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

[stormalong]

This discussion has been repeated many times on this forum. Here we go again.

A 12:1 patch is for structural repairs. It is not needed to seal a hole. A hole is not structural, it has very little stress on it.

This is what I have done a few times on my boat and some of them are over a decade old with no problems.

Use epoxy for all of the glass work. Each iteration should be allowed to harden up but not full cure - about two hours for standard cure epoxy.

First grind a bevel on the outside of the hole. 45 degrees is enough 30 degrees would be fine. No 12:1 ratio needed.

Then put a patch on the inside, about an inch wider than the hole. Light grind the perimeter of the hole to get a clean surface. One layer of mat and one of cloth. Let the patch set up.

Next lay up alternating layers of mat and cloth on the outside, about four mat and four cloth per iteration using slightly larger pieces bonding to the inside patch and the beveled edge.

Sand back around the perimeter of the patch so each successive set of patches bonds to the old fiberglass.

Keep building up layers until the patch if proud of the hole. Let it cure at least overnight.

Grind to shape of the hull.

Seal with either more epoxy or better yet Interprotect 2000. Find someone in the yard using the stuff and get a small quantity from them.

Then prep and bottom paint as usual.

I have done several patches below and above the waterline this way and you could not find them from the outside.

[s/v Jedi]

How thick is the hull and is it solid or does it have a core? If a core what are the thicknesses of outer and inner skins as well as the core material?

[Spot]

Yeah, I cannot see anyone single 12:1 tapering a 26" diameter crater for a 2" hole in a 1" thick solid hull...I could also see how a thinner hull would benefit from a more uniform tapered repair so as not to gain a stress-raising 'hard spot'.

For a cored hull, the tapers would be for each skin, not the cored thickness, yes?

[fivecapes]

Originally Posted by Spot

For a cored hull, the tapers would be for each skin, not the cored thickness, yes?

Correct

https://www.westsystem.com/wp-conten...anual-2015.pdf

page 38, section 5.4.1

[Fore and Aft]

Stormalong I like that set up. Maybe a slight improvement would be glassing the two inside layers with polyester resin so that way you can use flow coat on the inside as a finishing topcoat? The sweet thing about poly is it sets fast so you can get on with glassing the outside quicker with epoxy.
I have surveyed a yacht where they had removed a number of skin fittings, ground the inside but then forgot to fiberglass it. Just fibre glassing from the outside had worked well and you couldn't even see the repair.
Cheers

[SVTatia]

And then there is the question on how to laminate the hole, let me throw some gas into the fire:

Whats right and why?
Start with a large diameter piece and layer progressively smaller ones or start with a small piece and progressively increase to large diameters?

[Bowdrie]

Originally Posted by SVTatia

And then there is the question on how to laminate the hole, let me throw some gas into the fire:

Whats right and why?
Start with a large diameter piece and layer progressively smaller ones or start with a small piece and progressively increase to large diameters?

One picks their poison; it comes down to fairing-out the surface.
In the repair work I've done I've found in easier on the outside surfaces, (convex,) to start big and work small.
With inside, (concave,) starting small, working big.
As always, YMMV.

[Fore and Aft]

SVTatia there is bound to be someone reading this wondering what all the fuss is about. Two layers of polyester choppy glass, a bit of builders bog to save the extra layers of glass, two more layers of polyester choppy glass a bit more builders bog to fair it slap on a coat of Interprotect, antifoul and then write out the invoice for the client. With plenty of catalyst you can get the whole thing done ASAP.
Cheers

[stormalong]

Originally Posted by Fore and Aft

Stormalong I like that set up. Maybe a slight improvement would be glassing the two inside layers with polyester resin so that way you can use flow coat on the inside as a finishing topcoat? The sweet thing about poly is it sets fast so you can get on with glassing the outside quicker with epoxy.
I have surveyed a yacht where they had removed a number of skin fittings, ground the inside but then forgot to fiberglass it. Just fibre glassing from the outside had worked well and you couldn't even see the repair.
Cheers

Thanks for the comments Fore and Aft.

Everything I have seen suggests that for a secondary bond epoxy is best. I would not mix polyester and epoxy on the same fix. If something like what you propose was done I would thoroughly sand the surface of the one fully cured resin to remove any amine blush before changing to a different type resin. I don't wait for a full cure of the epoxy so amine blush does not have time to form. I suggested eight layers of alternating laminates because my experience tells me that this is the number of layers I can apply that will stay put upside down without sliding off and the endothermic heat is not too much. Yes, epoxy takes longer to cure. I consider that mostly an advantage. I can futz with the layup longer to get it right. It does slow down the job but I go do something else while waiting. No need to stare at it.

I don't particularly care that the inside of the hull has a visible patch. It is in a place that is not usually seen. I can use that as a way to identify where the patch is for future inspection. That first patch is simply the base to apply the remainder of the laminates.


Interesting remarks from Bowdre also.

I like progressively larger laminates because it is easier to clean up the edges of the original fiberglass before starting the next iteration. I have been taught to progressively overlap from small to large. The (minimal) water pressure will be pressing the patch towards the hull instead of away from it. I doubt it makes much difference just easier to do. In reality the strength on the initial inside patch is enough to hold back the water and then some.

If the patch is three feet down from the waterline (most are no where near that deep) the static water pressure is a mere 1.5 PSI (33 feet of water is equivalent to one atmosphere which is about 15 PSI).

As I said in my earlier post this is the method I have used several times over quite a few years now and the results are far stronger than it needs to be. I have never had to redo any of these.

I am certain that there are other solutions work just as well. I am just sharing what I have done that I know works.

[Spot]

Originally Posted by Fore and Aft

SVTatia there is bound to be someone reading this wondering what all the fuss is about. Two layers of polyester choppy glass, a bit of builders bog to save the extra layers of glass, two more layers of polyester choppy glass a bit more builders bog to fair it slap on a coat of Interprotect, antifoul and then write out the invoice for the client. With plenty of catalyst you can get the whole thing done ASAP.
Cheers

Almost spat out my coffee and still trying to Google 'choppy glass'...

Great summary of bad practice F&A!

[Spot]

Big piece first:

[Manateeman]

Hole in boat not structural. Well until hydraulic trailer pushes an arm against the hull right next to the repair.
There is always a reason to do the job correctly.
Mark

[mitiempo]

Originally Posted by 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.

12 to 1 refers to the hull thickness. If the hull is 1/2" thick the taper ground out should be 6" outside the hole.

I wouldn't grind that large a taper though - as posted above half that is ok. I would use epoxy and 1708 biax which is strong and builds thickness quickly. Large circle first then progressively smaller - this way any grinding to fair it will not go through the largest layer of biax.

[Bowdrie]

Originally Posted by mitiempo

12 to 1 refers to the hull thickness. If the hull is 1/2" thick the taper ground out should be 6" outside the hole.

I wouldn't grind that large a taper though - as posted above half that is ok. I would use epoxy and 1708 biax which is strong and builds thickness quickly. Large circle first then progressively smaller - this way any grinding to fair it will not go through the largest layer of biax.

You're right, I had a senior moment and thought the OP was equating the diameter of the hole with a ratio, rather than thickness.

[Cheechako]

It's a general standard really. A lot of things make a difference like you mention.
SO if your hull is .75" thick there, you are doing a 9" diameter grind. If it's a 1.5" dia throughhull, you have about 3.75 to grind all around. That sounds good. I'd want to grind inside also, at least some and layup inside and out. But maybe it doesn't add much really.

If your hull was 3/8" thick, I'd want about the same actually and inside and out.