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

[wsmurdoch]

OP,

I have one hole where you have more. Years ago, I asked the glass person, "Turtle", then at Sailcraft Services in Oriental, NC about repairing it. He said to leave the old fitting in place. The repair would be no stronger than a sound fitting, and I might need the hole for some other service (liquid in or out or for some yet un-invented future greatly desired electronic gizmo). Why bother? Just keep an eye on the fitting, after all, it is easier to inspect than a fiberglass repair.

[Spot]

The math has been suspect on this thread as previously posted by Zstine.
Some of the offerings are tempered from real world experience and I do not discount that wisdom...but the numbers should (!) calculate the same each time for each person applying the same formula which has been stated at 12:1. If you 'get' a different set of numbers then it is by practice a different formula...

-for a 1" thick hull the 12:1 ground radius (not diameter) is ...wait for it...(12*1) or 12 inches. The hypotenuse is 12.04 if you're being picky but who wants to figure out square roots and Pythagorean's theorem while glassing anyways.

-for a 3/4" hull this is 9" (12*.75) and this is added 2 times to the original hole diameter for the overall 'make it this big' diameter.

Above example would be 19.5" diameter ground from one face around a 1.5" starting hole (9+1.5+9) and 10.5" ground from both sides (4.5+1.5+4.5), which is half the ground radius but still add the original hole diameter.

In the post with the 5 CAD pictures I used a 2" hole in 1/2" x 15" square, at 12:1 my ground diameter is (6+2+6) or 14" from one side. It would have been (3+2+3) or 8" from both sides.

Once again, lots of experience on the forum, more than my own with thicker glassing, and well-founded thoughts as to when to 'hold em or fold em' as far as 12:1 goes.

[Fore and Aft]

Spot, choppy glass is chop strand mat that you can tear apart with your hands.
I have seen some fairly wild repairs done during my travels around various boatyards. One of the worst was a guy who had ground out some fairly large areas of osmosis then filled them with builders bog. I spoke to the guy and he wasn't worried about the bog absorbing water as he was going to seal the bottom with epoxy resin!
Cheers

[vpbarkley]

Sorry but the advice given here by most is complete lunacy. A full structural repair is not required for this non structural problem. Mountains out of molehills.

Cap the existing through hull if it is in good shape. If not use one of these:
https://www.marineeast.com/a_sch/sch...d=05&pid=05_04

If you prefer an all metal solution use a bronze through hull. Can't find a NPS cap, buy an NPS (straight thread) tap and in two minutes you have a NPS cap.

[s/v Jedi]

No, don’t just plug the hole or stick something in. At best you create a hard spot which is bad enough not to do it.

Here is an easier method that still does it right, assuming access to both sides:

1 put plastic food wrap on a thin piece of plywood, drill and screw it to the outside of the hole. Now fill the hole with epoxy fairing compound from the inside. Just flush with the laminate.

2 after cure and removing the board and plastic wrap, the grinding starts. Draw the 6 to 1 circle and try to imagine the area as a cupped area with the center as deep as halfway down the laminate. Use a drill or grinder to mark the center to half the thickness so you know where to stop grinding. Grind it out. I like to start with a small angle grinder (Makita 5” is the tool of choice because it’s lightweight) and a so-called stripper disc. The disc quickly wears out the outer edge, creating the perfect domed shape for this. Finish with 80 grit on a small random orbital sander.

3 cut circles from 1708 fiberglass. You can stack them to see how many layers you need: thickness doesn’t change much. Yes I like to start with the biggest one. Test with a flexible batten if another layer is needed but don’t go too thick. End with some epoxy with cabosil, trowel it on and use the flexible stick to form it to the shape of the hull. Then get the thin piece of plywood out, cut a circle somewhat bigger than the patch and drill and screw it down around the patch with new plastic wrap. Let cure.

4 remove wood and plastic wrap and say ooh and ahh. Countersink and fill screwholes with some thickened epoxy, then wash with water and soap, then sand with 80 grit ready for barrier coat and anti fouling.

5 repeat same for inside. The new fiberglass in the center is now the depth gauge. Finish with barrier coat without the antifouling

The trick with filling the hole first makes it very easy to grind and layup and screwing the wood down prevents the need for fairing.

[Fore and Aft]

VPBarkley they look bloody terrible those through hull blanks. Fair enough if next year your going to fit a water maker and need the fitting. But the OP is talking about removing seven old skin fittings. If I was surveying the vessel I would recommend removing all those blanks and doing the job properly. It's not even a hard job and then you are guaranteed a 100% waterproof seal that you never have to think about again. Not to mention a nice smooth locker finish and nothing for the dirt or equipment to catch on.
Cheers

[Rumrace]

Nice work. I do quite a bit of 3D work.

As far as fixing a hole yes. I patched a Donzi 18 bow hole. Rock hit ugly stuff. Since the boat sank water migration was a given. The Epoxy will not have the flex fibreglass had. It will have much more gloss. The key it to connect with each layer on glass so yes a strong patch 1” hole is at least 3” tapper.
I used laminated beer box on the inside to support new Matt. The beer box structure I just noticed Beneteau First 36’ design team using. It’s recycled paper coated in clay. It biodegrades in a year buried or turns to mud with a pressure washer after your glass work it done. If you use anything else for internal support it will be stuck to the Epoxy for 50 years or so. I vacuume bagged the patch. I used a piece of PEEK to replace part of a spar and connect a leading edge gap about 4”. The last coat of Epoxy I added primer gray and a heavy grade micro balloons. Water block sanded it back to the sexy Donzi entry. I like Auto Air out of California. So I mixed a candy red not Donzi Red but close for the stripes and Infinte white. I used the anti ox primer which I’ve used on a corvette. Seems silly but it’s such good primer I’m hooked. Since Donzi has only made subtle changes to the 18 over 40 years it looked like a new special edition.

[s/v Jedi]

Some pictures of 3 holes I recently closed.these were three Frigoboat keel coolers.

This hull is cored so a similar process is repeated on the inside.