Wood Backing Plate for Thru-Hull Soft and Rotten

[neilpride]

Very clear thanks, looks fantastic the tru hull, a real proper set up for a tru hull, trough bolted to a backing plate and with double hose clamps, for people with rotten backing plates this is the answer! Cheers.

[s/v Jedi]

I agree with Maine Sail: use modern backing block materials if you need a backing block; use a poly-sulfide sealant like 101 or lifecaulk (lifeseal is different); and use bolts (or possibly screws if the hull is cored and the inside fiberglass layer is thick&strong). The wall thickness of the thru-hull fitting is not enough for the bond and neither is 5200.

Valves on thru-hull fittings is what people do because they are either clueless or can't afford a seacock or they replace with the same (others made the stupid decision before them).

And again: the marelon valves that can't be turned in the shop are valves, without the flange. They are two parts screwed together with an O-ring and over-tightened. The flanged versions ("seacocks") are 1-part and will turn as easy as new bronze seacocks.

It's amazing how many sailors say they have sea-cocks when they have nothing more than a simple ball valve.

cheers,
Nick.

[Maine Sail]

Originally Posted by neilpride

double hose clamps, for people with rotten backing plates this is the answer! Cheers.

Double AWAB clamps. Yes, I have replaced far to many rotted wood backing plates to ever even consider it again. Actually making the fiberglass backing plates is quite easy. Even epoxy coating wood ones is not always fool proof..

For those who get queasy about through bolting you can also drill and tap a fiberglass backing block.

Tapping:


Bronze Studs:


Flanged Adapter (best boat invention of the decade!):

[S&S]

Originally Posted by Maine Sail

Double AWAB clamps. Yes, I have replaced far to many rotted wood backing plates to ever even consider it again. Actually making the fiberglass backing plates is quite easy. Even epoxy coating wood ones is not always fool proof..

For those who get queasy about through bolting you can also drill and tap a fiberglass backing block.

Nicely done. I agree that a glass fairing block makes sense for a glass hull. For us woodies, a wood one makes more sense as it'll show decay before the planking underneath gets punky. With a wood hull, best is throughbolting with carriage bolts. Next best is bronze screws into the planking.

[Christian Van H]

Originally Posted by Maine Sail

The seacock is also through bolted through a solid fiberglass backing block.

Maine Sail, if you get time, could you describe how you lay up your fiberglass backing blocks? Thanks, Christian

[clockwork orange]

For those who dont want to undertake making their own fiberglass backing plate material you can buy a sheet of G10 available from Mcmaster carr and other places im sure. Personally i question the need for backing plates at all in all boats,the theory behind them is that they spread the load over a larger area of hull,ok that made sense on a carvel planked hull where you were putting this tall thru hull/seacock/tailpiece etc assembly thru an individual plank that is say, 5-6" wide and is not attatched to the plank on each side,yeahyou need a big ass backing plate that extends onto the plank on each side so if someone,thing torques the assembly you are not working the seams. Now if you have a monocoque hull of any of the common materials there are no seams to work so wether or not you NEED a backing plate has more to do with how stiff the hull is,a thin glass hull of a coastal cruiser,thin plating of a metal hull,a cored hull,sure but if you have a 1/2" or thicker solid glass hull a little backing plate like ive seen in photos presented where they are bareley bigger than the seacock flange serve only one purpose and that is a way to attatch the flange bolts without penetrating the skin,you need much larger backing plates to effectivly spread the load,the one shown in the photo presented by dubhouse is too small to do anything.Of course i always use backing plates just to avoid any issues when some surveyor inspects the boat,not because it serves any purpose.
Steve.

[Maine Sail]

Originally Posted by Christian Van H

Maine Sail, if you get time, could you describe how you lay up your fiberglass backing blocks? Thanks, Christian

Usually on a sheet of waxed glass or a waxed Formica counter top piece I have in the barn. I usually make my sheets 24" X 36" as it is easy to lay up. I simply cut alternating layers of mat, roving or 1808 and cloth and mix up a large batch of resin and begin wetting out with rollers. The cloth is usually what I have on hand. The tapped one I think I used a lot of 1808..

Depending upon what I have on hand I will do it with either polyester or epoxy. I have a good distributor in NH for epoxy so I've been using mostly epoxy lately but the ones on my site were done with polyester. I like epoxy because it wets out a lot easier than does polyester..

It does not take much time at all. One side, the glass side, will be shiny and one side, the top side, a little duller...

G-10 was mentioned but there is also GPO-3 which is less money.

Check with a local plastics supplier as they may be able to get you regular fiberglass sheet that does not meet the ISO, fire ratings and other standards that add cost for electrical grade fiberglass sheet stock..

[Christian Van H]

Maine Sail, thanks for the instructions! As I read Clockwork Orange's post above, I had come to the same conclusion; why would you need a backing block on a seacock that has a base (possibly with thru-bolts) to seat against. The only thing I can come up with is this: if the hull under the seacock flange isn't perfectly flat, a backing plate could be formed (fitted) to the hull curvature. Your pre-made plates could be ground to fit. What do you think?

[s/v Jedi]

I once installed a thru-hull fitting and found that the inside surface wasn't parallel to the outside surface. I used a quick and nice method for dealing with that:

After cutting the hole, I wrapped the fitting in plastic wrap, put it in the hull and kept it there with a strut under it, making sure the flange of the fitting meet the hull all around. The inside area around the hole was sanded with 80-grit already. I made a decent batch of epoxy with a 50/50 mix of high density and colloidal silica fillers, to peanut butter consistency. I scooped that around the fitting and covered it with plastic wrap (hole in the middle for the fitting). Next, I screwed the seacock on until it squeezed a bit of epoxy out all around it. I shaped the sides a bit and unscrewed the seacock immediately and left the epoxy and plastic wrap to cure. After that it was just a bit of sanding to make it look pretty (wish I'd had my Fein multimaster then...) and I had a perfect base for the seacock.

cheers.
Nick.

[S&S]

Originally Posted by s/v Jedi

I once installed a thru-hull fitting and found that the inside surface wasn't parallel to the outside surface. I used a quick and nice method for dealing with that:

That's true for every thru hull on our boat. Mahogany plate and a belt sander is how I do it. Yours is a nice process of a glass or metal boat though.

[Christian Van H]

Originally Posted by s/v Jedi

...and I had a perfect base for the seacock.

Smart Nick! Very smart! I've got to try this. I'll add some chopped fibers too...not too much though, I dont want to spoil the perfect fillet!

[clockwork orange]

I replaced the 2 delaminated plywood backing plates on a C&C 24 i have last year and was astounded at the stupidity of the builder,picture this.They used the flush,countersunk type bronze thru hulls and had molded the recess as part of the hull layup,nice so far, however,of course this leaves a matching lump on the inside which doesnt allow for the backing plate to get anywhere near contacting the hull except on the very end of the protrusion,now you would think that since they had quite a few models all with this settup and really only a few thru hull sizes that they would have come up with a standard but custom recessed pad that they could use with all their boats that would go over the protusion and contact the hull properly right,but no ooo,they just stuck on a plywood pad that justsat on top of this lump with probably a 3/8" gap between it and the hull everywhere except the center,a contact area smaller than the nut,and btw white silicone was there goo of choice, actually neither of them were leaking and the plywood was not rotten but were totally delaminated because they tended to use a lot of interior plywood in their boats and these were obviously made from offcuts.I replaced them with 3/4" coosa board installed pretty much as nick outlined but of course with a thick milled fiber mix due to the large gap,i didnt recess the pad over the lump as i would have need a much thicker pad.
Steve.

[clockwork orange]

S&S,on carvel planked boats for both backing plates and replacing butt blocks i laminate up my stock of three layers with rescorcinal glue and of course shape the faying surface to the hull.I took to doing this to get some cross grain strength to prevent splitting as the most common reason i have to replace butt blocks is due to splitting along the grain caused by the screw fasteners,i dont use plywood as i would have to glue 2 layers anyway to get the thickness and i would end up with more cross grain strength than i need and not as much fore and aft as i want due to the many layers in marine ply.
Steve.

[S&S]

Originally Posted by clockwork orange

S&S,on carvel planked boats for both backing plates and replacing butt blocks i laminate up my stock of three layers with rescorcinal glue and of course shape the faying surface to the hull.I took to doing this to get some cross grain strength to prevent splitting as the most common reason i have to replace butt blocks is due to splitting along the grain caused by the screw fasteners,i dont use plywood as i would have to glue 2 layers anyway to get the thickness and i would end up with more cross grain strength than i need and not as much fore and aft as i want due to the many layers in marine ply.
Steve.

Good idea Steve. I haven't had any splitting problems (sound of knocking on wood) yet. 4/4 or 5/4 solid stock has worked well but I make the curvature match to the hull very close and set the grain fore-and-aft. Why the preference for rescorcinol over epoxy? (both are fine, just curious)

[Cheechako]

Ref page 2: Yes, 5200, 101, and butyl are all great.... used in the right places.
Mainsail: Very nice looking blocks.... but you are bolting your seacock just to the block? I guess that's a little better than no bolts... I kind of like it as a compromise.... but realistically, you could end up with a seacock/block broke free from the hull if the threads fail..... just like with no bolts right? I guess you get a little more "bonding area" if you are using 5200..... but with 101 it's probably not as strong as 5200 and no bolts.....