tight, but not about-to-strip-the-screw tight). And here it is completely installed.
If you managed to get the trim ring off without breaking it, wiggle it back into place. I broke pieces of mine, so I ordered a new one...it's a Bomar NT-2039 trim ring. The trim ring hides the rough edges of fiberglass and wood, but also gives you something to hold a screen (if you use screens).
This took me around 3-4 hours, but I was kinda taking my time. The hatch and trim ring aren't exactly cheap (around $400-500 for the hatch, another $60-80 for the trim ring), so I'd suggest taking your time, thinking it through well, and don't attempt it on a day when there's rain in the forecast for the same day or next day, etc.
Good luck!
Dave
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