Advice needed: closing old thru hulls, how to measure diameter of fiberglass material

[bacchant]

Hi sailors, I am a bit confused and in need of some advice..


I have several old thru hulls in my sailboat that I want to close.. When looking around online it seems common to make a "12:1 ratio bevel" around the hole. To me it seems like one measures the thickness of the hull through the hole and then makes a circle around the hole to be closed with a diameter 12 times this.. But in my head this is not very logical.. The diameter of the patch should take into account BOTH the thickness of the hull AND the diameter of the hole to be closed, no?! Am I misunderstanding something here...? Hope someone with experience can assists with some newbie advice!

[tkeithlu]

Two approaches, both of which mean getting down to the bare fiberglass first.

First approach is to bevel around the hole widely, and then fill with layers of fiberglass cloth. The second is make a plug to fit in thehole, the same thickness as the hull, and layer fiberglass on the inside as a patch. Fill the crack around the plug for a smooth surface and extra strength.

I prefer the second approach. It allows for even flat head bolts around the periphery of the large patch if needed, making the whole thing stronger than the original. The bevel patch on the outside does have the advantage of being on the side that the water pressure comes from, but is a bigger area to fair into the hull shape.

[tkeithlu]

Reading over your question again. Instead of trying to determine the exact meaning of the instructions you have found, consider it as guidance to apply to your unique situation. The 12:1 means basically that your patch needs a lot of area in order to grip the hull. You will forever after have a gap there where no fiberglass fibers cross. It's glued on, unless you use through bolts. That requires a large (and rough) mating surface. Don't skimp on it. It's your boat, so make a repair that you trust and are proud of, not simply having followed a rule.

[bacchant]

Thanks for the input. First I try to understand the recommended best practice including the rationale and logic behind it. Then I will trust the foundation of my knowledge is solid and that I am capable of completing a job that I can trust while crossing any ocean

[Bowdrie]

12 to 1?
If it was a 1-1/2" thru-hull you'd be looking at an 18" crater in the hull, and then trying to fair that out will drive you insane.
Any "interruption" of glass fibers would be far greater than if you use a plug, not to mention a huge amount of work, and an amateur would probably never get the fairness back that was original.
As was mentioned, using a plug with inside backing is preferable.
Set the plug ~1/8" below hull surface with epoxy, after it kicks then you grind a small bevel around the plug and patch that area.
Add some internal work, call it good, move-on to more important issues like sanding varnish whilst drinking beer.
12:1 is the standard ratio for joining spruce staves for building a wooden mast.

[psk125]

Tkeithlu is right that there is more than one way to skin a cat. The first method appeals to me more. It may entail more work with the beveling out the hole and then re-smoothing the fiberglass afterwards. That is not really necessary in the second approach, since the repair isn't going to be visible. (The seacocks were hidden behind joinery before you removed them, no?) Getting in to make the repair from the inside might be trickier though in the cramped inside space. The beveled join also makes sure you are bonding with clean, uncontaminated fiberglass. The old seacocks were probably somewhere in or near the bilge, and oily bilge water could have coated the fiberglass around them at some point, putting a solid bond in question. Either way, a solvent wash before layup would be recommended.

[Spot]

My understanding is thickness times 12, not hole diameter times 12...
So for an example of 1/8" thick hull, 1.5" hole, 12*.125=1.5" and add 2x that to original diameter and you have a 4.5" crater instead of 18".

For those inch challenged, 3.2mm thick x 38mm hole @ 12:1=114mm crater

[vpbarkley]

This is all you need. It's not like your boat is going to fall apart now that you're not using a through hull.

https://www.marineeast.com/a_sch/sch...d=05&pid=05_04

[Squanderbucks]

This is not rocket science. For a two inch hole I would grind a bevel in both sides so it was about 4-5” in diameter.
Put some metal aluminum air conditioning tape over the outside of the hull covering the hole. Press it in flush with the bevel.
Fiberglass the inside and let it cure.
Remove tape.
Fiberglass the outside and grind flush with just a slight depression that will allow fairing with epoxy fairing compd.
Seal with something like e2000 epoxy primer and paint with whatever you want for the bottom.

I like my bevels to be slightly concave as opposed to straight bevel.

[Manateeman]

Plastic? Why not use a few wine corks tied together and covered with duck tape.
There has been so much written on the subject of a proper repair...we can only conclude the owner in question has a serious allergy to epoxy or his search engine is derailed or unavailable.
We defer to the guy who holds his boat together with magic duck tape or the pillow guy. This is a joke thread...?
Seriously. Are we going to comment on fast setting cement?
We are TRYING to be nice, but all we ask is that the original poster will help by providing a bit of information as to why the published answers are not satisfactory, or do not apply to his boat.
I think a lot of ...let’s say...real boatbuilders are here to help posters with complex problems. It’s exhausting to answer the same question time and time again.
Marine How To had an excellent answer. Perhaps the forum should have something like that built in to direct new owners to tutorials on subjects.
Lastly, our objection to posting numbers vs definitive expertise.
One gets stars, colors...whatever...based on the sheer number of posts NOT on credentials. What?
The manatee crew holds a five star Michelin rating for pizza.
Just kidding.
Pizza bazooka distance that is. LOL
Mark

[zstine]

I haven't seen anyone quote the correct taper. If the glass is 1 inch then you need a 12 inch radius or 24inch diameter to get 12:1. or the 1.5in thick example above is not an 18 inch diameter bevel, it is 32 inches in diameter. this is a bit excessive if using epoxy, which I'd recommend since it bonds better and is easier to work with IMO.
you just need to cut circles of fiberglass starting slightly bigger than the penatration hole diam and progressively bigger until you get to full bevel diameter say 20inches or whatever. divide the thickness you need by the thickness of glass. so 1708 for example is .044in thick, if you need 1 inch thickness, you'd have to build up 23 layers... so maybe some 24oz roving would be good to work in there to reduce layers. also that's pretty thick and could get hot if in a hot climate. so depending on how warm the weather is, you may need a slow hardener. if below 60deg, use medium.

[Fore and Aft]

Bacchant there is no way I am doing any serious grinding inside the yacht if I can help it. Usually I will do a light grind around the old hole, just enough that the new fiberglass will adhere well. Then the rest of the grinding is done on the outside. Even before I grind the outside I tape the old hole well so no fiberglass dust enters into the yacht. I usually grind a bevel about 50mm from the hole edge and then start glassing new glass in with slow hardener.
I have never heard of using a fiberglass plug but I don't see why not? You could lay the plugs up in an old plastic yoghurt container etc. Then I would hold them in place with two layers of fiberglass inside and outside of the hull. Just typing this up makes me realise that is probably more work than just laying fiberglass directly onto the hull.
I have fiberglassed countless holes and never had one fail. I have never even thought about ratio bevels.
Dugong lover it seems you and your crew are a little bitey today?
Cheers

[guyrj33]

As many previous posters have mentioned the 12:1 bevel ratio refers to the thickness of the fiberglass being repaired.
Use epoxy it's a lot stronger than polyester resin when making secondary bonds.
The reason for the large ratio is to ensure there is enough bonding area for the patch to have a strength similar to the original layup.
So yes if your hull is 1.5 inches thick you need a bevel that's 18 inches wide and a gazillion layers of glass to make your layup 1.5 inches thick.
Most likely your hull is about 1/2 an inch thick and your 12:1 bebel will need to be about 6 inches wide.
Use epoxy it's a lot stronger than polyester resin when making secondary bonds.
Remember this repair is being made below your water line, you need it to be strong. Think about a stormy night and falling off a wave when you're thinking about how strong it needs to be.
You can reduce the width of your bevel if you repair from both sides. Then the bevel only needs to be wide enough to make the 12:1 ratio for the thickness you're patching from that side.
Use epoxy it's a lot stronger than polyester resin when making secondary bonds.

[guyrj33]

I think plugs held in place with a too thin layer of glass are a really bad idea.
Yes they will hold most of the time. But then that's the problem isn't it.

[mike d.]

Use unwaxed resin,until last layer then switch to waxed ! Taper as required 12 to 1 ! So if you have 1/2 inch hull thickness, you would taper to about 3 inches, on the outside and inside ! I did mine after consulting with surveyor, and he said to go this way ! I used stitch mat ! I went with going small, larger, larger, until I had the thickness needed ! Then went to inside and went opposite of previous !