Skip to content

DIY materials and bonding for monofin water rails

Context

The starting point was a photograph of a CETMA carbon monofin with raised side wings or water rails. The questions were what those rails were made from, how they contributed to lateral stability, and how a similar structure could be made and attached in a DIY carbon-blade project.

The exact compound could not be identified from the photograph. The response inferred moulded rubber or TPU from appearance and common construction practice. CETMA's own monofin warranty refers separately to “water rails and rubber parts,” which supports rubber as the broad material class but does not establish the exact elastomer, hardness, reinforcement, or manufacturing process:

https://cetmacomposites.it/warranty-monofin.pdf

No material, adhesive, rail geometry, or construction method was finally selected or tested.

Functional requirement

The intended outcome was good lateral stability: reducing unwanted blade twist, wobble, and sideways movement during the kick so the fin tracks predictably.

The response proposed that side rails can:

  • influence water escaping sideways from the blade;
  • guide flow near the blade edges;
  • alter torsional behaviour;
  • protect the carbon edge from impact; and
  • help the blade remain aligned through the kick cycle.

These effects were qualitative claims. Their magnitude and even their desirability depend on rail height, length, taper, stiffness, mounting, blade flex, kick cycle, and flow conditions. No hydrodynamic or mechanical measurement was made.

Material options discussed

Preformed rubber strip

Buying an existing rubber edge profile or cutting a strip from sheet material was presented as the easiest DIY route. A strip could be fixed beside the blade or wrapped around its edge in a U-shaped profile. It would avoid the need for a mould and could serve as an inexpensive geometry prototype.

EPDM or another firm elastomer

EPDM was proposed as a durable, moderately firm material for outdoor and marine exposure. Its actual suitability would depend on grade, hardness, section geometry, and the available adhesive system.

Neoprene

Neoprene was described as easy to cut, bend, and glue, but probably too soft for substantial structural stabilisation. It remained useful for padding, edge protection, or early shape experiments. Foamed neoprene and solid neoprene should not be treated as mechanically equivalent.

Cast polyurethane rubber

A custom rail could be cast from a liquid polyurethane elastomer using a mould. This would permit control of profile and hardness but would require material selection, accurate mixing, mould preparation, bubble control, curing, and a validated attachment method.

3D-printed TPU

Flexible TPU filament was proposed for repeatable custom profiles and U-shaped channels. This would require printer and material tuning, suitable wall and infill design, and testing of print orientation, fatigue, water exposure, and bonding.

Industrial TPU profiles

Injection-moulded or extruded TPU was considered realistic for production but not an easy one-off DIY process. An off-the-shelf profile could still be used if its shape and material were suitable.

Carbon reinforcement

Because the blade itself was carbon fibre, carbon reinforcement was considered for increasing rail stiffness. Proposed arrangements included:

  • carbon cloth laminated onto the blade edge;
  • a separately moulded or precut carbon strip;
  • a carbon element embedded in rubber or TPU; and
  • a carbon structural wing with a softer protective or trailing edge.

Using carbon on a carbon blade does not automatically produce a compatible structure. Fibre direction, laminate thickness, resin, attachment geometry, and stiffness transition determine whether the wing supports the blade or introduces a new stress concentration.

Bonding advice received

The response suggested abrasion, cleaning, adhesive application, clamping, and full curing. Candidate products included:

  • polyurethane marine sealants or adhesives such as Sikaflex 291 and 3M 5200;
  • West System G/flex epoxy; and
  • contact adhesive for prototypes or lightly loaded joints.

It also suggested optional primers, flame treatment, and temporary clamping with tape, bands, or weights. These were generic suggestions rather than a validated process for a known elastomer and blade resin.

Editorial corrections

  • Adhesive compatibility cannot be inferred from the labels “rubber,” “TPU,” or “carbon.” The exact elastomer, carbon surface resin, adhesive, primer, cure conditions, and immersion environment need to be tested as one system.
  • Epoxy does not generally form a dependable flexible bond to arbitrary rubber or TPU.
  • Sikaflex 291 and 3M 5200 are not universal rubber-to-composite adhesives. Manufacturer substrate guidance, surface preparation, primer requirements, bond-line thickness, and cure conditions matter.
  • Acetone can attack, swell, or extract components from some elastomers and should not be recommended without material compatibility data.
  • Flame treatment is chemistry- and process-specific. Passing a lighter or propane torch over unidentified rubber is not a generally valid preparation method.
  • EPDM and neoprene are cross-linked elastomers and cannot be permanently heat-formed like thermoplastic TPU.
  • Screwing or stitching through the carbon blade edge creates holes and stress concentrations and should not be treated as casual reinforcement.
  • Carbon reinforcement can create an abrupt stiffness mismatch, a peeling load at the bond edge, or a hazardous sharp edge if its taper and finish are not designed.
  • Neoprene foam compressibility, generic rubber-strip stiffness, and rail performance cannot be ranked without actual grade, geometry, and load data.
  • The commercial “underplanter” description concerned the footbed or foot platform, not the side water rails, and was unrelated to this design question.

Proposed comparative experiment

Build removable or sacrificial rail samples with the same external geometry and compare:

  1. the bare blade;
  2. a firm rubber rail;
  3. a printed TPU rail;
  4. a carbon-reinforced elastomer rail; and
  5. a thin carbon rail with a soft protective edge.

Record:

  • blade tracking and perceived side-to-side movement;
  • blade twist under a controlled asymmetric load;
  • overall flex pattern and the location of stiffness transitions;
  • perceived effort at a fixed pace;
  • rail drag or changes in kick feel;
  • bond peeling, fretting, cracking, or water ingress; and
  • durability through repeated pool sessions.

The experiment should separate rail material from rail geometry where possible so that a result can be attributed to one variable rather than a simultaneous change in height, stiffness, and profile.