Ted Van Dussen on fasteners and modification of carbon masts

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folotp
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Ted Van Dussen on fasteners and modification of carbon masts

Post by folotp »

Hello,

There are a lot of different opinions of what can and cannot be done to carbon fiber masts, and to which extent it impacts their strength and structural integrity. There are also lots of opinions on the best way to fasten hardware to carbon fiber masts. My masts having been replaced around 2006, I asked the builder, Composite Engineering, what was their recommended way of fastening to the mast. Ted Van Dussen replied.
We recommending drilling and taping threads in the mast using coarse threads. When the fasteners are installed some epoxy should be put in the hole. This strengthens the laminate disturbed by the threading, helps reduce corrosion of the fastener, and prevents it from working loose. If you can find fasteners made from 316 or Nitronic stainless steel, they would be less likely to rust that more common stainless steel alloys.

Even a small hole increases the stress in the mast locally by 2.7 times. The holes should not be located in high stress areas unless extra carbon is laminated in the area to increase the wall thickness about 50%. Another alternative is to bond blocks of fiberglass to the surface of the mast that have threaded holes for the fasteners.
I asked how to remove machine screws that have been epoxied as recommended.
It is difficult to get a strong bond between epoxy and stainless steel. There is also a difference in expansion rates with temperature, so the bond is always being stresses. Most times we find that the torque to turn the bonded fastener out is roughly 20% higher than unbonded ones. If the fastener is threaded deeply into the material, the bond may be too strong and the heating is required. The stainless steel conducts heat poorly. so it takes a few minutes for the heat to travel down the fastener. I make a sheet metal mask to protect the laminate so that I can heat just the head with a torch and almost always can get them out.

If I know that I will have to remove the fastener soon, I will put a light coat of release agent on the screw and bond in with epoxy. The good fit by casting the threads in epoxy makes a stronger connection and they come out like normal unbonded screws.
I'm also looking at improving the lightning protection of the boat. It is often now recommended to exit the down conductor at the deck instead of at the mast step, so that the lightning current does not pass through the cabin but around it. I asked Ted Van Dussen what would be the impact of drilling a hole right under the deck for a 2AWG wire. Thank god I asked... catastrophe avoided!
Thank you for checking in with Composite Engineering for our recommendation before making changes to your mast.

Putting a hole near the deck would be the worst place to have one. The mast would only have about 40% of its present strength. It might be just strong enough to get by, but that would be giving up a large margin of safety that has been engineered into the mast.
Hope this helps someone. Cheers!
Pierre-André Folot
s/v “Farfelu” - Freedom 39 PH 1983, hull #14
Sailing the Thousand Islands and Lake Ontario
Canada

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