filling holes in an unstayed carbon mast

[marlinspike]

Yep, is that staining on the lower two screw holes cosmetic or delamination?

Cosmetic, these seem to be surface rust stains from the stainless fitting that was once bolted here. The faux wood paint on the mast is also failing.

What is the advantage of filling the holes with screws instead of thickened epoxy? Shouldn't silica thickened epoxy also be strong enough in compression?

 

[jehardiman]

What is the advantage of filling the holes with screws instead of thickened epoxy? Shouldn't silica thickened epoxy also be strong enough in compression?

No need to have anything on the inside and no voids. That mast (unstayed CF) is going to be about 1/2" or more thick, so filling the hole completely with no voids has issues.

 

[fallguy]

Cosmetic, these seem to be surface rust stains from the stainless fitting that was once bolted here. The faux wood paint on the mast is also failing.

What is the advantage of filling the holes with screws instead of thickened epoxy? Shouldn't silica thickened epoxy also be strong enough in compression?

Just difficult to fill all the way. It can be done with syringe and open needle tips. Sometimes, I put the volume expected to fill on the hole and then keep pressing with a clean knife until the putty is used up or close. Otherwise, sometimes you may end up only filling 1/8” of 1/2” hole, etc.

 

[Fellies]

We have not had much commentary from the one composites expert lately.

Probably need to ask those senior members who never actually built a boat for advice
The true professionals
If it were me i would remove the screw, counter sink it and put new screw and fill iver them will MF and silica. The faux is nice on a mast though must say

 

[marlinspike]

I did receive some advice from Ted Van Dusen, the engineer who designed and built the spar.

He says drilling a hole in a carbon mast approximately triples the stress on the material at the edges of the hole, even for very small holes, so ideally you should not add holes at all to a carbon spar.

Also, both adding a bolt/screw, and filling with epoxy are insufficient to protect it from compressive forces, because the screw won't fit tight enough, and epoxy is too compressible to carry any load compared to the surrounding carbon.

Apparently, the correct method here is to create a custom tapered carbon fiber plug the thickness of the mast, taper the hole to fit, and hammer in the plug with epoxy for a tight fit. Then, sand the surface down to bare carbon around the holes and epoxy on a carbon patch with tapered edges of 1/3rd the spar thickness, using alternating layers of unidirectional carbon cloth going vertical (lengthwise) along the mast, and woven mat.

This raises the concern/question about the fact that the sail track on this boat is already attached from the factory with 120 small holes, and filling and patching all of those to regain the full mast strength would be a huge endeavor. I am not quite sure what to think/do about that fact, as apparently the larger holes and smaller holes weaken the mast by a similar magnitude. I am going to ask for followup advice to see what I might do about that. However, all ~20 of these boats built, plus pretty much all other CF mast boats out there have holes for the sailtrack and aren't failing, so I feel like I am most likely fine to not worry about that.

 

[Fellies]

the sail track on this boat is already attached from the factory with 120 small holes

You should be alright then

 

[Fellies]

correct method here is to create a custom tapered carbon fiber plug the thickness of the mast, taper the hole to fit, and hammer in the plug with epoxy for a tight fit

I dont understand this.

 

[marlinspike]

I dont understand this.

You use epoxy cloth and carbon to build up a small stack as thick as the mast, let it cure, cut it into a tapered cylinder, and epoxy it into the hole to fill the hole in the mast. If that still isn't clear, please provide more info about what is unclear.

 

[Fellies]

You use epoxy cloth and carbon to build up a small stack as thick as the mast, let it cure, cut it into a tapered cylinder, and epoxy it into the hole to fill the hole in the mast. If that still isn't clear, please provide more info about what is unclear.

So you make a stack of 1 cm carbon, cut it to a taper and wack it in?

 

[marlinspike]

So you make a stack of 1 cm carbon, cut it to a taper and wack it in?

Yes, that is my understanding- you also need to add a taper to the hole, and epoxy it in.

 

[fallguy]

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..

 

[marlinspike]

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..

I don't think a carbon plug will be a big deal, I have a plug cutter drill bit set that will be able to cut a small tapered plug from a stack of carbon. I'm more concerned about the labor and resulting ugliness of building up a thick tapered patch on the outside.

He told me that epoxy without carbon fiber cloth is so flexible/compressible compared to the surrounding carbon, that it won't carry any of the load, and the edges of the hole will still experience about 3x the stress just like they do with an open hole. I was told that apparently the same is also true for a metal screw, as it has to fit fairly loose to have the clearance that allows it to be screwed in, thereby never actually loading up the screw in compression. I do then wonder about what if you epoxied in a metal screw, using epoxy with some sort of carbon dust in it to make it non-compressible? Or what about actually using a carbon fiber screw or threaded rod?

This stuff is fascinating, surprising, and counter intuitive- it makes me want to go pick up a composites engineering textbook so I can get a handle on what is actually going on with this stuff.

I also realize I'm probably overthinking this, after all the boat has been holding up fine for a long time as-is, as are almost all of the other unstayed carbon fiber mast sailboats out there with spars full of holes.

 

[fallguy]

I don't think a carbon plug will be a big deal, I have a plug cutter drill bit set that will be able to cut a small tapered plug from a stack of carbon. I'm more concerned about the labor and resulting ugliness of building up a thick tapered patch on the outside.

He told me that epoxy without carbon fiber cloth is so flexible/compressible compared to the surrounding carbon, that it won't carry any of the load, and the edges of the hole will still experience about 3x the stress just like they do with an open hole. I was told that apparently the same is also true for a metal screw, as it has to fit fairly loose to have the clearance that allows it to be screwed in, thereby never actually loading up the screw in compression. I do then wonder about what if you epoxied in a metal screw, using epoxy with some sort of carbon dust in it to make it non-compressible? Or what about actually using a carbon fiber screw or threaded rod?

This stuff is fascinating, surprising, and counter intuitive- it makes me want to go pick up a composites engineering textbook so I can get a handle on what is actually going on with this stuff.

I also realize I'm probably overthinking this, after all the boat has been holding up fine for a long time as-is, as are almost all of the other unstayed carbon fiber mast sailboats out there with spars full of holes.

This is not fully true.

There are high strength epoxies that would be highly compressible. Not as good as a 1/2” stack of carbon, but there is a practicality problem. How many layers of carbon laminated well? And you cannot practically go 2” square, so probably 5” square and that is a lot of work….

The 610 epoxy is probably about 6000psi compressive strength and your carbon stack is ? 30,000 psi, but in

I don't think a carbon plug will be a big deal, I have a plug cutter drill bit set that will be able to cut a small tapered plug from a stack of carbon. I'm more concerned about the labor and resulting ugliness of building up a thick tapered patch on the outside.

He told me that epoxy without carbon fiber cloth is so flexible/compressible compared to the surrounding carbon, that it won't carry any of the load, and the edges of the hole will still experience about 3x the stress just like they do with an open hole. I was told that apparently the same is also true for a metal screw, as it has to fit fairly loose to have the clearance that allows it to be screwed in, thereby never actually loading up the screw in compression. I do then wonder about what if you epoxied in a metal screw, using epoxy with some sort of carbon dust in it to make it non-compressible? Or what about actually using a carbon fiber screw or threaded rod?

This stuff is fascinating, surprising, and counter intuitive- it makes me want to go pick up a composites engineering textbook so I can get a handle on what is actually going on with this stuff.

I also realize I'm probably overthinking this, after all the boat has been holding up fine for a long time as-is, as are almost all of the other unstayed carbon fiber mast sailboats out there with spars full of holes.

610 high strength epoxy has compressive strength over 6000psi

That carbon stack (done well) … maybe 20-40,000.

I apologize, I don’t know how to calculate the forces on the hole well, but I do know laminating a 1/2” carbon stack say 5” square is not easy.. At 0.020 per, you need 25 pieces; ideally zero air which means vac bagged and probably two gos as it will heat at 1/2” thick.

I’d use high strength epoxy on further review.

Edit: my post vanished and then reappear; least I was somewhat consistent it appears

 

[marlinspike]

This is not fully true.

There are high strength epoxies that would be highly compressible. Not as good as a 1/2” stack of carbon, but there is a practicality problem. How many layers of carbon laminated well? And you cannot practically go 2” square, so probably 5” square and that is a lot of work….

The 610 epoxy is probably about 6000psi compressive strength and your carbon stack is ? 30,000 psi, but in


610 high strength epoxy has compressive strength over 6000psi

That carbon stack (done well) … maybe 20-40,000.

I apologize, I don’t know how to calculate the forces on the hole well, but I do know laminating a 1/2” carbon stack say 5” square is not easy.. At 0.020 per, you need 25 pieces; ideally zero air which means vac bagged and probably two gos as it will heat at 1/2” thick.

I’d use high strength epoxy on further review.

Edit: my post vanished and then reappear; least I was somewhat consistent it appears

I think you are comparing compression yield strengths, but it is necessary to instead compare the tensile/youngs modulus and compressive modulus to tell if they will share load, such that the epoxy would carry load and protect the surrounding carbon. 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. The epoxy won’t break but it will stretch and compress so easily that the carbon will still carry all of the load the same as with an open hole.

Good point about the problems laying up a thick pile of carbon. Perhaps I can just purchase a small carbon bar from McMaster Carr or something and cut the plug out of that. I don’t have any way of doing vacuum bagging at home, I’ve never done it before. My composites experience is limited to using west 105 with fiberglass cloth.

 

[Fellies]

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. If you could get the mast off and lay up a insert from the bottom to where the holes are, you could somewhat probably have the best luck.