Hi everyone,

Even though I have invested most of my thinking on aluminum telescopic mast, I can't seem to stop thinking about a diy carbon fiber mast for a junk rig conversion. I need a reality check on whether my proposed build method is reliable enough, or if it’s a recipe for failure.

A professionally manufactured carbon mast is out of the question—it costs at least three times what the boat is worth. On the other hand, due to the lack of adequate off-the-shelf dimensions, I am struggling to get a 3-to-4-part telescopic aluminum mast below 3.5% of the boat's displacement. The mast specifications are 13.5m total (11.7m above deck, 1.8m bury) with a design bending moment of at least 70,000 Nm at the partners (2.5x safety margin).

Because I cannot build a dead-straight, rigid mandrel that won't sag, and I am short-handed in the workshop, I am considering casting the mast in two halves using long, leveled "gutters" (split molds) on the workshop floor.

The Halves: Laminate each half independently. The layup will be a heavy core of longitudinal UD-carbon fibers sandwiched between thin biaxial skins, trimmed perfectly flat to the mold edges, vacuum-bagged and cured.

Once both halves are cured and sanded, both halves will be epoxied together and the inside will be covered a circular triaxial/biaxial weave (CF, glass or basalt) The halves are brought together, clamped hard from the outside at short intervals using custom clamping collars to prevent misalignment, and an internal bladder is inflated to press the inner weave tightly against the walls of the tube. This will then be cured completely.

The external seam is sanded flush, and the whole mast wrapped in a continuous triaxial carbon weave, and vacuum-bagged. The goal is to lock the flat butt-joint in a "vice-grip" between the inner and outer hoop-strength layers to block shear and torsional forces without creating thick, asymmetric internal flanges.

PS. I have also considered using a 6 metre aluminium mast, and then making the top part only from carbon fibre. Electrically isolating it with fibreglass from the aluminium, in the same style as some make a telescopic aluminium or an aluminum-wood-mast. If such a mast build would fail, it would not cost nearly as much.

My Questions:

1. Would it be acceptable to go for the much easier and more affordable aluminum, even if it is very heavy?

2. Is this split-mold approach mechanically sound for a freestanding mast under continuous pumping, or will the longitudinal seam inevitably fail over time?

3. Do you have any suggestions to how I could do this easier or more reliable?

4. What is the most reliable bladder setup for an internal 13.5m taper to ensure even pressure without getting permanently trapped after curing?

Thanks,

Thomas