Medium Fins - V1 - Training Fins¶

Medium training fins, short enough to fit in the gym bag but long enough to have proper water resistance.

Planning¶

Foot pockets ready¶

Make sure your foot pockets are on hand before you start. If you still need to choose a pair, follow the steps in Choosing the foot pockets. Once the pockets are sorted, lay out a fresh cutting template with Laminated paper cutting template.

Heads-up: The dimensions below assume 170 mm of blade will slide into the foot pocket. Measure your pockets to confirm before cutting.

Specifications / Dimensions¶

Target outline for each blade:

Width: 18 cm
Total length: 17 cm + 43 cm = 60 cm
- 0–17 cm: inside the foot pocket (flat section)
- 17–60 cm: free blade to the trailing edge

Layer schedule (one blade)¶

Reserve 17 cm from the heel line for the bend zone.
Layer 1: 20 cm × 60 cm
Layer 2: 20 cm × 35 cm
Layer 3: 20 cm × 45 cm
Layer 4: 20 cm × 60 cm top ply


Expanded Laminate View	Laminate Thickness Profile

Cutting plan¶


	Cutting plan for both blades

Estimating the flex¶

Start with the Flex predictor modelling workflow to sanity-check the layup. Adjust the layer stack and bend allowance until the predicted deflection matches your training goal.


Bending Calculation	Bending Profile

Predicted:

Load required for 90° = 9.7 N (0.99 kg)
Hydrodynamic resistance score 5N = 3.54 units
Hydrodynamic resistance score 10N = 2.17 units

The predicted code for this fin would be (see hydrodynamic resistance codes):

C430-T10-R35-F14

Reference images¶

TODO	TODO
Footpockets	Cured Laminate

TODO	TODO
Complete fins	Nice and compact

Time needed¶

Technique	Implementation	Waiting
Foot pockets Choosing the foot pockets
Base Support Creating a laminating base	0.75 h	0 h
Laminating Carbon Creating the carbon laminate ×3	1.5 h	12 h
Vacuum Bagging Reducing the resin percentage of the laminate	0.75 h	0 h
Cutting Cured Carbon Producing the final shape	1 h	0 h
Finishing Carbon Finishing the carbon laminate surface	1 h	4 h
Gluing Fin Rails Gluing Fin Rails	0.5 h	12 h
Total	5.5 h	28 h

Bill of Materials¶

Technique	Material	Quantity	Unit Cost	Line Cost
Foot pockets Choosing the foot pockets	Bifin Footpockets Reusable Pair of angled freediving pockets with short rails	1 pair	£36.00 - £58.00 per pair (recorded range)
Foot pockets total				£36.00 - £58.00

Base Support Creating a laminating base	Acrylic sheet (A3, 2 mm) Reusable Six panels for laminating base	1 6 pack A3 sheet (2 mm)	£16.00 per 6 pack A3 sheet (2 mm)	£16.00
	Thick cardboard Consumable Cardboard sheet used to form a base	1 sqm	Inexpensive
	Protective plastic sheet Consumable Disposable sheet to protect the bench surface	1 sheet	Inexpensive
	Electrical tape roll Consumable Wide PVC tape to join acrylic seams	1 roll	Inexpensive
Base Support total				£16.00

Laminating Carbon Creating the carbon laminate	Carbon Fiber Fabric Consumable 0.3 m² of 200 g/m² 3K 2/2 twill cloth	0.9 m² (1 m wide) scaled ×3 — base 0.3 m² (1 m wide)	£17.00 per m² (1 m wide)	£15.30
	Laminating Epoxy System Consumable 150 g mixed resin (approx. 150 ml)	0.75 500 ml kit scaled ×3 — base 0.25 500 ml kit	£15.00 per 500 ml kit	£11.25
	Pva Release Agent Consumable Two thin coats on the laminating base	0.525 200 ml bottle scaled ×3 — base 35 ml (two thin coats)	£5.00 per 200 ml bottle	£2.63
	Peel Ply Consumable Two layers cut to the blade outline	0.36 5 m pack (1.5 m wide) scaled ×3 — base 0.6 m (1.5 m wide)	£12.00 per 5 m pack (1.5 m wide)	£4.32
	Consumables pack Consumable Gloves, mixing sticks, acetone wipes	3 pack scaled ×3 — base 1 pack	£3.50	£10.50
Laminating Carbon total				£44.00

Vacuum Bagging Reducing the resin percentage of the laminate	Vacuum Bagging Kit Reusable Manual pump with heavy-duty storage bags for low-cost vacuum pulls	1 kit	£9.00 per kit	£9.00
	Butyl Sealing Tape Consumable Continuous bead around the laminating base	0.4 15 m roll (12 mm)	£6.50 per 15 m roll (12 mm)	£2.60
	Breather Cloth Consumable Under-bag airflow path and resin catch	0.6 m (1 m wide)	£12.00 per 5 m pack (1 m wide)	£1.44
Vacuum Bagging total				£13.04

Finishing Carbon Finishing the carbon laminate surface	Laminating Epoxy System Consumable Thin finishing coat (approx. 100 ml mixed)	0.2 500 ml kit	£15.00 per 500 ml kit	£3.00
	Clear Coat Spray Consumable Three light lacquer passes for UV protection	0.3 400 ml can	£8.00 per 400 ml can	£2.40
	Sandpaper set Consumable 400, 600, and 1000 grit sheets	1 set	£3.00	£3.00
	Printable vinyl decal sheet (optional) Consumable Gloss white adhesive sheet for logos Optional branding layer	1 sheet	£1.50	£1.50
Finishing Carbon total				£9.90

Gluing Fin Rails Gluing Fin Rails	Rubber Fin Rails Consumable One pair of soft rubber rails sized for bifin blades	2 m (enough for four rails)	£6.00 per metre	£12.00
	Marine Adhesive Consumable 25 ml of 3M 5200 Marine Adhesive	0.3 88 ml tube	£13.29 per 88 ml tube	£3.99
	Fold-back clips set Consumable 20 mm binder clips for clamping rails during cure	1 set	Inexpensive
Gluing Fin Rails total				£15.99
Consumables subtotal £73.92	+	Reusable subtotal £61.00 - £83.00	=	Grand total £134.92 - £156.92

Tools Required¶

Tool	Techniques	Purpose
Brushes	Laminating Carbon Finishing Carbon	Apply resin and coatings evenly across the laminate
Digital scale	Laminating Carbon	Weigh resin batches for accurate mix ratios
DIY Vacuum gauge	Vacuum Bagging	Estimate when vacuum reaches 80% atmospheric pressure
Dust mask	Cutting Cured Carbon	Prevent inhalation of carbon dust during cutting
Gloves	Laminating Carbon Cutting Cured Carbon Finishing Carbon Gluing Fin Rails	Protect your hands from resin, adhesives, and carbon fibers
Isopropyl alcohol	Base Support	Clean acrylic edges before applying tape
Junior hacksaw with a fine-tooth metal blade	Cutting Cured Carbon	Cut the cured carbon cleanly without fraying
Metal file	Cutting Cured Carbon	Smooth cut edges and remove burrs
Metal roller or rounded tool	Vacuum Bagging	Press the butyl sealing tape firmly to the base
Mixing pots	Laminating Carbon Finishing Carbon	Measure and combine resin and hardener accurately
Mixing sticks	Laminating Carbon	Blend resin and hardener thoroughly
Plastic finned roller (75 mm)	Laminating Carbon	Consolidate layers and push out trapped air
Protractor	Base Support	Measure angles
Safety goggles	Cutting Cured Carbon Finishing Carbon	Protect your eyes from dust and splashes
Sandpaper	Finishing Carbon Gluing Fin Rails	Prepare and smooth surfaces for better adhesion and finish
Scissors	Base Support Laminating Carbon Vacuum Bagging	Cut materials like fabric, film, or templates to size
Tape measure	Base Support	Check dimensions and spacing during setup
Window cleaner	Vacuum Bagging Gluing Fin Rails	Degrease surfaces before bonding or sealing

Instructions¶

Build a 1000 mm × 600 mm laminating base following Creating a laminating base so both blades can be laminated at the same time.
Lay up the carbon according to the schedule above, using the steps in Manual wet layup stack.
Pull the stack under vacuum to tighten the fiber volume, referencing Edge-sealed bagging.
Trim the cured laminate to the template with the Junior hacksaw method.
Seal the surface with the approach in Epoxy and clear coat finish.
Bond the rails using the guidance in Marine adhesive.

Results¶

Desired vs Predicted vs Actual¶

I've recorded the flex using the Kitchen Scale Test.

	Desired	Actual	Notes
Free blade size	TODO	TODO
Load for 90 degrees	TODO	TODO

Water trial¶

Overall the fins performed pretty well but favour a particular style of kicking:

feels lighter than Cressi Clio
slightly heavier than swim training fins
sprinting feels good as that engages the blade properly
feels too light (low surface area) for slow, hip-driven movements

	Desired	Actual
Hydro resistance	~2.7 units	2?

Plugging the numbers into the Flex calculator, I get the following numbers:

Hydrodynamic resistance score 5N = 2.54 units
Hydrodynamic resistance score 10N = 2.30 units

Judging from the above, the code for this turned out to be (see hydrodynamic resistance codes):

C230-T50-R25-F02