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Multihull designer Bernd Kohler shares his method of building rudders and foils in this short article. The post about his first ever small trimaran design has generated a lot of interest and I am very glad to be able to spotlight one of his time-saving building techniques here.

Please keep in mind, as you read, that English isn’t his first language. I did very little editing of the words below; I think he does a pretty good job at communicating the concept and instructions. I hope lots of folks are able to glean from it and, hopefully, at some point, save a whole bunch of time when making pieces like this.

Be sure to click on images below to enlarge them. (Thanks for sharing this with us Bernd!)

Update – 10-31-14: An earlier article (essentially the same info, except with a few additional photos) was published at Duckworks here – http://www.duckworksmagazine.com/07/howto/rudder/


How to Build Rudders or Dagger Boards
by Bernd Kohler

There are many ways to build dagger boards and rudders. I am a multihull designer/builder who has built, I do not know how many, rudders and boards. I was trying all sorts of building methods, plywood glued together, foam sandwich, building them up from wooden pieces. But all of them needed a lot of hours. Till I found this way. The rudder on our PELICAN catamaran where the first. The rudders are 2,20m (7,2ft) long.


I prefer this method over all the others I have tried out. Because the construction time is short. The boards or rudders are light and strong. The surface is smooth and maintains the chosen section over the whole length.
They can be made in the shape of most sections. Or a good approximation. See as example the rudder and the dagger board for the LITTLE TRI (image below). For easy terminology I call them from now on foils.



The illustration shows the set up of such a foil. The foils are build up as follows. The base is in this case 4 mm plywood. The wood piece determines the thickness of the foil. Pay attention to reduce the width according to the UD carbon. With 300 grams reduce with 2 x 0,8mm. The wood is situated at 30 % of the chord of the foil.


— First, build the jig as shown in on the photo. Use a straight piece of wood as base. Mount particle board on top. The side pieces should be about 180 mm high. The inner angle of the pieces should be not more as 26°. Otherwise the plywood will crack when bending into shape. Make at least three of these pieces and mount as shown. Mount the pieces 4 mm from the center line. The length of the jig should be about 20 mm longer as your foil. Isolate the surfaces with plastic tape.


— Cut the base for the foil, use 4 mm plywood.
— Scarf the trailing edge according to plan. At the end about 1,5mm.Apply Epoxy on one side of the panels. This will be the inside. Stick the two panels, which will form the leading edge with plastic tape together. Place the panels in the jig.

— Fold these to the jig inner flankes. Use the board cut outs from the jig as wedges.
— Close the front and the end of the panels at the leading edge with plastic tape.

Now, as shown in the photo below (brown material)…
— Mark the height for the epoxy mix.
— Make a mix of epoxy, micro spheres and fused silica.
— Use a spirit level to place the jig level in both directions.
— Fill in the epoxy mix (see also picture . Any spilled epoxy has to be removed. Watch for correct height (12 mm) in this case. Let the Epoxy mix properly cure.
— Prepare the inside spar.


Now, as seen in the next photo (below)…
— Wrap the stringer in UD carbon (300 grams) and apply epoxy and let cure.
— Mark the position of the spar. Mount the spar to one side with wood screws.
— Push the trailing edge together using lumber and big clamps. Screw the foil together. Mark all positions.
— Remove the screws in the spar on one side.
— Carefully open the foil. Remove the spar complete.


Now we can assemble the foil.
— First, isolate the lumber with plastic tape on the surface which will be on the side of the foil.
— Apply Epoxy glue to the spar and the gluing area inside the foil.
— Find the holes for the screws and fasten them.
— Apply Epoxy glue to the other side and the trailing edge.
— Close the foil with the lumber and clamps. Fasten the screws on the opposite side of the foil.
— After curing remove the clamps. Remove the screws.
— Fill the screw holes with Epoxy putty.
— Round of the leading edge. The result will look like this…


— Close the ends with wood pieces and shape to your liking.
— Apply a layer of glass cloth. We use for small rudders and boards 120 grams glass.
— Finish the surface to your liking and paint.

Here as example the rudder on LITTLE TRI