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Building Foils

Disclaimer

This document is compiled by an amateur, not a professional. It has been compiled in good faith, but almost certainly contains errors and inaccuracies. “Best practice” also changes frequently with changes in technology and materials. None of the procedures listed are guaranteed to work, and some or all of them may be hazardous. If you feel unable to take responsibility for your own actions and errors and therefore may resort to litigation then you are advised not to read it, let alone build anything based on information here. In any case you are advised not to build a foil without someone experienced in the materials to contact for advice.

The Shape

The planform and section shape of the foil can make a big difference to the drag and efficiency. The selection of this is dependent on the use to which the foil is intended i.e. rudder or centreboard for more ideas about the shape see foil_design.

The Core

The choice of core material will have a big impact on how you build your foil and will also affect the final weight. Traditionally foils were made from a solid wood core strong enough not to require any kind of skin to provide the stiffness and strength (this was often a hard wood such as Mahogany). However, by reducing the density of the core and adding a high strength/stiffness outer skin the weight of the foil can be reduced. With a modern foil the strength comes from the skins, and the core just acts to hold the thin skins in place, and transfer the shear forces from one skin to the other while resisting crushing.

Wooden Cores

The first steps away from the monolithic wood core was to use a low density but strong wood such as Western red Ceader. This can still be worthwhile as for a single project small quantities of wood may be more available than small quantities of high density foam, and many people find wood more pleasant to work with.

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To build a Wooden foil prepare an even thickness blank from timber strips approx. 50mm wide. Quarter sawn strips are preferred as they are stronger. Organise the strips so that alternate ones are turned end for end the grain should run in different directions to reduce the warping of the timber. Warping can happen long after the foil is fully coated and complete, particularly if moisture gets into the foil through damage to the skins or pivot holes.

The timber strips should be bonded together with epoxy and clamped to a flat surface. When cured plane the blank to an even thickness. Cut out the profile of the blade, but make it 5mm smaller than the intended finished size at trailing edge. (Apart from anything else this ensures that minor damage doesn't expose a wood core) Shape the foil, allowing about 1.5mm undersized for the laminate, fairing and painting. Make the trailing edge as sharp as you can, because the fibres will overlap here to create the true trailing edge.

Plywood

Plywood is a medium to high density engineered wood product with the wood fibres alternating in direction by 90 degrees in each ply. Wood is significantly stronger along the length of the fibres than across them, however in plywood many of the wood fibres will be at 90 degrees to the load direction so will not contribute much to the strength or stiffness of the foil, yet will still be adding weight. Plywood has been used recently as the core for carbon skinned lifting foils for T-foil rudders, where the small size and thin sections make the core difficult to shape out of foam, and crush resistance is important. As only a small quantity of the plywood is used it's high density is not such a problem, and the main strength comes from the carbon skins.

Foam Core

The best core material is probably high density PVC foam (200 kg/m³). If you're really keen the core can be done with two different densities of foam using 80kg/m3 near the bottom. Failing that you can use 80kg/m3 foam but with a spar made from wood or high density foam to spread the compression loads into the blade. It is also possible to create a carbon web joining the skins together, or rout several grooves in the core, and fill these with UD carbon to provide support to the skins. If you are using an engineered core it is important to taper the high density part to avoid point loads at the end. Lower density core is quite flexible and delicate so it can become difficult to shape on thinner sections particularly close to the trailing edge.

One of the most common failure modes with light weight foam foils is for the core to crush against the sharp bottom edge of the boat, this takes the skin on the compression side out of column and quickly leads to a broken foil, which is the reason for using either the higher density core, or dual density core.

Foam is typically sold in quite large sheets, of different densities, and thickness. It is generally possible to get a sheet of the required thickness, sufficient to make several foils. However if this is not cost effective, you can bond two thinner sheets together. The foam resists shear forces from one side of the foil to the other so if using two thin sheets the bonding between the layers is important for the final strength of the foil.

Shaping the Foil

The shape of the core will determine the final shape of the foil so getting this right is an important first step. It is also important to remember when shaping the foil that the laminate will add around 1-2mm to the thickness of the foil shape, and that the skin will need to protrude several mm past the end of the foam, or wood to allow for a solid trailing edge.

There are a few techniques used to judge the shape while you are removing material.

Templates

Templates of the final shape are made from wood or other stable material, if there is significant taper on the planform then extra templates are required for the smaller sections. These templates need to be smaller than the final shape by the thickness of the laminate. The Centreline is marked on the blank, once the shaping is complete the template should match up against it. Initially material can be removed quickly using a belt sander as you get closer to the shape a flexible long board can be used to avoid dimples and hollows. You should check regularly with the template to ensure that you are removing the correct amount of material.

Hot Wire

Commonly used for making model aeroplane wings a polystyrene block can be shaped by cutting with a hot wire. The wire is held under tension using a bow, it is heated by passing a current through it from a battery (this needs careful control to avoid melting or softening the wire). The bow is drawn across a template fastened to each end of the blank to cut out the shape of the foil. If the wire is not tight enough it can move away from a straight line causing a hump, or hollow in the middle of the foil. This technique can only be used for a parallel or straight taper foil, although you could hand shape a more complex tip shape after cutting. Polystyrene foam is very low density so a foil built from it will need either a spine, or additional reinforcement to prevent crushing, just using a thicker laminate may not be sufficient.

Routing Contours

A router can be used to cut grooves along the length of the blank at the required depth for the position along the chord. The contours for each depth are marked onto the foil, and groves cut with the router following the appropriate contour. When sanding it is easy to see if you are approaching the required depth as the grooves will start to disappear. Once the core is rough sanded the final sanding should be done with a flexible long board to avoid bumps and hollows.

Routing contours printed on paper and glued to core A Partially routed foam centerboard core Core after routing and shaping ready for layup

CNC Machining

The shape of the foil can be designed in a 3D cad program which is then sent to a workshop where it is used to program a CNC milling machine that cuts the shape out, a step distance of 1mm is typical. A higher density foam is often used for this as it needs to remain stable during the milling process. A light sand will be needed to remove the ridges from the milling process before the foam blank is ready for laminating. This is probably going to be the quickest option, and for a complex shape may give a more accurate blank, however that will depend on the design, and on the accuracy of the machining. To get an accurate shape the blank needs to be held solidly. If you mill the foam away to a thin edge, then it is likely that the blank will distort under the cutting head, ruining the shape. Small concave shapes are also hard to cut due to the shape of the cutting head. For this reason CNC machines may be more suited to making plugs, or moulds, rather than the finished foam core.

In a Mould

It is also possible to build a foil in a mould, this is typically done with two half shape moulds for the skins, these are then bonded together around a roughly shaped core. There is a large time investment required to build the moulds so this technique is good for building a large number of foils of the same shape, rather than prototyping a new shape. Particular care needs to be paid to the join between the two half skins to avoid splitting. An internal flange could be used, packing could be used on the leading edge to allow you to laminate a strip of cloth on the outside of the blade, and then fair it in. If the foil is built in a mould then expanding epoxy foam can be used to form the core of the blade. There have been some L shaped T-foil rudder moulds made, these use two different Ls for the rudder and top surface of the lifting foil, and a separate flat mould for the lower surface of the lifting foil. This allows you to make the joint smaller, which should reduce drag.

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The Layup.

Skins of unidirectional carbon and woven carbon cloth, will give lowest weight and maximum stiffness. Start with layers of 200 g/m² unidirectional carbon, one layer over all of the foil, plus a second layer of 200 g/m² unidirectional carbon over the top half, and a third layer of unidirectional carbon, approx.150mm wide, over top part of the foil, extending approx.100mm past the bottom of hull when the foil is right down. Then add one layer of 200 g/m² woven carbon cloth, and finally a layer of 86 g/m² glass cloth, both covering all of the foil. If you wish you can use white pigment in the top layer to enable you to produce a white foil without painting it.

Laminated foil ready for trimming and finishing

Laminating it

It is often a good idea to skim the board with a mix of micro-balloons and resin to fill the broken cells in the foam with a lighter mix than pure resin, it can also help to get good adhesion between the skin and the core. Suspend the blank with the leading edge horizontal by means of screws in timber supports at the head and tip of the board. Cut the carbon oversize, approx. 50mm wider than the board. Using a roller, wet out the board with resin, apply the UD carbon, aligning the fibres along the leading edge. Wet out the fibres with epoxy, leave a few minutes to soak in and then wet again. Next comes the layer of carbon cloth. Again wet it out off the foil, wait a few minutes, and the roll on more resin. Finally add the layer of glass in the same way. Squeegee the excess resin away, remove air from the laminate, then squeeze along the trailing edge overlap to remove air. Check for bubbles under the glass, and squeegee out.

After approx. 1 hour it’s time to add filled epoxy which will be used to fair the foil. Make up a reasonably runny filled epoxy mix that will roll on nicely. If you're going for a pigmented foil then use glass bubbles and white pigment. If you're going to paint it anyway leave out the white pigment, and if you have ambitions for a clear finished foil (your laminating better have been really neat if you're attempting this) use silica in place of glass bubbles. Anyway roll on a coat of filled epoxy and repeat twice more ( 3 coats resin/glass bubbles). When the layup is part cured (still a little flexible, but no more than that) trim the excess glass from the head and tip with a sharp knife. Leave the foil to cure overnight

Remove the supports and screws, and trim the trailing edge to the finished size, using a jigsaw with a carbide tipped blade. Trim and sand the head and tip areas. Now it’s time to glass coat them. Apply 2 layers x 200g/m² glass to the head edge, and 3 x 200 g/m² cut on bias at ±45° around the tip of the foil. Cure overnight.

File / sand all the edges to shape. Apply 2 coats of resin/glass bubbles to the head and tip. Cure overnight, and then the next day post cure the foil at approx. 45°C for 3-4 hours (A wooden box & a fan heater does this nicely).

Vac Bagging

If you have the facilities to vacuum bag your foil then this can help to consolidate the laminate and remove excess resin, see the Vac Bag how-to for more information.

Finish

Now the hard work begins! Start by spraying on a guide coat ( car paint ) of some contrasting colour with the layup (bright red perhaps). Sand all the paint off! You should have enough filler to sand pretty aggressively (Andy Paterson uses a belt sander with 40 grit abrasive) without cutting (much) into the glass. On no account whatsoever cut through the glass, and cut into it as little as you possibly can. If it goes black stop immediately!

Now suspend the foil with the screws and blocks as before, leading edge up. Finish the foil with 2 coats of neat epoxy (with white pigment if appropriate. Leave the first layer to cure for approx. 1 hour, then a 2nd coat. The epoxy will fill and flow over the big scratches. When this has cured fill suspension holes, and apply 2 coats of epoxy on the head edge. Apply another guide coat of paint and sand, sand, sand, the finish coats. Wet sand it starting with 100 grit, working through 180/280/400 and then finish off with 600 wet sanded to give smooth matt finish. Most people think this is all the finish required, but feel free to carry on with 800 grit and 1200 grit and then an abrasive polish to get a real mirror finish. It will last at least a day on the beach!

Further Reading


tech/foils.txt · Last modified: 2013/06/25 15:55 (external edit)