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Argh! Great pictures of the #1 cause of death for Good Old Boats! This just makes me angry, that boat designers are so bad that they do this kind of crap.
Some years ago I did research on sealing aluminum tracks to concrete. What I found out is that Silicone is the sealant of choice for this task, hands down. A properly applied silicone seal will move + or - 100% in compression, tension, and shear! Note that this is 100% of the thickness of the sealant. That means that if the chainplate passes tightly through the deck then the sealant thickness will be minimal to nonexistant. It also means that ANY motion of the chainplate will break the seal! There are boats out there with chainplates that do not move I would guess, but not that most of us can afford.
If you can't keep the chainplate from moving then the obvious answer is to make the sealant thick enough to allow the motion without breaking. So simply opening up the deck all around the chainplate to 1/4" or 3/8" and filling the gap with silicone will allow the chainplate to move up, down or sideways 1/4" to 3/8", and never break the seal! To keep silicone from leaking below while it cures you simply stuff the bottom of the gap with the foam backer rod made for this purpose.
Oh, and that cover plate? What a STUPID idea! Lets add some screw holes to leak too. And while we are at it, let's SHEAR the sealant right off the chainplate as it moves!
Finally, you've done all this work. How do you KNOW you still don't have a leak? One way that is easy and pretty fool proof is to use Joy dish washing detergent in a spray bottle, and look for air bubbles on the outside of the boat. To generate air pressure inside the boat just close all the hatches and vents, and install a piece of plywood in the companion way with a hole for a big shop vac hose, or a better yet a leaf blower. You can check every fitting on your boat in about an hour.
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