Wooden Hydrofoil

This is an instructable for making a wooden hydrofoil to attach to the bottom of a surfboard for kite surfing or paddle surfing. This is low tech and does not require any fancy tooling to make. Its designed to "fly" at low speeds so is good for use in waves or light wind. This is my 3rd foil, all of them have been slightly different design, all of them have been very rough and ready in construction but all of them have been heaps of fun to ride. This is not a high speed / performance foil, for that you need to go to much slimmer dimensions which requires exotic composite materials to achieve the necessary stiffness.

Ive taken a photo of the required timber dimensions for each of the components.

Ive used 20mm thick pine plank for most of the parts but any similar timber will do. Ive constructed the mast out of thinner (15mm) pine previously but it requires more glassing afterwards to make it stiff enough.

Do not use ply wood for the mast as half of the laminations have the grain running the wrong way which will compromise the stiffness. Ply wood works ok for the wings and stabiliser but make sure to use exterior grade.

The mast Ive made is 90 cm long, 60 cm is a good length for learning on, for the surf or in shallower water.

Once your pine plank is cut to length mark a pencil line down the centre of the leading and trailing edge.

Next mark pencil lines on each face where you will plane the wood back to.

These marks ar critical in ensuring you will shape a symmetrical mast.

The picture above shows the end of the mast Ive made. Note how the leading edge is a steeper taer than the trailing edge.

Ive cut out tenons where the mast will mate with the base plate, which attachs to your board, and the fuselage.

The fuselage is made from a central 20mm thick core . Ive cut out a recess to receive the tenon of the mast, Ive then laminated another piece of thinner timber on either side as cheek pieces. This creates a pocket to receive the mast ( this is easier than cutting out a mortice accurately).

The fuselage has cut outs made to receive the wing and tail stabiliser.

Note that the aerofoil is inverted for the tail stabiliser.

The flat bottom surface of the wing and flat top surface of the stabiliser should be parallel.

Shape the wing and tail stabiliser using the timber dimesnions supplied.

The aerofil profile should be basically flat on bottom, relatively blunt leading edge and tapered trailing edge. I made a profile gauge for the wing and stabiliser from card, you can use this to ensure the wing is a consistent shape and also use it top cut out the receiving profile on the fuselage.

I use a carpenters block plain for all shaping ( followed by a light sand with coarse sand paper) as power planers are too aggressive.

Ive went for a basic flat wing template for both.

Some commercial design have curved wings or delta shapes but these are harder to shape and I'm not convinced its necessary.

I used an offcut of the same plank used to make the mast for making the base plate.

The 20mm thickness is thick enough to cut a mortice in to receive the tenon of the mast.

I drilled 10mm holes to receive the 8mm carriage bolts that will secure the base plate to the board.

Now that all the components are made perform a dry fit up to confirm everything looks good.

Disassemble the parts and glass fibre them using two part epoxy.

The epoxy and glass protects the wood from the sea water and contributes to making them all stiffer.

For the wings glass the top curved surfaces first. Allow the glass to dry and then trim the glass using a sharp knife to within about 2mm of the wood edge.

Flip all of the parts over and epoxy glass the other side.

Allow to dry and trim off the excess

I use thickened epoxy to glue the wing and stabiliser to the fuselage. Commercial designs use bolts so that they can be disassembled easily but I managed to rip a wing off using that technique. I screw the wing to the fuselage (holes pre drilled) to clamp them together while the glue sets.

One the wing is tacked on to the fuselage use thickened epoxy (you can use saw dust to thicken your laminating epoxy) to create a fillet at the 90 degree join between the two parts, lay on a few strips of glass while the epoxy is still soft to stiffen the joint. (see photo).

Wet out the tenons on the mast with epoxy glue and mate the mast into the mortice in the fuselage and base plate.

The highest stress area of the whole assembly is the joints at either end of the mast . Stiffen this interface with an epoxy fillet and multiple 6" wide strips of glass fibre. Once the glas fillet is set it is easy to sand / shape to the desired profile ( see picture).

Correct placement of the foil on your board is critical. The centre of lift of the front wing should be just in front of your back foot, this can be achieved by attaching the mast / base plate pretty much directly under the back foot strap (see picture of my mark 1 foil).

I drill 4 large oversized holes (22mm) in the board then fill it with thickened epoxy . Once the epoxy sets you can then redrill the 10mm hole to receive the 8mm carriage bolts. Use stainless hardware, large washers to spread the load and locking/nyloc nuts.