bedding/backing through hulls

[pasty63]

I don't like the looks of the 1/2" plywood squares that have served as backing on the through hull fittings on my FRP hull for 22 years - what's the best replacement? UHMW? Is 5200 the splooge of choice?

Thanks -.b

[FAST FRED]

If there well above the WL, most anything will do.

IF there near , at or below the WL I would sugest a proper marine seacock BOLTED to the hull.

THe thru hull screws into the seacock with dolphinite (an easily removable sealant) and can be replaced easily if needed.

You should be able to stand on the seacock when installed properly.

FF

[kenJ]

Most of the DIY instructions on through hulls suggest marine ply as the backing plate. Pre cut, drill and ensure proper fitment, then seal with epoxy before attaching it to the hull. Most folks glass/epoxy in place. 4200 or 5200 should work fine also. Do not use Starboard, it is too slippery, won't stay in place.

[Crag Cay]

the 1/2" plywood squares that have served as backing on the through hull fittings on my FRP hull for 22 years
Your answer is in the question. Decent plywood backing plates are good for a quarter of a century. If you want to go longer than this, than epoxy coat them as suggested. 5200 is fine for mating the pads to the hull and the through hull itself. I know it's an 'adhesive', but I've removed hundreds of fittings from boats that have been bedded with it. Realistically, you are fitting these pads 'for ever', so it's fine.

[pasty63]

Your answer is in the question. Decent plywood backing plates are good for a quarter of a century.

Well - it's been almost a quarter century for them now - the factory just didn't see that epoxy was necessary. To Fast Fred's point, the through hulls in question are the engine seacocks, and are bolted through. I'll bolt them in again of course - after the backing pads are in place:)

I'm fitting the seacocks with additional ball valves, elbows, and pick up strainers for inside the hull. I like the idea of using the raw water pumps as emergency bilge pumps in the case of a hull rupture. Kind of cost prohibitive unless you compare it to the cost of carrying an independently powered high capacity pump along just in case.

Thanks for all the replies -.b

[PAR]

You have to use backing plates to spread out "point loading" when the gland nut on the fitting is torqued down. Plywood works fine for this, as does aluminum and stainless steel, though plywood has a few advantages over the metals. It's easy to cut, can flex repeatedly without fatiguing, bonds well and is light weight. Don't use HDPE or other high density plastic, they don't "goo" well at all.

I wouldn't use Dolfinite as a bedding on 'glass boat through hulls, but rather an adhesive/sealant, such as 3M 4000, 4200 or 5200 or my preference with underwater on wood joints polysulfide (3M 101).

[Ophion]

I am replacing a sea cock from ¾ to 1_1/4 and need ideas for sealant through the hull I am using a grocko style Bv 3 bolt style flange
Also should I re use the backing board or replace it boat is a 1964 do I use same sealant if I replace backing board ?

[sabahcat]

Glassed in fiberglass tubes with threaded end for seacocks is my choice

[FAST FRED]

Drill the hole for the thru- hull slightly oversized and seal the cut edges with EPOXY .

Mount the seacock to a ply pad , 5200 is fine on the pad and thru bolts (3/8 in diameter) minimum , bronze.

Screw the seacock into the thru hull from outside , to trim it to proper length , which is most of the threads in the seacock , but not all.

Then goop it with your favorite EZ to REMOVE goop and screw it in and tighten lightly with the proper tool.

The tool will be needed to break the goop bond when you need to inspect it.

Good Seacocks usually have drain plugs , which is std pipe thread.

Not a bad idea to install a temp grease fitting and give a shot of waterproof grease before moving the handle , if its been a while.

FF

[Ophion]

thank you fast fred do you mean 5200 is the same as epoxy
also would it be wise to coat bolts with it?

[PAR]

3M 5200 is a polyurethane adhesive/sealant and not even remotely similar to epoxy in physical properties.

3M 5200's best use is under parts that will not be removed very often, say every decade or longer. It is highly adhesive for a sealant, but nothing like epoxy. 5200 is stiffer then all the common sealants (less flexible) but shouldn't be used as a structural glue, it's a tenaciously sticky sealant not a glue.

Backing plates are a necessary evil, but relatively easy to make from several materials.

Plywood is a good choice, because it's cheap, easy to machine and bonds well.

Metals can be used, though machining and costs rise sharply.

Pre-manufactured 'glass plates, like G-10 is a good alternative, but again costs will rise. It's a lot easier to machine then metals and bonds exceptional well.

Bonding the fasteners (drilling over size holes, back filling with epoxy, then drilling fastener holes) can insure potential leaks or moisture ingress can't harm laminates, wood or core materials. This technique also dramatically increases compressive strength, which is very handy on cored structures.

[Ophion]

fast fred would you not recomend stainless hardware to mount seacock i do agree 3/8 will be stout enough?

[FAST FRED]

"would you not recomend stainless hardware to mount seacock i do agree 3/8 will be stout enough"

NO NEVER !!!

Most SS is poor underwater , its far better as a stanchion or deck fitting.

USE a good grade of marine bronze , look at Jamestown Supply as a source of type and pricing , then match locally IF you can.

FF