Deck plate in balsa core water tank

[hlev00]

My Beneteau has a water tank that is built into the hull under the v-berth. The top of the tank is mostly 1/2" balsa core except where Beneteau installed the sender, fill port and vent. I am installing one or maybe two 6" inspection ports in the top to access for cleaning and possibly to install some baffles to reduce sloshing. I'm planning on using a Beckson, or maybe West Marine equivalent, screw in deck plate. These all say to use only a pure silicone sealant to not damage the plastic.



First, after cutting out the hole I plan to cut back 1/8" or so of balsa, not as far as the fastening screws. Then seal with epoxy, probably two coats, and finally fill back to the edge of the GRP with thickened epoxy.



For the fasteners I am considering 1/2" #8 screws, instead of through bolting, drilling only through the top/outer skin. My thought is that this will eliminate a source of leaking through the inner skin. The tanks are all inside the boat so there should be no water on the outside of this deck plate. Will putting silicone in the hole in addition to under the flange before screwing it down protect the balsa in this installation, or should I use epoxy in some way? My secondary concern is the pressure on the seal and screws from the water when the tank is full or any flexing when underway.



If I through bolt I'll oversize the holes, seal with regular epoxy followed by thickened epoxy, and re-drill the holes to the final size for the through bolts. I will fill the bolt holes with the silicone sealant for final assembly.


I am interested in feedback on my approach to this project, including other deck plates or access ports to consider. I've looked at some like the Vetus solution, but it and others seem to be geared towards single walled tanks.



Thanks,
Harry

[hlev00]

Or, use the oversize hole method for the self-tapping screws, with or without drilling through to the inner skin.

[tkeithlu]

As I understand your description, the 1/2" (SS) sheet metal screws will hold the flange to the tank and thread through the outer layer of tank glass, into the balsa, but not be long enough to penetrate through the second layer of glass and into the tank itself.

My impression is that you plan to use inspection plates such as this one:

https://www.amazon.com/MARINE-PLASTI...28070031&psc=1

Why use the screws? You've got a bunch of surface area both between the flange and the horizontal surface of the tank, lots between the outer part and the edges of the hole in the tank, and you can each your hand into the tank to make sure that the epoxy or 3M 5200 you use to join the two is spread evenly. You can set a brick on it while it cures. The load on the port is sloosh only. The screws, I'm inclined to think, largely just create the problem you have been wrestling with, that of water penetration into the balsa.

[hlev00]

Thanks for your suggestion.
Yes, basically an inspection plate like that, though screw in. Just gluing it down is interesting, but all the manufacturers seem to be consistent on their insistence that only silicone be used as a sealant. After too many years, I've finally learned that silicone is awful stuff on a boat in most circumstances, but it seems to be a requirement for that application. That leaves me with fasteners.

[tkeithlu]

Are they concerned about flex?

If you must use fasteners, I would vote for SS through bolts, with the holes slightly oversized and filled with epoxy. You can get to the bolt heads/washers by reaching a hand in through the port. I'm inclined to stay away from the sheet metal screws.