Explanation of Countersinking
“RC why on Earth do you countersink or chamfer the deck?”
#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 Bed-It 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. Doing the math shows us 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 a sealant failure.
By countersinking around the bolt holes you’ll 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 a sealant 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 very simple step and has numerous benefits. For instance, you can install the fitting and tighten it down now with no waiting. This means you can seal deck hardware alone and not wait for a marine sealant to change states or cure.
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 would discourage anyone from using the Don Casey “two step“, “wait to tighten and form a gasket” method of bedding deck hardware with a curing marine sealant.
Is this because it can’t or won’t work? Absolutely not? Is it because I don’t like Don Casey? Absolutely not! Don is a great guy and tremendous asset to the boating community. Then why?
In my experience this method is probably one of the leading causes of deck core rot on the planet. This is NOT to say that when done properly it won’t work, just that it is VERY, VERY, VERY, VERY difficult to actually do this properly. 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 you can’t get the mechanically fastened hardware tight enough, against the deck, because of compression resistance, the hardware will move, flex and also fail, so again, you lost the battle.
I mean no disrespect to Mr. Casey, he writes tremendous marine books and articles, but the method he made popular is very difficult to nail down repeatably because cure-rates change with humidity, temp and far too many factors to get a good reliable procedure you can easily repeat.. I have witnessed far too many cases of core rot due to DIY & professionals using a two-step bedding procedure than I would have liked to.
So where did I lean this countersinking method? I learned it from 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.