hole repair, HELP!

[SurryMark]

[quote="
Mark this yard is clearly "in the darK' on this one. ALL bonds to cured polyester resinated laminates are SECONDARY bonds meaning no chemical cross linking has occurred.
..[/quote]

Well, M'Sail, that's very interesting. So this means that if the prep is done right - sanded to bare fiberglass, with no gelcoat - epoxy will stick better to the substrate, and polyester can be put over it, if necessary? Why would polyester be used again? I don't know, except expense, but just about every fiberglass yard I have run into prefers polyester for everything. I won't name the yard I mentioned in my post, but it's one of the more productive in Maine, with a line of fishing and pleasure boats in the 30- to 50-foot range.

The repairs done by the previous owner were so poor that power washing took off slabs of patches. I loved the saucy look of that little boat, but it would have been stupid to trust anything on it. Still, I know that there are lots of sound boats with polyester repairs, so there must be a reasonably right way to do it. Correct?

What, by the way, is considered sufficient bond strength for repairs that will meet the wear and working of a fiberglass hull?

[S/V Necessity]

Epoxy has the superior bond and strength. However polyester has *adequate* strength **if used properly**. It's also less expensive. Some feel it's easier to work with (however I think this is only the case if you are EXPERIENCED with it, as there is more room for error) And probably it's biggest benefit? You can gelcoat over it. Polyester (including gel coat) does NOT like to adhere to epoxy. If you use epoxy, you have to finish with paint.
Most yards are already experienced with polyester resins, and thus prefer to use them.

[Maine Sail]

[quote="
Mark this yard is clearly "in the darK' on this one. ALL bonds to cured polyester resinated laminates are SECONDARY bonds meaning no chemical cross linking has occurred.
..

Well, M'Sail, that's very interesting. So this means that if the prep is done right - sanded to bare fiberglass, with no gelcoat - epoxy will stick better to the substrate, and polyester can be put over it, if necessary? Why would polyester be used again? I don't know, except expense, but just about every fiberglass yard I have run into prefers polyester for everything. I won't name the yard I mentioned in my post, but it's one of the more productive in Maine, with a line of fishing and pleasure boats in the 30- to 50-foot range.

The repairs done by the previous owner were so poor that power washing took off slabs of patches. I loved the saucy look of that little boat, but it would have been stupid to trust anything on it. Still, I know that there are lots of sound boats with polyester repairs, so there must be a reasonably right way to do it. Correct?

What, by the way, is considered sufficient bond strength for repairs that will meet the wear and working of a fiberglass hull?

You wouldn't put polyester over epoxy...

While there are a few yards still using polyester for repairs it is actually quite rare for high load critical repairs. Most builders still use polyester for laminating and building but this is primary chemical bond territory not secondary. I can name about 20+ yards here in Maine alone that use Epoxy for most all repair work. While some still use polyester for "quick" repairs, in non load or structural areas, most any reputable yard will not use polyester for critical load areas when it is a secondary bond.

My yard uses epoxy at about 98% and buys it in 55 gallon drums from West System.

Yards still using polyester for critical repairs are doing so on the cheap and it's usually relegated to work boats.

I have seen many secondary bonds of polyester fail and many of these were done by the builders. If you've spent any time crawling around bilges, as I have with a friend who is a surveyor, one thing you'll see often is failed tabbing. We see it on about 2 out of 10 boats built in the 60's, 70's & 80's.

Many tabbed bulkheads, or other items, were tabbed after the polyester of the hull had fully cured during the build process. Unfortunately after the laminated hull sat for a while during the build process and then the bulkheads are laminated in they become a secondary "glue" bond rather than a primary "chemical" bond and thus a reason why you read about so many tabbing failures. If you ever wondered why you read about so many tabbing failures as opposed to hull failures the differences is a chemical bond and a glue bond. Polyester is not a good glue and in fact is less of a glue than 5200 when use as a secondary bond...

Epoxy is more costly but easier to work with and significantly stronger and also much more water proof than polyester. If I am laminating up a sheet of fiberglass, for say thru-hull backing blocks, I still sometimes use polyester. If I was repairing a hole in the bottom of my hull there is no doubt that I would use epoxy..

[Maine Sail]

And probably it's biggest benefit? You can gelcoat over it. Polyester (including gel coat) does NOT like to adhere to epoxy. If you use epoxy, you have to finish with paint.

This myth has been debunked with laboratory peel strength tests. I personally have repaired and gelcoated over epoxy many, many times and never once had a failure, ever.. If done right there is NO issue going over epoxy.

Remember a gelcoat repair over cured polyester or cured epoxy is a secondary bond in BOTH instances and 100% reliable on the gelcoats adherence as a secondary bonding agent.

Gelcoat sprayed into a mold, when a boat is built, is a primary chemical bond not a secondary bond..

Debunked:
http://www.westsystem.com/ss/polyester-over-epoxy/

[Troy Scott]

I'm glad I got this started. This is good. Thanks everybody!

[Troy Scott]

Folks,

I just discovered Paul Ring's article in the Nov./Dec. '08 Good Old Boat. In the last section, on page 27, he describes his recommended method for filling an unwanted through-hull hole. He suggests using epoxy resin, six ounce cloth and a SIX-TO-ONE taper (scarf). This seems quite reasonable to me.

[Troy Scott]

Folks,

Please take a look at the photos accompanying this article.

http://www.sailnet.com/forums/gear-main ... -hole.html

They refer to a 12 to one taper, but I think there is some confusion in their measuring and/or their thinking. If that hole they are undoing is 2 inches in diameter, then the repair is about 7 inches in diameter. If the hull is 1/2 inch thick at that point, then the entire repair will be 14 times the hull thickness. There will be a less than 6 to 1 taper on each side of the hole. If the hull there is 3/8 of an inch thick, the ratio becomes about 7 to one, which is still WAY less than the 12 to 1 West calls for and the 14 to 1 Don Casey calls for. I'm beginning to think these good folks are referring to the ratio of the diameter of the repair to the thickness of the hull in the area of the repair. Thoughts?

Actually, the repair in the photos looks reasonable, like many boatyard repairs I've observed over the years. If I do say so myself, mine look much better. A 12 to 1 taper all around a 2" hole in a 1/2 inch thick hull would require an amazing FOURTEEN INCH diameter repair. I'm beginning to believe that these numbers people throw around are often-repeated and passed around, but rarely scientifically adhered to in practice. Thoughts?

[Matt Cawthorne]

Troy,
I did not see a link to the article so I can not comment on it directly.

The reason for the taper ratio is to limit the shear stress in the bond. In a highly fatigue loaded component, with low modulus materials (like glass cloth) and some form of bond line thickness control, the 10:1 ratio is a reasonable choice. If you are using carbon plan on doubling the taper ratio. If you are using low quality boat resin perhaps it is better to go larger. For a relatively lightly loaded structure, you can get away with less. If it is holding your coffee cup to the binacle, even less.

Since you are patching a hole in the bottom of your boat, I would not skimp. The beauty here is that if you put half of your patch on the inside and half on the outside of the hull, you have two separate bonds so each patch is 1/2 as thick and the two resultant patches will be about 7 inches in diameter.

If you really want to prove me wrong, however, patch the hole in the bottom of your boat with a 5:1 patch and take the boat far out in the ocean for a while and find a really good storm. If you make it you can report back

[Troy Scott]

Matt, and anyone else who noticed I forgot to put in the link: SORRY. I was in a hurry. I should (and usually do) always proofread.

[Troy Scott]

Matt, and all,

Really, I'm NOT skimping. I'm working very hard to make this boat as good as it can be. I buy the best materials and use good methods based on common sense and many years of experience. I have increased the ratio well beyond what I believe would be plenty. It's "extra work", but I want to have the photos on file to prove I did it "right". If (God forbib) she ever sinks, it won't be because of the way I've repaired these holes.

What I'm trying to get at is: How do most folks REALLY do this......

[S/V Necessity]

I've heard of many many poor repairs done, often by professionals. Simply because others do something one way does not necessarily mean it's correct. I suspect in reality many pros don't even bother to calculate the scarf angle.
One thing to consider in this whole process is "safety factor." I used to do rigging work for a local indoor stadium. Everything was required to have a safety factor of 10. This means that to hang a 500# speaker, I had to use a chain rated to 5000# (and keep in mind the manufacturer of the chain probably rated the chain well below it's actual breaking strength.) Would a chain rated 500# have worked? Probably, but what if it failed? (and eventually there would have been an accident)
To consider an extreme, would a 2:1 scarf work? probably. But what *if* it fails? The a 12:1 or 15:1 ratio may seem excessive, But so is hanging a 500# speaker with a 5000# chain. Just consider the extra angle "safety factor."

[Matt Cawthorne]

Troy,
I do not doubt that you do good work. We can see your intent from your posts. There is no need to defend yourself. In fact, if I ever find myself without a boat, I want to buy yours!

Matt

[Troy Scott]

Necessity,

Thanks for your thoughts. I agree that we should "go overboard" (not literally!) for safety. Please don't take offense while I get a little extreme as I hijack your speaker chain analogy: I think to make the chain analogy really work, you would have to actually remove a substantial part of the indoor stadium in order to accommodate the chain. That part of the stadium would have nothing wrong with it; it would be removed only because of it's proximity to the chain and to provide a connection point for the chain. Of course there should be an optimum size for this chain, and this size would certainly stop well short of being so large and so heavy that the roof would not be able to support the chain, never mind the speaker! It's not inconceivable (considering some government contracts that have been let) that some misguided "expert" might specify a chain so large and so strong that there wouldn't be any room left for the activities for which the stadium was designed.

This is how I felt when I drew on my hull the huge circles representing just how much material I was expected to grind away just to repair a two inch hole. It looked even more ridiculous with the one inch hole! I really felt like I was about to knock a huge hole in a wall so I could fit one new brick. I eventually removed considerably more material than I felt was really appropriate. This was my concession to "conventional wisdom". So now I have four "over-done" repairs to my originally "over-built" Cape Dory sailboat. BTW, these repairs, while obnoxious, were insignificant compared to removing the glassed-in steel engine bed, welding up a new bed (properly protecting the steel this time) and glassing the new bed in place....

Over the years I have built a lot of things with composites and with ordinary FRP. I often make a "sample" as I make a part or a repair, so I will have something I can "test" later to give me some confidence in my work. On rare occasions the result after bending or whacking one of these "test" pieces has led me to go back and re-work something. But when the test piece breaks somewhere other than in the new joint, I feel especially good about it.

[Oswego John]

Hi Troy,

As the Brits might say, "Different strokes for different blokes". For quite some time now, I have hesitated from chiming in on the subject. Tonight I said "What the hey, go for it".

Call me old school if you want, but I will usually only grind the bevel in a hole in fiberglass from one side. The side to be beveled is from the outside. The reason for that goes back many years, whether it is right or wrong in todays thinking.

I don't expect many of the readers to know this. Standard equipment on wooden lifeboats was a package of assorted diameter, wooden or cork, cone shaped plugs and a mallet. They were to be used to plug the holes from the inside when the boat was straffed.

Because of the unique circumstances, it was indicated that it was one of the rare times that a hole was to be plugged from the inside of the hull. Any other time a patch would be applied to the outside and allow water pressure to apply force to the patch to help keep it in place. I guess that's the reason I'm in the habit of having the wide part of the bevel to the outside

Of course with fiberglass, I suppose we're talking about a whole different enchilada so.....

FWIW
O J

[Troy Scott]

John,

All four of these repairs were in areas where tapering/scarfing on the inside were not really good options. One of the holes was behind a liner with access through a small, five inch hole. The others were all near bulkheads which would have severely limited the diameter of a tapered area. In all four areas I was able to get well bonded backing plates installed, but these were simply bonded to the cleaned and roughed-up inside of the hull. I did all of the tapering on the outside.

However, WRT the viability of doing this kind of repair by scarfing in new glass after tapering both inside and outside, I think it would probably work just fine. I believe the major engineering concerns are for the repair to be as strong as the original structure, and bonded thoroughly enough that normal (or even extreme) hull flexing won't cause it to break loose. To me, if this kind of repair is done right, the force that would be required to break the repair loose would be dramatically greater than the force of water pressure on that area of the hull, whether the bevel is inside or outside.