So why on earth do I countersink or chamfer the deck side of the skins?
#1 it does a lot to prevent gelcoat crazing! By feathering the edge of the gelcoat, through the use of countersink bit, you are less likely to start a crack or craze mark in the gelcoat.
#2 By creating a bevel or countersunk recess in the surface of the deck the marine sealant or butyl tape has a cavity to fill. Without a bevel the hardware would compress the sealant to about 1/64th of an inch thick after the fasteners are tightened. Marine sealants do have some flexibility but not as much as you would guess. Follow me for a moment; Let's say you have a marine sealant that has a rating of 400% elongation before break on a 1/64" thick joint. Simple math shows you that 400% of 1/64" is only 1/16" of total allowable joint movement before a joint failure or leak starts. In the case of a stanchion base 1/16" is not much allowable movement before failure.
By countersinking around the bolt holes you increase the maximum thickness of the sealant, at the bevel, to roughly a 3/32" depth at it's deepest point. Using the same math as above 3/32" X 400% gives you a total joint movement before failure of 3/8". If a mechanically fastened joint is moving 3/8" you have more problems than just a deck leak! So 1/16" allowable movement before failure at 400% without countersinking or 3/8" allowable movement at 400% before failure with countersinking. Even a small bevel will drastically increase max allowable movement before failure. Countersinking to a mere 1/16" depth will give you 1/4" total movement around the bolt before failure. You don't need a deep bevel to make a large difference between a failure and a seal.
#3 Countersinking is really a no-brainer, and has many many benefits. For instance, you can install the fitting and tighten it down NOW with no waiting. This means you can seal deck hardware alone.
I am going to inject some strong opinion here so please move on to the next photo if you don't want to hear it. I truly dislike and disagree with the Don Casey "two step", "wait to tighten and form a gasket" method of bedding deck hardware with a "curing" marine sealant. In my opinion this method is probably one of the leading causes of deck core rot on the planet. Believe it or not but I'm not a conspiracy theorist! I do however feel as if Don Casey wrote that technique to guarantee boatyards future revenue. This is NOT to say that when done "properly" it won't work just that it is VERY difficult to actually do this properly and many a DIY and pro have messed this up and ruined decks as a result.
Think about it? If the sealant cures, to form a "gasket", and you then move the bolt while tightening it down on the "second step".... you will likely lose. If you create a gasket too thick, it cures, and then can't get the mechanically fastened hardware tight enough against the deck because of compression resistance, the hardware will move, and also fail, so again.... you lose.
Sorry, I mean no disrespect to Mr. Casey, and he writes some great stuff, but this method is perhaps some of the worst advice I've ever read in any sailing/boating related book. I have witnessed far too many cases of core rot due to DIY & pro "two step" bedding procedures than I would have like to.
So where did I lean this countersinking method??? Hinckley Yachts. Sure this extra step takes all of about 30 seconds per stanchion but apparently it's too much "extra" work for most production builders.
OK I'm done with my rant...