Patching holes

[p.hammar]

Hi
I have five holes after thru-hulls in my GRP boat that need patching. Three are, say, 45mm wide and two are around 16 or 20 (sorry, not by the boat just now). Hull thickness 15-20mm solid GRP (this is all down where it is really thick).
I know the conventional wisdom of tapering the edge of the hole from both sides to a pointy edge, and end up with two concave areas, one in and one out, then building up to surfaces again with resin and glass. Makes good sense.
But grinding away that amount of GRP is untempting, and feels like overkill.
Can I cheat, and only excavate, say, 5 mill each side and then build up to surface? I would still get a taper, and about 150mm diameter patching.
Admittedly, I would end up with a ”puck” or cylinder in the middle, which I would have to fill up with thickened epoxy.
Especially with the 15-20mm holes it seems overkill to grind away to a sharp taper.

Kind greetings
Per

[dcstrng]

I've been known to cheat a bit well above the waterline, but in the hull proper (especially below the waterline), I'd suggest not taking a shortcut. Further, an holes any larger than 1" (25mm), the WEST manual requires a backing of some sort (in section #4 of their old manual) before fitting the repair/patch. Candidly, other than some extra grinding, the glass work for the required taper is not much more than on less...

[Cheechako]

Yeah, I just dont like to taper out several inches myself. Tapering inside and out would be best. Dont fill anything with thickened epoxy, make sure it all has glass in it.

[Phantom Jim]

If you do it, do it to the best that can be done. Repairs under the waterline that fail can have very dire repercussions, particularly chronic leaking and flooding should the patch come loose or completely out.

Your boat is a significant investment, it should be treated as such. If you are going to skimp, do it on something that could not be catastrophic if it failed.

[chris95040]

I’d be inclined to install capped through hulls, bronze or marelon, and move on. Nothing preventing you from glassing it later. And since it sounds like you are feeling half hearted about doing it and looking for shortcuts (believe me, i know the feeling) I’d wager a properly installed and capped through hull is a superior fix to some glass work done half heartedly.

[Cheechako]

Originally Posted by chris95040

I’d be inclined to install capped through hulls, bronze or marelon, and move on. Nothing preventing you from glassing it later. And since it sounds like you are feeling half hearted about doing it and looking for shortcuts (believe me, i know the feeling) I’d wager a properly installed and capped through hull is a superior fix to some glass work done half heartedly.

Yeah, the boat in my avatar I eliminated a few seacocks/holes. By the time I was done with all the work I felt a little bit like the boat may be less safe than having the original seacocks/holes! It was fine, but combined hoses running under the bilge etc into Bronze Tees which created overhanging weight on a large seacock etc.... one wonders.
A repair from inside and out which bonds to itself is not going anywhere though... even if it's not a wide grind pattern. The outer plug would be held in place by pressure even if it wasn't bonded. I just used normal glass and resin like most all repair people do and laid up the inside and outer while wet for a complete bond. It ain't going anywhere.

[C420sailor]

Do it once, do it right. You want a repair that moves with and flexes like the factory glass around it. That ‘puck’ in the middle will separate over time, and it’ll be on the glass sandwiching it to keep it together.

Andy with Boatworks Today on YT has great videos on it.

[Spot]

Would using a 10mm thick piece of G10/FR4 as a tight fitted and epoxied 'plug' be a reasonable start in a 20mm hull ??? Do the big taper and patch job on the outside and then a thinner one on the inside? I take it that inside access might not be 100% conducive to grinding full tapers. Just asking, not an expert...

[Manateeman]

You specifically asked “ can I cheat”...which tells me that you DO know the proper method. How can we advise you as to what degree of risk you should take with your own vessel. I think the little voice inside you knows best.

The objective of tapering is to increase the surface area of the bond between the layers of glass. One is trying to attain most of the strength of the original layup...most, not all... because to do that, would require a very, very, large patch.
So it’s a compromise to begin with.
So now you ask if it’s all right to compromise the compromise.

The ideal patch should try to attain most of the structural strength and flexibility of the original hull layup schedule. This schedule is important and changes in different areas of the hull. You will be in good shape if you can find out exactly how the boat was built but that’s not often the case.
When you taper grind, wet it with acetone and you might see the layers better.
It’s not critical to have the inside taper as large as the exterior. Keeping the glass dry and clean and the taper angle shallow is more important. A big plug is exactly what to avoid. Layer after layer...increasing in size. Don’t “push” the cure time which just makes for a brittle patch.
Does this all take time and a lot of work...yes.
A good patch should sound like the hull and not a dead thunk .Excessive filler and patches with a big center plug have little strength. There are not a lot of shortcuts in boatbuilding. It’s a big job but you’ll never have to worry if it’s done right.
Happy trails to you.
Captain Mark and his manatee crew building a micro brewery in the engine room using excess heat from the turbo to speed fermentation. You think you have problems.

[dcstrng]

Mantateeman -- yep...

[smj]

When doing a patch like this always start with the larger piece of glass and then each individual layer should get smaller. Doing it this way makes it easier to get the air bubbles out but also places the largest more structural layer of glass away from the top of the repair where it may be compromised by the sanding and fairing process. Read the West Systems bible.

[Phantom Jim]

SMJ, I think maybe got the large piece/small piece layup reversed. The inner layer should be the smallest, with each subsequent layer about 1/2" larger than the earlier one. The purpose is so that each additional patch layer covers all the edges of the previous layer. The result is that there will be only one edge to the final repair. My reference book is "This Old Boat" by Don Casey.

[smj]

Originally Posted by Phantom Jim

SMJ, I think maybe got the large piece/small piece layup reversed. The inner layer should be the smallest, with each subsequent layer about 1/2" larger than the earlier one. The purpose is so that each additional patch layer covers all the edges of the previous layer. The result is that there will be only one edge to the final repair. My reference book is "This Old Boat" by Don Casey.

I’m sure there’s more than one way to skin the cat! I was taught larger piece of glass first for the reasons I stated above. This is also the way the Gougeons describe it in their fiberglass repair manual.
Even when tabbing in stringers or supports.

[Phantom Jim]

Maybe it does not matter that much then. The Gougen's are the gurus of epoxy. Maybe my schedule was for fiberglass resin?

[Spot]

My copy of Don Casey's book shows patches going into the tapered opening from smallest to largest. Same plates are in the article here: https://www.boatus.com/expert-advice...iberglass-boat


The largest patch first seems to make sense in that there are longer fibers running through the resin that is in direct contact with the old tapered material.