With a bulkhead hatch, you don't need a vent hole. Just don't dog/screw/toggle it down fully tight. Then air can bleed both directions, while water will be held out pretty well, even submerged.

You can go with a nice (expensive) genuine boat hatch if you like. But if all you want is a vent (because a 4" hole isn't access), go to the hardware store and buy a few pieces of threaded PVC pipe components. One cap, one nipple, and one collar. Screw the collar onto the hatch with some caulking under it, drill the wood (good idea to seal the exposed edges), screw the nipple into the collar (seal it and screw down firmly), screw the cap onto the nipple by hand. You're done. Screw the cap on loose and it'll bleed air. Screw it down hard and it'll be air tight. Use a longer nipple to move access further back until you're comfortable with it. If you want to run it back all the way to the combing, you might want to use a piece of wood to support it for that long run. That would get you away from needing any special tool.

Another thing to consider if you really want access is to make a very large cutout on your bulkhead, and then essentially make another complete bulkhead piece to act as a door to cover the whole hole. That's what I did on my boat. My structural bulkhead is 2 pieces of 1/4 luan laminated to 1/2". I then drew an outline line about 3" inside the outer edge, and cut that. Now my bulkhead looks like a lifesafer candy. Built the boat with those bulkheads. I also cut out a second set of bulkheads from the 1/4 luan as covers or doors. used a shoulder on the bottom to locate the door there, and two thumb wheel screws to pull the door tight against the bulkhead at the top. I planned to use gaskets, but found I really didn't need it. Pulled tight, the doors do a fine job of blocking moving water. The 3" bulkhead also does a dandy job of blocking standing water. I've gone out after leaving it in the rain to find upwards of a foot of water in the boat, bailed it, and found only a small puddling of water in the aft compartment. None in the bow. Spraying with a garden hose with all sorts of enthusiam never got a single drop of water around the doors.

> So, I guess it's going to be a 4" screw in inspection port
> in the bulkhead. I'll have to make a tool to reach it though since the
> bulkhead is about a foot beyond my reach. I think I can put a small hole
> (like 1/8") in the center of the inspection plate to prevent any 'oil
> can' effect. A small hole shouldn't let in much water even in extreme
> situations (which I hope to avoid). Which leads me to wonder, how small is
> too small to provide an adequate air flow? I'm not too concerned about the
> awkward location for screwing the inspection plate in and out since I
> should have to do this infrequently (like off season storage). The tool to
> screw it in and out should be challenging, but I have a couple of ideas.
> Anyway, thanks to all who provided input to this discussion.