Reducing hole size for thru hull

[s/v Jedi]

1708 for closing a thru-hull hole? Remember that for a straight epoxy laminate, layers of cloth is stronger than anything else. It may be impractical, too thin per layer etc. but for the weight of glass added, nothing beats it. For a small repair like this it is no problem to add a couple layers extra. The repair will be so strong that you can't even knock it out with a big hammer if you try.

[ramblinrod]

Originally Posted by Cadence

Just my humble opinion. The use of mat as an intermediate layer is essential, even if light mat since it has no real tensile strength but makes the bond better . Roving or cloth for a small repair, cloth would probably fair easier. Roving for large. I guess they make things like triaxle products today and I cannot comment on them.

I'm wondering about buying such small quantities

Based on my understanding, the resin is solely responsible for adhesion (bond). The purpose of the material (cloth, mat, roving) is to add tensile strength, while
reduce weight, increase flexibility without fracturing. Mat, while having little tensile strength in free air, increases tensile strength of the composite by about 5 times.

The issue with cloth and roving, is that it increases tensile strength in only two directions, along the bias and fill, whereas mat is omni directional. (When mat is not used, it is good practice to rotate alternating layers of cloth or roving by 45 degrees.

Tensile strength is everything when it comes to cored composites, so if mat contributed no tensile strength, it just couldn't be used.

[s/v Jedi]

Originally Posted by ramblinrod

Based on my understanding, the resin is solely responsible for adhesion (bond). The purpose of the material (cloth, mat, roving) is to add tensile strength, while
reduce weight, increase flexibility without fracturing. Mat, while having little tensile strength in free air, increases tensile strength of the composite by about 5 times.

The issue with cloth and roving, is that it increases tensile strength in only two directions, along the bias and fill, whereas mat is omni directional. (When mat is not used, it is good practice to rotate alternating layers of cloth or roving by 45 degrees.

Tensile strength is everything when it comes to cored composites, so if mat contributed no tensile strength, it just couldn't be used.

of-course mat contributes to strength... the point I tried to make is that cloth contributes much more. We're talking about a 2" diameter hole, not a hull. If you cut the circles from cloth, of-course you can rotate them around a bit (I do random) to get "diagonal" direction fibers but for such a small repair, the 2" patch done with cloth and epoxy is so much stronger than the rest of the hull that it's funny to even discuss fiber direction

[ramblinrod]

Originally Posted by s/v Jedi

With epoxy the cloth on cloth layup is the strongest one can make. Fiberglass mat, even when you get the type compatible with epoxy, is only good for preventing print-through.

Also, there is no question that epoxy bonds stronger, much stronger, to a cured polyester laminate than polyester. That doesn't mean that a repair with polyester fails... only that it would fail sooner than when epoxy was used. I would use polyester for very large area repairs so as to keep things equal (no hard spot) in the hull. For small repairs like this one, the hard spot is already there and has been since cutting the hole.

Done properly, a 2" diameter hole in 1/2" fibreglass results in a 14" diameter fibreglass repair area. In 3/4" fibreglass, 20" diameter.

For a reliable repair (as strong as the original composite) the bonding agent needs to be at least as strong as the materials bonded.

Polyester resin is as strong as polyester resin.

The "average" epoxy has about 20% greater adhesion capability than the "average" unwaxed polyester resin.

For bonding materials stronger than polyester resin FRP, this additional strength may be of value.

However, when polyester resin based FRP is used properly for repair, the bond is as strong as the materials bonded. Any additional adhesion strength is worthless. Whether using polyester or epoxy resin, at that point of bond stress, a failure occurs.

The only advantage to repairing with epoxy, is that the adhesion surface area could be 20% less, to have the same adhesion strength as the same repair with unwaxed polyester resin. Big deal. Mean while, epoxy resin costs 4-5 times as much, needs various hardeners for different temps / cure times, risks catching fire when curing, and risks the user developing an allergic reaction of contact dermatitis.

There is absolutely no good reason to perform this kind of repair on polyester resin FRP with epoxy (regardless of any epoxy manufacturer's propaganda).

[rwidman]

Put a thru hull fitting in the existing hole, cap it off and drill a new hole and you're done in a couple hours. Pretty simple.

[ramblinrod]

Originally Posted by rwidman

Put a thru hull fitting in the existing hole, cap it off and drill a new hole and you're done in a couple hours. Pretty simple.

It then one ends up with a potentially catastrophic failure node for no reason other than minimal cost or laziness.

[ramblinrod]

Originally Posted by ramblinrod

It then one ends up with a potentially catastrophic failure node for no reason other than minimal cost or laziness.

But then...

[rwidman]

Originally Posted by ramblinrod

It then one ends up with a potentially catastrophic failure node for no reason other than minimal cost or laziness.

Nope, not at all. Where is the "potentially catastrophic failure node" ?

Answer is; There is none.

[Cadence]

Originally Posted by ramblinrod

Based on my understanding, the resin is solely responsible for adhesion (bond). The purpose of the material (cloth, mat, roving) is to add tensile strength, while
reduce weight, increase flexibility without fracturing. Mat, while having little tensile strength in free air, increases tensile strength of the composite by about 5 times.

The issue with cloth and roving, is that it increases tensile strength in only two directions, along the bias and fill, whereas mat is omni directional. (When mat is not used, it is good practice to rotate alternating layers of cloth or roving by 45 degrees.

Tensile strength is everything when it comes to cored composites, so if mat contributed no tensile strength, it just couldn't be used.

I brought up tensile strength. It probably is not that important for a small patch job. I myself would build up a sizable area inboard over the hole. Wouldn't even need to be pretty with roving and mat. CSM could probably used in the hole.

[ramblinrod]

Originally Posted by rwidman

Nope, not at all. Where is the "potentially catastrophic failure node" ?

Answer is; There is none.

Every thruhull fitting is a potentially catastrophic failure node. If the fitting fails for any reason, the vessel could be lost.

I can imagine a goal to reduce unnecessary thruhull fittings, but never one to needlessly increase.

[ramblinrod]

Originally Posted by pcmm

For repair work 1708 biaxial is the only way to go. It has epoxy compatible mat and stitched unidirectional fibers. much stronger than woven roving and produces a much smoother finish that requires less fairing work.

I prefer to use individual products so that I can select the best combination for the application. Ie, I determine the thickness of the lay-up required, and then choose the layers of 18 oz roving, 6 oz cloth and 1.5 oz mat to make the required thickness.

[ramblinrod]

Originally Posted by Cadence

I brought up tensile strength. It probably is not that important for a small patch job. I myself would build up a sizable area inboard over the hole. Wouldn't even need to be pretty with roving and mat. CSM could probably used in the hole.

When filling a 2" diameter thru-hull hole, after properly feathering the edge of the hole, the repair area is not 2", it is 2" + 24 times the hull thickness.

As previously mentioned, a properly feathered 2" hole in 1/2" fibreglass is 14" in diameter, prior to repair layup.

The feathering must be done to increase the surface area of the repair secondary bond, to maintain vessel hull integrity in that location.

It is absolutely essential that the repair be about the same strength (neither significantly more or less strong and stiff) as the original lay-up before the thru-hull hole was drilled.

The potential consequence of repair failure (total vessel loss) is just too great to cut corners on repairs below the waterline.

[Cadence]

Originally Posted by ramblinrod

When filling a 2" diameter thru-hull hole, after properly feathering the edge of the hole, the repair area is not 2", it is 2" + 24 times the hull thickness.

As previously mentioned, a properly feathered 2" hole in 1/2" fibreglass is 14" in diameter, prior to repair layup.

The feathering must be done to increase the surface area of the repair secondary bond, to maintain vessel hull integrity in that location.

It is absolutely essential that the repair be about the same strength (neither significantly more or less strong and stiff) as the original lay-up before the thru-hull hole was drilled.

The potential consequence of repair failure (total vessel loss) is just too great to cut corners on repairs below the waterline.

He is only trying to fill a very small gap on each side of the new transducer. Achieving original hull strength and drilling it out?

[ramblinrod]

Originally Posted by Cadence

He is only trying to fill a very small gap on each side of the new transducer. Achieving original hull strength and drilling it out?

Yes!

Absolutely!

Correct!

Any hull modification should not be made significantly weaker (nor stronger) than original design.

It makes absolutely no difference if the spot will be re-drilled and a smaller diameter transducer be inserted than if the original hole was just being filled.

The hull around the new hole, has to be as strong as if there was never a hole there before.

Would filling the gap with silicone sealant be enough?

No! Of course not!

Would filling the gap with Bondo be enough?

No! Of course not!

Would filling the gap with a 2" diameter cylindrical FRP plug be enough?

No! Of course not!

This would not be nearly as strong as the original hull.

The strength of a cylindrical plug adhesion is based on a secondary bond surface area = diameter x pi x hull thickness.

For a 2" diameter hole in 1/2" FRP, this is a secondary bonding surface area of only 3.14 sq. in.

Whereas when the proper feathering technique is used, the secondary bonding surface areas is (pi * 7"^2) - (pi x 1"^2) = 150 sq. in.

(Actually a bit more due to the angle, but whose counting? ;-).

Big difference kids; HUGE!

If one doesn't feather out the FRP repair area, the surface area of the repair secondary bond to the original FRP is too small and the hull integrity is compromised. One sharp whack or serious hull flex and the vessel sinks.

This is exactly why I do not recommend DIY FRP repair for critical structures, especially below the waterline.

If one hasn't built the boat from the keel up, they are probably not expert enough, and even some of those who have aren't, if they just followed instructions without knowing why they were doing what they were doing, every step of the way.

[s/v Jedi]

Videos have been posted, West System has a very good Boat Repair guide that shows this exact job in detail as well. A simple Google finds it. If one wants to use polyester (who still uses that, they all switched to vinylester years ago) then do as the polypeople say and get the mats out :-)