Shaping Dinghy Foils

This started as a post in the series on building the Argie 15 on Dudley's Blog. It became too long and involved for that blog, so I simplified that post and left the more in-depth information for this post.

The first part of making any wooden yacht foil, whether a rudder, daggerboard, centreboard, keel, leeboard or whatever, is to make the blank that will be the material of the core. For the Paper Jet I used cedar or poplar planks, depending on the size of the particular foil, edge-glued to each other to achieve the required width. For the Argie 15 we used a combination of cedar and poplar strips that were cut from planks. These were laminated to take advantage of the colour differences of the two species to produce very attractive striped foils. For some designs we use multiple layers of plywood, bonded together. This post is mostly based on the striped poplar/cedar boards of the Argie 15.

Gluing blanks for the Paper Jet foils from cedar planks. Click on the images to enlarge them.

Laminating the Argie 15 daggerboard blank from strips of cedar and poplar.

The blank must be as straight and flat as possible. That means keeping the strips or planks all in the same plane, without steps at the joint lines. Any variations will reduce the thickness available for shaping the foil because it must first be sanded flat on both sides.

Once glued-up, smoothing of the blank can be done with a belt sander with a coarse belt (80-100 grit), a hand plane, or a combination of the two. If you have a planer/thicknesser available with enough width for the foil that you are shaping then that is the obvious tool to use.

A note about sanding belts would be appropriate at this time. Your belt sander will remove a lot of wood before your foils are fully shaped, so it is worthwhile to pay a bit more for good quality belts that will last the task. I used an 80 grit ceramic belt that is expensive compared with conventional belts but well worth the extra. Whereas conventional belts wear out and normally get thrown away because they have lost their cutting power although still in one piece, my first ceramic belt was still cutting well when the joint parted, having already lasted about as long as five conventional belts.

When the blank is true and flat, draw the outline shape onto it and cut it out with a jigsaw or bandsaw. If it has a rounded toe, don't round it off at this stage. Cut it with a sharp corner because this will make it easier to draw the foil section correctly. The blanks that Kevin laminated for me had been cut to shape already before I received them, so the photos of the striped foils in this post have the toes already rounded. Leave cutting the rounded toe until the foil has been shaped.

Once the outline shape has been cut, mark onto the surfaces both sides the part of the foil that will be shaped to a foil section shape. For a daggerboard or centreboard this will normally coincide with the bottom of the hull. For a rudder it is normally aligned with the bottom of the rudder stock.

The next step is to mark the foil section shape on the bottom edge of the blank. First draw lines exactly along the centreline of the blank along the forward, bottom and aft edges of the blank. When shaping the blank, make sure that you don't lose these lines, redraw them in exactly the same position at any time that they are becoming faint from sanding etc.

If your plans have foil section patterns then cut each out as a rectangle large enough to fold down both sides of the blank to secure it. Position the pattern over the bottom edge of the blank exactly aligned with the centreline of the pattern on the centreline of the blank, with the trailing edge of the pattern at the aft edge of the blank. Tape the pattern securely in this position or hold it down with push pins.

Paper Jet rudder foil pattern wrapped over bottom edge of blank, ready for transferring the shape. 

Using a bradawl or push pin, prick through the pattern along the lines that show the section shape, making holes in the wood. Make the holes close enough on tightly-curved sections to clearly show the shape but they can be further apart where the shape is more straight. Remove the pattern from the blank, then draw the curve by following the holes pricked into the wood. That gives you the shape for the bottom end of the blank.

If the foil that you are shaping is foil-shaped all the way to the top then repeat the process at the top of the blank. That will set you up to shape the ends exactly to the patterns and to use a straight-edge to ensure that all points between the ends are also shaped correctly. To do this, rest the straight-edge on the foil, aligned so that each end is at the same relative position on the two ends of the foil, eg. 25% from leading edge at both ends. The surface should be straight top-to-bottom, so you should feel no end-to-end rocking of the straight-edge (indicating a convex surface) nor should you see any light between the straight-edge and the foil (indicating a concave surface). Use this as your guiding principle when shaping the foil.

If your foil will not be shaped all the way to the top then you need to make a template to show the foil section shape at the top end of the shaping. This can be from plywood, stiff cardboard, plastic or sheet metal. It must have a concave edge accurately drawn and cut to the foil shape so that you can lay it against the foil to check that you cannot see light between the template and the foil.

If your foil doesn't have straight leading and trailing edges, i.e. the plan shape is elliptical or curved along the leading and/or trailing edges, then you cannot use a straight-edge to check fairness. The surfaces will be curved instead of straight as long as at least one of the edges is curved. In that case you will need to make multiple templates, showing the shapes at various positions. Mark all of those positions accurately with small diameter holes drilled deep into the surfaces near to the leading and trailing edges, so that you won't lose the positions by sanding. (These holes will all be filled with epoxy when the coatings are done later.) Check the templates against the foil frequently while shaping, to give you accurate shapes at those points. For the spaces between the templates you can rely on your senses of sight and touch to get a smooth surface.

This is the rudder of the CR33 design, with locations of templates used for shaping. The green curves are template shapes and the red lines connect the points of maximum thickness. In this case they are straight lines because the leading and trailing edges are straight lines. That makes it possible to use a straight-edge to check accuracy of the foil shape.

The next line that must be drawn onto the blank before shaping begins is the location of maximum thickness of the foil. This position will vary depending on what section shape is being used but is normally from 30% to 35% aft of the leading edge of the foil. The exact position of maximum thickness comes from carefully researched and documented shapes that produce the least drag under different operating conditions. The roots of the research methods date all the way back to wind tunnel testing by early aircraft pioneers like the Wright brothers and have been refined over the past century and more by NASA and other authorities. They have tested hundreds of shapes to establish their lift/drag characteristics for designers to compare and select what will best suit what they are designing.

That research was for aircraft but is equally valid under water as above. Sailboats have three or more wings, some operating in air in the form of sails, the others in water in the form of rudder, keel etc. Sailboats are the only forms of transport that operate with wings in two different media, in the process deriving power and motion from both.

OK, back to the foil section and marking the point of maximum thickness. Look at the two sections in the diagram below. Both are the same chord length (distance from leading to trailing edge). Leading edge is to the right, trailing edge to the left. Both are the same thickness but the points of maximum thickness differ. The upper section is a NACA 000 series foil, which has maximum thickness 30% aft of the leading edge and it is fairly full in the forward sections, with almost straight lines fairing into the trailing edge. The lower section is a NACA 64A series foil, with maximum thickness 35% aft of the leading edge, finer forward sections and slightly hollow (concave) surfaces fairing into the trailing edge.

NACA 000 series at the top, NACA 64A series at the bottom, with very different distribution of the thickness.

For us operating a sailboat, we may not feel a difference between the two shapes as they move through the water but they make a difference to performance. The 64A section creates less drag than the 000 section when water flow is closely aligned to the direction of travel through the water but it stalls out easily at low speed and also if the direction of flow diverts more than a few degrees from the centreline of the foil. In contrast, the 000 section can divert a lot further out of alignment with the water flow and it is much harder to stall. The water, if coming from a few degrees off to one side, can flow more easily around the more rounded leading edge of the 000 section than the sharper 64A section. That makes the 000 section more forgiving. The result of these differences in characteristics is that the 000 section is well-suited to rudders because they are out of alignment with the water flow most of the time, alternately out of alignment to one side or the other, due to helm corrections. It is also suitable for keels on slow boats that can benefit from increased lift at low speeds. The 64A section is best used for keels on fast boats, to benefit from the lower drag.

Mark the point of maximum thickness onto both sides of the foil at each template position. If your foil has straight leading and trailing edges, draw a straight line connecting the points at top and bottom of the foil. If the foil has multiple templates then use a flexible batten to draw the curve that connects all of the points of maximum thickness. These lines are important for guiding your accurate shaping of the foil, so don't sand them off. If they start to fade while you work then draw over them again to keep them clearly visible. Don't sand them off until  you are happy that the shaping is complete and you are doing the final clean-up before coatings start.

Our Argie 15 daggerboard with the upper limit of the shaping marked with a line across the board and the line of maximum thickness drawn lengthwise on the board.

The area in front of the maximum width is curved, so it is most easily shaped with a sharp hand plane, taking off a fairly small amount at a time. This tool gives good tactile feedback to your hands so that you can judge the angle that the blade is removing wood and also where there are lumps and bumps. You can feel this through the changes in angle of the plane against the wood, along both the length and width of the plane. The plane allows you to gradually work down to the template shape in very small and accurate increments.

The area behind the maximum width is fairly flat and needs a lot of material to be removed. Here I find that a 3" or 4" belt sander is the most useful tool. An 80 grit belt will remove a lot of material quite quickly and with good control. The long base plate gives a good flat surface to remove material over a large footprint, without lumps, bumps and gouges as happens with a disk sander and much faster than can be done with an orbital sander. As with the hand plane, the belt sander gives tactile feedback that can be felt through the hands as minute changes in angle that can identify mounds, hollows and other irregularities in the surface.

The belt sander can be used with the belt cutting along the length of the foil, across the foil or any angle between. It can also be moved in any direction relative to the direction of belt movement, so you can use it with the belt running across the board but the sander moving sidewards along the length of the board. This gives your hands feedback about changes in surface angles both along the board and across it at the same time.

Back to the actual shaping process, initially you can use rough-and-ready methods to remove excess material. If you are shaping a large rudder or daggerboard then a power plane can remove a large amount of material very fast but be wary of this machine, particularly if the foils are dinghy-size. It is a brutal machine that will very quickly turn your intended foil into scrap firewood. Use it only for rough-forming the blank then put it away. I prefer to keep it away from dinghy foils.

The closer that you progress toward the final shape, the more gentle and controllable the method used must be. With a belt sander, change to a finer belt so that it will remove less material with each pass. After that change to sanding by hand with a firm sanding block with the sandpaper wrapped around it or sandpaper folded double without a block.

Whichever method you use, do it with a light touch. If you push hard against the surface you may feel large irregularities but you won't feel the small ones. The lighter your touch the more you will feel the smaller variations in angle that identify highs and lows in the surface.

Working with a light touch with a wood plane or a belt sander you will feel changes in angle through changes in pressure transmitted through your hands into your arms. With practice and by paying careful attention to what your hands and arms are telling you, you will be able to shape surfaces that are surprisingly smooth and accurate.

Stop working every now and then and run a hand lightly over the surface, with your fingers "loose" and able to follow the surface. You will feel variations that you can't even see.

Eventually you will arrive at a smooth and regular surface that closely follows the templates. If your foil is full-length of the blank, your shape will now be down to the lines that you drew onto the ends of the blank and your straight-edge will show a straight surface between the ends. If your foil has multiple templates then you will be able to set each template against the surface in the correct position and see no light coming through between the template and the surface.

The Argie 15 daggerboard with shaping completed, ready for glassing.

It is up to you to decide how close you want to get to that perfect surface. For a racer where every second counts and all unnecessary drag must be eliminated, work to achieve a perfect surface. For a family daysailer that is built for fun above speed, work to a level of accuracy and finish that makes you happy.

If you will laminate the foil blank from plywood then you have the opportunity to save plywood. To do this, cut each layer of plywood only as large as needed for that particular layer. Only the centre layer is full size, all others will each be smaller that the layer inboard of it. Laminate in layers that are stepped down in size as you move outward, reducing the area of plywood needed and also the amount of waste material that must be removed. Draw the plywood layers onto your templates to establish where the edges of the layers need to be. You can also use the layers of veneer as a rough guide for shaping a smooth foil. I say a "rough guide" because there are often tiny variations in the thicknesses of the veneers, particularly with less expensive plywood. Those tiny variations can result in wavy lines when exposed at the small angles that result from shaping a foil. Trying to smooth out those lines will result in an inaccurate shape.

This is the rudder blade of the Didi 38 "Black Cat". It has a central shaft of solid iroko, with plywood leading and trailing edges. I laminated only enough plywood to get to the surface of the finished rudder, reducing the amount of waste that I had to shave off. This rudder is big enough to have most of the shaping done with a power plane and the finer shaping with a belt sander. 

The completed rudder on "Black Cat".

The trailing edge needs special attention because it can be the source of vibration that can transmit itself throughout the boat. That vibration is caused by turbulence off the trailing edge in the form of eddies flowing off one side then the other, then back again in a repeating sequence. The eddies and the vibration create a large amount of drag, so much that one can feel the braking effect on the boat as soon as it starts. Eliminate the vibration by shaping the trailing edge in a way that it will not generate those alternating eddies.

One way to prevent the alternating eddies is to shape the trailing edge perfectly, to a clean and sharp edge, with no rounding off. This is an easily-damaged edge though, so it is normal to have a very narrow flat instead of a knife-edge. It is normally OK also to have a very narrow flat filed onto the trailing edge, 1-2mm wide, with a sharp corner both sides. If the edge is 3mm wide and the corners are rounded you will likely get vibration.

Another way to stop the alternating eddies is to purposely create an imbalance between the two sides of the foil. This will make the eddies consistently flow off the one side and not the other, so no vibration. The way to do this is to either file the flat onto the trailing edge at 45 degrees to centreline instead of 90 degrees, or to round off one of the corners of the trailing edge and leave the other one sharp.

This post is already very long, so I will leave glassing the foils for a separate post in a week or two.

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