Easiest way of making attachment hardpoints on a carbon fiber mast?

If I were doing the job there would be an insert inside the mast and an extra ply on the outside.No concerns about the depth to drill and tap because there would be an extra ply in pace.There are lots of other parts of a boat that might be needing attention.

 

I'd drill and tap metal inserts before glueing them in. It's easier and safer to clean out a pre-threaded hole of cured resin with a tap, than drilling and tapping in situ, in case the drill heats up the surrounding resin, compromising its bond, or if the tapping doesn't go well first time. An opposite cleat through bolted to the one required may foul the boom or sail, but Gonzo's idea with just a low profile plate opposite instead, would work with the reinforcing crush tube idea. Don't think about recessing the inside of the mast, keep it uniform, and do the 'letting in' with your mast inner form instead. Eg., 3D print your inner form with a slot or box for your threaded plate or crush tube or other ideas; make the threaded plate / insert; put thread studs in it (with anti seizing compound) with pointed ends that will poke through your fibre layup, so you can find the holes by unscrewing the studs with pliers / allen key, or something, then trim / tidy the holes and bolt in your cleat. I personally would just use a wood or aly mast, not much heavier, but a whole lot easier to work, and to repair if damaged. Made in hours instead of days. I hate having needles between my fingers after glassing and sanding. But I know you're into the CF, so good luck.

 

If building boats had to pass the test of making sense, we would have to play video games instead.

 

OK, Gonzo has summed it up succinctly. While I wholeheartedly agree with this assessment, I tire of video games eventually and have to go out to the shop to waste more wood.

I admit to not having read, studied and taken notes on every post in this thread.

Thermal expansion: I will start here as it is closest to Gonzo's comment. Glass fiber does expand more than carbon, but there is a lot of resin and the thermal expansion in the fiber direction is far smaller than it is in the z-axis.

Cleats: Following closely on the worries about thermal expansion, do an image search on jam cleat, v cleat, and cam cleat. The consensus will be clear. Good advice, whoever admonished us against peeing into the wind.

Galvanic corrosion: Yes, carbon is cathodic to everything. There are a few metals that are compatible, titanium being the only practical and relatively affordable one. Ti has a very wide range of electrical potential where it will be passivated rather than corroded, and C will passivate Ti because it is in that range. So said the corrosion prof at MIT. Everything else needs to be isolated and the customary solution is a thin outer layer of fiberglass scrim. So customary, that it was very hard to convince a supplier to leave it out when we wanted a static dissipating surface.

Drilling holes: It takes a hard, sharp bit, preferably diamond or carbide, but according to some outfit I worked with down in Florida,

Having quoted that I, as a wood butcher with little practical experience, was handed a fiberglass part and a hand drill by an experienced machinist. He was amused. You have to pull the bit out and clear the debris very often to avoid binding and burning. Fiber reinforced composite does not act anything like wood or even metal.

Carbon fiber tubes: I didn't notice any mention of braided sleeves. They are ±45° which is not the most efficient for tensile strength, but it does make for a nice surface, easier handling of the layup and a good way to wrap the unidirectional layers in a hand layup. The video does say that you can get better fiber orientation by minimizing the diameter. There are also unidirectional tubes available, but I have to admit to not knowing mush about handling them. Mush was a typo, but it fits. I will also recommend the carbon/Kevlar braid based on ignorance and appearance. I would like to say that it would be more durable, but I would be speculating.

Backing plates and inserts for holes: Lots of good ideas already.

Tension and punching holes in the bottom: This is a 17' boat with an unstayed mast. How strong are you that you can pull that hard on the lines? I am impressed.