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C‑Flex fiberglass planking (for 'one‑offs' over a male mold)

C‑Flex® is a type of fibreglass 'planking' that was created by and is still sold by Seeman Composite Inc. since the 1970s. It's made up by using continuous, flattened longitudinal glass rods with roving in between, and it's all woven together with a light fiberglass cloth to keep things in position, and this allows it to be worked on the bias to create compound surfaces.

The small 'rods', spaced approximately on ½" centers, are pultruded like some fishing rods, so that makes them a far denser and stronger product than normal glass roving and resin. C‑flex is sold in rolls up to 250' in length and comes in just two sizes or weights—both being 12" wide. See photo.

Though the lighter weight stuff (defined as CF‑39) would be too heavy for say a kayak, it could be used for a solid dinghy or certainly for the main hull of a trimaran and that's the justification for being included here. It can be pushed around to mold over many shapes and even compound curves—certainly far more than most sheet materials. Although heavy compared to composites of foam or even plywood, it works out somewhat lighter than an equivalent boat built of solid fibreglass and significantly stronger longitudinally.

Typically, it is stapled over a male framework of wood station templates and only after being in place, is the resin rolled on. Typically, a casting (mold) resin is used, as this shrinks far less.

The 'planks' can be cut with tin snips or when bunched together, with a fine metal saw. The CF‑39 plank needs a transverse frame not farther than 14" apart and 12" is recommended as this can reduce the fairing later on. The outer edge of the 'planking' is stapled while dry to the temporary frames, typical with small stainless or monel staples about 38" long with a small (316") crown.

Above is a hull where the planking was started from the sheer and then the bottom filled in after the bilge was covered. As mentioned, the FG rods within the plank are quite small and flat, but still have far more glass in them than say a woven roving and that is what give a C‑flex boat such great longitudinal strength.

A small boat like this would typically have only need one layer of C‑flex planking, though a light mat with woven roving would be added to the interior to complete the skin, after the boat was turned over and the temporary frames removed.
The use of C‑flex will nearly always benefit from a more elaborate mold, as the more support the soft material has, the less fairing will be required later. The comment 'C‑flex can require extensive fairing' is one of the things that has been levelled against the product in the past. One of the more consistent supporters of C‑flex over the years has been the fine designer Bruce Roberts from 'down under'—though he works primarily with monohulls. Here is a sketch from his on‑line data, showing longitudinal battens first laid over the station templates and then overwrapped with wide flat battens (glass or wood) from sheer to keel, to provide much more support to the dry C‑flex than would just the templates alone. This method also permits easy fairing of the exterior surface with shims etc, in the event that the stations are not computer generated and perfect.

C‑flex is one of the few ways of building a one‑off design in solid glass without a sophisticated mold, and as such has been used by monohullers where weight is generally a less critical factor than on multihulls.
Although this material can hardly be called 'lightweight' with its 1/1 ratio with resin (giving about 1.1 lb/sqft for a complete lamination on a small boat with a 18" skin), it might justify being used for a long slim boat requiring great longitudinal strength—let's say to support the bending stresses of a large sail rig. In such a case, a bottom and deck of C‑flex might be worked into a hybrid design to achieve this, much like flanges on a wide‑flange beam.

By using a milled fiber paste (~75% resin) instead of the normally specified chopped strand mat required between the C‑flex and woven rovings, Seeman Composites claim that the weight of the mat portion of the laminate can be cut by 50%. That's still a relatively small part of the total laminate though.
More specific information is available from the Seeman Manual at this site:

  1. Probably the only proven method to build a one‑off hull in solid glass+resin (FRP).
  2. Creates a boat with high longitudinal strength, as the fibreglass 'rods' in C‑flex are not only relatively large and solid compared to all other glass fibres but they are also laid in as one piece from bow to stern.
  3. C‑flex lays on as multiple lengthwise battens and therefore provides basically fair lines over the construction form.
  4. The cloth that binds the solid 'rods' permits a lot of flexibility to follow compound hull shape and the 'planking' can be spread or compressed a little to fit.
  5. C‑flex can be totally laid on dry and positioned easily and rapidly with simply a staple gun.
  6. By stapling down each rod to the mold, the material can be made to lie in concave areas and therefore works for boats that have a flare.
  1. A solid hulled boat is going to be relatively heavy by today's standards and even though C‑flex is no heavier than a custom built solid fibreglass hull (and arguably lighter for the same strength), its weight must be considered a disadvantage compared to other, perhaps less rugged methods.
  2. Surface fairing has been reported as a problem as the C‑flex can dip between the station forms*, especially while being wetted out and left in a heavy state to cure. This then means a lot of filler and even more sanding than normal. Arranging under‑glass support much closer together (as per this monohull photo with battens 9" apart) can lower this risk and reduce fairing work.
  3. A solid glass boat is not only heavy but will also require added interior insulation to avoid condensation. Many of the alternative systems will not have the same issue and also have a better natural finish for the interior.

* This 'dipping', while caused slightly by resin weight on horizontal surfaces, is principally from resin shrinkage on curing. This has the effect of pulling a curved surface somewhat straight between supports, then requiring more filling to make the hull surface fair. For this reason, only low‑shrinkage molding (casting) resins should be used for the critical first layer of C‑flex. There is a slight premium a pay for this but little option. Even then, expect a slight tightening of the C‑flex and the justification for using closer supports than normal.

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