filling holes in an unstayed carbon mast

[marlinspike]

You would need to get inside the mast to actually be able to fix it without breaking the mast which is impossible and then lay a insert.. Those screw holes are minor to the amount of work needed to fix it to an Americas cup version again. How do you laminate the outside without breaking the inital weave.. I think hes right with a tapered stack of carbon and use a glue like spabond or silica with micro fibre and call it a day. If the boat is planning on winning races no one would ever have screwed holes in it. Curse the man that did such a thing

Interestingly long before I owned it, but after the holes were added, this particular boat consistently won so many PHRF races, both offshore and inshore that it caused a huge amount of chaos and headaches for the rating committees- many wanted the rating drastically lowered, and others accused those people of trying to rate skippers instead of boats. The boat is so well known that it’s hard to work on at the dock without a stream of people coming by to talk about its racing history. I very much hope to do a lot of offshore racing in this boat and hope to win, so I want to do the repairs properly. Of course, it’s no Americas cup boat, just an amateur PHRF “plastic classic.”

The holes were added to race with a large asymmetric spinnaker, which turns it into a planing boat in high winds… but the spar engineer does not recommend doing that, so I will be sailing it with no spinnaker. It kind of defeats the purpose of a catboat, which is easy shorthanded sailing to turn it into an overpowered spinnaker boat that now requires a large crew to handle.

 

[Fellies]

Interestingly long before I owned it, but after the holes were added, this particular boat consistently won so many PHRF races, both offshore and inshore that it caused a huge amount of chaos and headaches for the rating committees- many wanted the rating drastically lowered, and others accused those people of trying to rate skippers instead of boats. The boat is so well known that it’s hard to work on at the dock without a stream of people coming by to talk about its racing history. I very much hope to do a lot of offshore racing in this boat and hope to win, so I want to do the repairs properly. Of course, it’s no Americas cup boat, just an amateur PHRF “plastic classic.”

Sounds like you got a winner mate I dont think you have anything to worry about except filling the holes with a good epoxy and silica mix. Dont change whats working

 

[marlinspike]

Im still building a floor, you leagues ahead on the trophy

This boat needs a lot of work, including repairing a 2’ square some idiot cut out of the bottom of the hull, but what you’re doing does look like a lot more work. I built a 6 foot folding dingy and it took me a full year to complete, so don’t think I should be allowed to begin a 200 year long big boat building project…

 

[Fellies]

This boat needs a lot of work, including repairing a 2’ square some idiot cut out of the bottom of the hull, but what you’re doing does look like a lot more work. I built a 6 foot folding dingy and it took me a full year to complete, so don’t think I should be allowed to begin a 200 year long big boat building project…

I raced a farr 38 that just beat everything for years. Was just perfect. Had something so in tune not one person could describe. If you got that boat then you got the boat.

 

[Fellies]

The professional does not perhaps understand the hole size. Let’s say those are number 12 screws. In order to make a plug; you need to get a plug cutter and probably going to be 0.250”. Now you cut the plug from the stack and it will perhaps be stronger under ideal conditions and lamination quality, but major effort.

Much easier to use West System 610 or similar high strength epoxy. It may need some hair dryer gentle warming or post cure to avoid changes after normal cure and showing through the sticker..

Can only be a true professional using west system :/

 

[fallguy]

From chatgpt

Material​	Approx. compressive strength​
2:1 Cab-O-Sil/epoxy	~4,000–8,000 psi (estimated)
Neat Six10 epoxy	~9,700 psi
Neat System Three epoxy	~10,000–13,000 psi
Carbon/epoxy laminate, quasi-isotropic	~50,000–100,000 psi
Carbon/epoxy, fibers primarily in compression direction	~80,000–150,000+ psi

 

[fallguy]

just add milled carbon fiber to epoxy into a hole drilled to the od of the screw

It will be very strong.

If you want to test it; use 3 samples with varying amounts of cf and hammer test them

 

[Fellies]

Its probably counter intiuitive to use a fabric theres no ways you will get the strength back without breaking the mast in the first place and cracking the layup so Maybe just use proper resin like ampreg with silica? Your mast is probably 5 to 10 x stronger than what you will get with a filler.. Your bond line will be shiity.You need a flexible structural buffer that can shift actually. You would have to sleeve the entire inside which is impossible. I cant see how you would lay this up with fabric

 

[marlinspike]

I raced a farr 38 that just beat everything for years. Was just perfect. Had something so in tune not one person could describe. If you got that boat then you got the boat.

I think it was more the skipper than the boat, but certainly shows there is nothing wrong with the boat.

 

[marlinspike]

From chatgpt

Material​	Approx. compressive strength​
2:1 Cab-O-Sil/epoxy	~4,000–8,000 psi (estimated)
Neat Six10 epoxy	~9,700 psi
Neat System Three epoxy	~10,000–13,000 psi
Carbon/epoxy laminate, quasi-isotropic	~50,000–100,000 psi
Carbon/epoxy, fibers primarily in compression direction	~80,000–150,000+ psi

Those numbers seem about right, but as I was saying, that isn't the spec that matters for plugging a hole, from what I understand. That is the compressive yield, the pressure where it will crack or permanently deform, but strength of the plug material isn't the issue here, the issue is how much will it compress or stretch in a non-permanent way (e.g. compressive and youngs/tensile modulus) with lighter loads, because if it compresses or stretches a lot more than the surrounding carbon, then it doesn't carry any of the structural load, so doesn't protect the carbon from failure.

You might be right that adding milled carbon (carbon dust/powder) would help there, but I'm not finding good numbers on what the specs of that would likely be, and do not trust LLMs to provide engineering specs.

 

[wet feet]

This is the sort of thread that I find entertaining;lots of replies,each describing a more "perfect" way to fix a problem that doesn't really exist.If it did,the mast would have failed a good while ago.

 

[jehardiman]

Raw epoxy is strong but also stretchy and compressible with a youngs modulus of about 3 GPa vs 135 GPA for unidirectional carbon fiber composite.

I don't think Young's Modulus is what you should be looking at here. A CF mast that old (most likely woven pre-preg with linear overlay) in compression fails at the matrix-fiber bond (either in matrix compression or interlaminar separation). This is very different than modern pre-stressed mandrel tension wound spars.
Rather than wade through half a hundred papers I know about, here is an AI summary based on the prompt "carbon fibre compression failure table"

AI Overview:
Typical carbon fiber reinforced polymer (CFRP) composite failure values in compression vary by fiber orientation, resin type, and manufacturing quality. Longitudinal compressive strength is generally 50% to 60% lower than longitudinal tensile strength because failure is governed by fiber micro-buckling, kinking, or shear crippling rather than direct fiber breakage.

Typical Compressive Failure Values
Unidirectional (UD) High-Strength Carbon/Epoxy (0° fiber direction): 1,200 to 2,000 MPa (compressive failure strain ~0.9% to 1.2%)
Unidirectional (UD) High-Modulus Carbon/Epoxy (0° fiber direction): 600 to 1,000 MPa (compressive failure strain ~0.5% to 0.8%)
Standard Carbon Fiber Woven Fabric Laminate (0/90°): 300 to 600 MPa
Transverse Compression (90° across fibers): 110 to 280 MPa (heavily dependent on matrix/resin properties)

Key Compression Failure Modes
Micro-buckling: Localized wiggling or bending of adjacent fibers within the polymer matrix under high end-loads.
Kink Bands (Shear Crippling): Progressive localized rotation and fracturing of crystalline carbon planes creating a sharp failure band.
Matrix Crushing / Debonding: Micro-cracking of the epoxy binder under multi-axial or high compressive loads before total structural collapse.

This is the sort of thread that I find entertaining;lots of replies,each describing a more "perfect" way to fix a problem that doesn't really exist.If it did,the mast would have failed a good while ago.

True, the damage is done. The issue is preventing more damage. Any unsealed cut fibre end is a water entry and delamination point, and an empty hole is a stress riser. The screw heads are not only a cosmetic and snagging issue, but the compressive strength member (which is why they were left in). Epoxy here is only the sealant, the NI-CU screw is the strength ( which has a Young's Modulus of 172 GPa btw.)

 

[marlinspike]

True, the damage is done. The issue is preventing more damage. Any unsealed cut fibre end is a water entry and delamination point, and an empty hole is a stress riser. The screw heads are not only a cosmetic and snagging issue, but the compressive strength member (which is why they were left in). Epoxy here is only the sealant, the NI-CU screw is the strength ( which has a Young's Modulus of 172 GPa btw.)

What you are saying makes a lot of sense, and I am more confident in my ability to pull it off correctly than the advice quoted by the spar engineer of creating a tapered plug plus a very thick external patch, and I feel like enlarging the holes by tapering will cause additional loss of tensile strength that the patch won't repair. I am leaning towards doing exactly what you suggested. If you don't mind me asking, what is your background, are you a composites engineer?

Your idea of a monel stud epoxied in seems to solve both the problem of the epoxy being too compressible, and the problem of a regular screw being too loose and not actually taking any compressive load.

 

[marlinspike]

CF will corrode regular SS

I hear this often, but I am not convinced it's true... my Sage 17 sailboat is a carbon deck and transom, and there is zero corrosion on any of the 316 stainless deck fasteners when I rebed them, with the boat now being 16 years old, and largely used in marine environments. Most of the carbon mast and carbon racing sailboats in my marina/yacht club are also using 316 stainless fasteners in CF.

I am particularly interested in confirming this issue, because I'm also trying to figure out how to attach a new sail track, and that does require what would be many thousands of dollars in fasteners if I needed to use titanium or monel instead of 316.

 

[jehardiman]

I hear this often, but I am not convinced it's true... my Sage 17 sailboat is a carbon deck and transom, and there is zero corrosion on any of the 316 stainless deck fasteners when I rebed them, with the boat now being 16 years old, and largely used in marine environments. Most of the carbon mast and carbon racing sailboats in my marina/yacht club are also using 316 stainless fasteners in CF.

I am particularly interested in confirming this issue, because I'm also trying to figure out how to attach a new sail track, and that does require what would be many thousands of dollars in fasteners if I needed to use titanium or monel instead of 316.

Depends on it being active or passive.... i.e. did you pay for the good (passivated) stuff.

Galvanic series - Wikipedia

en.wikipedia.org

Edit: Not a composites engineer but a NA that worked on systems with lots of composites. Across the isle from me and my NA/structural tag-team partner was Dave Mancebo who previously worked for Tom Wylie doing design and composites and he worked with the MINS "plastic" shop to develop a lot of light weight high strength composite items for deep submergence. Let's put it this way, in the fallout of the A-12 program...the Ocean Engineering department was singled out by McDonnell Douglas/Boeing and General Dynamics as not providing information known by the Navy that "could" have made the A-12 program successful (NOT!)...

McDonnell Douglas A-12 Avenger II - Wikipedia

en.wikipedia.org

Edit 2: Boeing merged with MD not GD...