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Finger foil

A whole miniature hydrofoil, board to stabilizer, sized to print on a desktop FDM printer and push round a bathtub with one finger in the deck recess. Free, no account needed. Open it at /finger-foil.

What is in the kit

Download print kit builds every piece as a solid, cuts the joints with the parts in place, turns each one onto the bed, and zips one binary STL per piece with a README of print settings, hardware and assembly steps.

• Board: flat deck, rocker on the bottom, soft bottom rails, a finger recess, and a pocket underneath for the mast plate. Prints deck down.
• Mast: constant symmetric section with a plate on top that reaches sideways. Prints lying on its trailing edge, plate standing beside it.
• Fuselage: an extruded side profile, flat sided, so it prints on its side. The mast socket, the cross pin and both wing saddles are cut into it.
• Front wing and stabilizer: built by the same wing engine and CAD path as a full-size Winghopper wing. The trailing edge is straight in plan so each prints standing on it, leading edge up.

None of these orientations needs support. Parts that stand on a trailing edge need a brim. If a piece does not fit the bed it is split: the board across its length with filament alignment pins, a wing at its centreline with its pins moved off the cut.

Joints

• The mast goes into a socket shaped like its own section, so it only fits one way round, and a cross pin through the fuselage holds it.
• Each wing presses up into a saddle cut from its own root section, held by two pins or screws from below.
• The mast plate sits in a D-shaped pocket under the board and is fixed through the deck.

Pins can be offcuts of 1.75 mm filament (nothing to buy), 3 mm steel dowels, or M3 screws. Clearance is the gap per side on every mating face; 0.2 mm suits PETG on a tuned printer. If a joint is tight, sand it rather than forcing it.

Print minimums

Small chords print thin. Min trailing edge floors the trailing edge everywhere; Min thickness floors the section thickness, and the wing's thickness percentage is raised automatically at any chord where it would print thinner than that.

Why there is no polar here

At bathtub speeds the front wing runs at a Reynolds number of a few tens of thousands. Full-size foils run in the millions. Winghopper's lift-off, cruise and L/D readouts add a section drag and a stall angle that describe full-size foils; at this scale the flow over the section is laminar and separates early, which changes both. Those readouts would be confidently wrong, so they are not shown.

What is shown is arithmetic that does not depend on that:

• To lift: all the plastic, less the buoyancy of the parts still under water once the board is out. Your finger adds to it.
• CL needed: the lift coefficient the front wing needs to carry that at the readout speed.
• Lifts off: the speed at which it carries the load at CL 0.6. That working value is an assumption for sections at these Reynolds numbers, not a solved one.
• Balance: the finger spot, the centre of weight while foiling (board at full weight, underwater parts net of buoyancy) and an estimated neutral point, in mm aft of the front wing's root leading edge. Keep the first two ahead of the third for a pitch-stable ride.
• Tail volume: stabilizer area times its arm, over wing area times mean chord.

Sections are NACA 4-digit: thin and well cambered suits low Reynolds numbers better than the thick sections full-size foils use.

Floating

Printed plastic is denser than water, so the foil parts on their own sink. The board prints at low infill and floats the whole thing at rest; At rest shows the margin.