Cutting tips off carbon foils

A hydrofoil’s front lifting and rear stabilising foils are usually made of carbon with a foam core. Carbon fibre is incredibly light and stiff but on impact it shatters! If you damage a tip badly you can trim off and sand smooth the damaged section; and replicate on the other tip for symmetry. If you are cutting near the tips the chances of encountering the foam core are minimal.

Scratches in carbon foils are common and can be sanded out using 600 grit wet and dry, in wet mode, to prevent inhalation of carbon fibres.

In this blog, I trim 40mm off each tip of an Armstrong high aspect HA1325 v1 to improve roll and safety. I was guided by forum discussions describing the modification process. The latest v2 model of the Armstrong (and many other brands) high aspect range is a hybrid style, good turning/low stall foil design which sacrifices some top end speed for early lift and glide.

Hydrofoils pitch forward as speed increases and models like the HAv1 are prone to abrupt downward pitching. The latest stabiliser designs can moderate pitching somewhat. The latest profiles used in the HAv2 include a concave along the trailing edge of the foil to improve pitch control, and create greater lift from a smaller foil area.

The HA1325v1 has a span of 1164mm, chord of 114m, aspect ratio (span*span/area) of 10.2, and an area of 1325 sq cm. Its large span provides tonnes of glide and makes it tricky to gybe but it has good top speed. By trimming the tips I am less likely to skewer myself if I fall during a gybe and the foil will roll easier reducing the tracking effect in turns.

To start the trimming process, I marked a straight line 40mm in from each tip. Masking tape is used to reduce splintering effect when cutting and provides contrast for the marker pen. I found a plastic cap off a can of spray paint was a good size to trace a rounded end for the wing tip.

I put a fresh cutting disc on the grinder for a neat cut. Cut directly across the foil first then do the curves. Use a coarse grit paper, 180 or less, on the first pass to smooth out any lumps and bumps. Progress to 400, then 600 grit. You can go finer but the law of diminishing returns applies. Sanding down carbon doesn’t take that long so go easy. Ensure both tips are similar shape.