marine plywood as thru hull backing plate ?

K

KEVIN D TOBIN

Member
Jun 12, 2018
69
Boat Info
1993 290 sundancer, 7.4L mercruiser repower 2018
Engines
7.4L Bravo ll drive
is 3/4 marine plywood ok to use as a seacock backing plate ?
 
Yup, that'll be fine. Are you leaving it bare, or will you be glassing it in - you probably have other "pads" already in place in the hull bottom like that. Ideally, fully encapsulate it with resin and glass it. Even regular plywood can be used if you're fully encapsulating it, first - especially if you're oversize the holes for screws and fill with epoxy.
 
That make a round backing plate 3/4" with brass nuts built into it just for this purpose, I used 610 thickened epoxy to glue it to the hull. Not sure what kind of material it was but it was in between fiberglass and plastic, will never rot.
When I was done it came out perfect.
gro-bb-1_1__52437.1558175583.jpg
 
When I installed my AC, I used regular plywood. Coated real good with expoy resin. Seemed to work fine.
 
Use a quality, bronze flanged seacock. Before mounting your plywood backing plate, drill the holes for your seacock and countersunk bolts slightly oversize and immerse the plywood in epoxy resin so it can penetrate the wood, especially the edge grain. Clean the hull in the area, insert the bolts (tape the threads) and position it. Then bed it against the hull with thickened resin so it oozes out all around the plate, then scrape up the excess. Once cured, drill through the hull. Thoroughly coat the seacock flange and threads of the pre-sized through-hull fitting with 5200 or similar so it squeezes up through the drilled hole. Snug up the through-hull, tighten down the seacock mounting plate, also using 5200 around the bolts and washers, tighten the nuts and clean up the excess caulking. This will last the life of your boat.
 
Last edited:
Once you have your parts, do a dry fit install that Juliery describes. You want to make sure the bolts for your backing plate are the right length (not to short, not to long). Check the orientation of the seacock - will the handle open/close the way you have it setup? Find a way to mark the backing plate/hull so when you do glass it in - the orientation is proper. Dont forget your green bonding wire, attach to one of the seacock flange bolts. You may need to cut to length the actual thru-hull. Find a wrench, piece of wood, or something to turn the thruhull so you can tighten it nice and snug. I would clean the outside hull down to the gelcoat, and suggest scuffing/scarffing up the inside hull so the epoxy has something to set/grip/bite into. Lots of paper towels when it comes time for the sealant - stuff sticks to everything.

https://marinehowto.com/replacing-thru-hulls-and-seacocks/
 
One quick note, Kevin... the only thing you want to use "permanent" methods of attachment on (such as epoxy or 5200) is going to be the backer plate. Everything else is mechanically held in place. For those things, use a good quality sealant. You could back down to 4200 - or something like LifeCaulk (there's others, as well) would be perfect. The reason for this is, number one, the primary method of attachment is mechanical - no need for super glue - there's advantage to using it. Number two, it's possible that these may need to be removed in the future (damage, failure, whatever) and you'd only be making that unneccessarily harder than it has to be.

A fiberglass piece of "wood", like that G10 stuff, is certainly a nice option. Plenty of options out there for you.
 
You must log in or register to reply here.

Forum statistics

Threads
113,210
Messages
1,428,658
Members
61,108
Latest member
rvlewis
Back
Top