Countersunk thruhull nut?

[Buchido]

Hi,

So I needed to replace a thruhull on our boat and when I took it off I was surprised to find that the backing nut was counter-sunk into the fiberglass. I hadn't seen this before as how would you access it except with a long specialty socket to tighten.

Anyway, the new thrull nut is a larger diameter also, which causes some problems as there will be a large void underneath and the seating area will be small.

So in order to fix this I was thinking of making a plastic ring to act as a spacer and fill the void and provide the backing for the seating of the thruhull nut (see attached picture). I would probably add some 5200 to the spacer ring to fasten it down to the hull.

Does anyone have any other ideas, opinions? I though about filling with epoxy but I think that would just be more messy and time consuming and not necessary any stronger/better.

Thanks in advance for any advice.

Steve

[Buchido]

Just thinking to myself, maybe a better option would be to take a hole saw and enlarge the countersink for the thruhull nut. But then again how would you hold the nut in place to tighten it?

[UNCIVILIZED]

For the areas in red, & any other voids in the region, I'd fill them with a thickened structural epoxy. Probably some bog consisting of epoxy, & milled (fiberglass) fibers. Followed by bonding a G-10 or FRP backing block to the hull, & then installing a proper seacock. As what's drawn there is just a through hull mushroom fitting, to which a lot of folks & even builders, simply attach & valve, plus a hose barb.
Do a search on seacocks & you'll get lots of info, & also visit Maine Sail's page.

FYI a plastic ring inserted as drawn is truly quite weak. Though such an installation is better than a through hull with a countersunk nut. Which was utter crap, kinda' dangerously so.

[hellosailor]

Buchido-
You might want to bite the bullet and do a proper repair and installation. You take out the old fittings and with an angle grinder you feather out the fiberglass all around the hole. Then you lay in multiple layers of overlapping fiberglass to fill it completely, just as if you were patching a hole in the hole. (Instructions and free advice on the West System / Gougeon Bros. web site.) Now you have restored the proper structural integrity of the hull, and you can simply drill and install any conventional fitting, with confidence that it will not break out because there was some kludge job on the hull.

Yes, you can kludge it. Fill with MarineTex, add a big backing plate, maybe everything is fine for another 40 years. But...sometimes it pays in the long term to do it right. The next survey, or buyer, might see a kludge job and just quietly walk away.

[topmast]

Compared to trying to lift the sunken hull with airbags, fixing it right (#4 ) is so easy.

[UNCIVILIZED]

Originally Posted by hellosailor

Buchido-
You might want to bite the bullet and do a proper repair and installation. You take out the old fittings and with an angle grinder you feather out the fiberglass all around the hole. Then you lay in multiple layers of overlapping fiberglass to fill it completely, just as if you were patching a hole in the hole. (Instructions and free advice on the West System / Gougeon Bros. web site.) Now you have restored the proper structural integrity of the hull, and you can simply drill and install any conventional fitting, with confidence that it will not break out because there was some kludge job on the hull.

Yes, you can kludge it. Fill with MarineTex, add a big backing plate, maybe everything is fine for another 40 years. But...sometimes it pays in the long term to do it right. The next survey, or buyer, might see a kludge job and just quietly walk away.

Technically speaking the above may be correct, if done in conjunction with a backing plate & true seacock, etc.. Though it may not necessarily be wise.

See, here's the thing. The hull's cored, right? Type unspecified. But bog as described will have 10x (or more) the properties of any core who's place it's taking. Then when you bond on a 1/2" thick (or better) G-10 "backing block" to the hull, on top of this. Which the G-10 by itself has several times the physical properties of the entire hull laminate. Then where's the hazard or overall strength loss?
There are types of G-10 out there who's strength numbers surpass aluminum plate of the same thickness.

I'm all for doing things correctly. However in order to fully, & properly fill that hole in the described manner, you have to grind 12:1 bevel for scarfing in the patch. Who's circumference will, I guarantee you, be much, much larger than any backing plate (hull reinforcement) which you may or may not later put on.

So then because that large patched area is a secondary bond, it likely constitutes a much bigger hazard than does my initial recommendation.
Since if you want to use solid glass to fill in the area as you're describing, & the hull (including the core) is only 1" thick there. Then you'll have a patch that's over 24" in diameter, non? To which you should then bond a G-10 backing plate, drill the hull & backer for the mushroom barb. And install it in conjunction with a proper seacock.

Albeit I'm uncertain where the thinking of fitting a solid glass patch to a cored hull is coming from ???


PS: How much interior furniture do you figure will have to come out so that you can grind for a 2' diameter patch? And will any stringers, frames, or ring frames have to be cut & or rebuilt due to this? Or maybe an engine bed?

[topmast]

If I were doing this , the first thing is to cut back the core and build up solid area to full thickness. Include a backing plate and drill hole for fitting. Goop good.

[thinwater]

I think the OP is talking about a hull liner, not a cored hull. Need clarification because the fix is different.

But yeah, not a place for HA repairs.

[Jim Cate]

thinwater is on the right track IMO. Look at the drawing again: This is a bennie, so no cored hull, rather he shows the hull, some "filler" which is likely the plexus glue and then the liner, supposedly stuck to the hull with the Plexus. I suspect that the "countersink" he reports is simply a clearance hole in the liner, not actually into the FRP hull material.

If this is so, then the issue of grinding a scarf out 12 inches is not feasible, nor is it necessary, for the hull has not been compromised. I'll leave it to those better able to suggest a proper fix, but IMO all the draconian measures suggested don't fit the realities of this boat.

Jim

PS I bet that the skin fitting was put in place before the liner was glued in, thus allowing a spanner to be used on the nut. What a wonderful way to build a boat (not).

[Buchido]

Thanks so much for all the feedback!

In order to better describe it, it is not a cored hull just a liner which I believed is glued to the fiberglass hull. As Jim suggested, they must have installed the thru-hull before gluing in the skin. However, this construction method makes for a tricky replacement!

I don't think re-glassing is the best option. 1) as it would be bonded to a secondary liner/skin (non-strutural) and 2) Unfortunately, the through-hull is also in a very tight spot under the head sink, which would require "remodeling" of the whole head. Maybe the wife wouldn't mind this! but we only have 2 weeks to get back into the water.

I originally wanted to install a backing plate and proper seacock as everyone correctly suggested; however, once I saw the original method I had second thoughts about installing a backing plate on the liner. Even if you could fill the void where the original nut was, the backing plate would still be attached to a "loose" liner and may not be strong enough to handle the compression and weaken over time.

That is why I decided to go back to the nut concept and just put a spacer for the nut to bite down on and give me access to attach a spanner to tighten it.

However, now I'm thinking the best way is to cut the skin off down to the hull and then install a backing plate directly to the hull.

As the liner doesn't add to the structural integrity, I don't see why cutting a 5" to 6" hole in the liner would matter. And then I can attach the backing plate directly to the hull and I don't have to fiddle around with re-glassing or trying to fill voids with epoxy. Seems the simplest and the most robust option. Comments?

Also, if I get a G10 sheet for the backing plate what is the best way to attach it to the hull? Epoxy, 5200 or something else?

Thanks again,

Steve

[UNCIVILIZED]

Steve,
Some liners do contribute to a hull’s strength, & stiffness. Remarkably so.And many liners/grids are designed with just this purpose in mind.So use caution with regards to this, & about cutting holes into a liner.

As to backing plate installation, it’s possible to fit some pretty large ones despite small access holes. There are some tips on this on the WEST System site, & in their user manuals. Along with plenty that aren't.

[Buchido]

Alright, so cutting off the liner might not be the best idea. Although the backing plate (thicker than the liner) would help to put back some strength/stiffness to the hull if that was the case.

Also, I think the glue used to attach the liner would be difficult/impossible to remove. But maybe all the unevenness would provide a good surface for the epoxy to grab onto? Although I don't know what the material of the glue is and would be concerned about incompatibility?

So I think Uncivilized's original concept is the current option leader (i.e. fill in void with structural epoxy and bond a backing plate to the liner with a proper seacock). This has got to be way more robust than the original installation, which btw lasted ~30 years without failing.

Thanks again,

Steve

[UNCIVILIZED]

FWIW, there are ways of structurally bonding the liner & hull together. Particularly in key regions such as in the vicinity of a through hull. So that there would then be no chance of any shearing loads being applied to the through hull as the boat flexes, causing the liner & hull to move slightly in relation to one another. Thus having this movement force the liner against some part of the through hull.