Construction of Rudders...
By Bernd Kohler - The Netherlands

... and Daggerboards

There are many ways to build dagger boards and rudders. Some are already show in Duckworks Magazine (see links at end of article). I am a multihull designer/builder who 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. 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 any section.

The example shown is for a NACA 4412 section. The picture shows one of the rudders for our PELICAN catamaran which is 10,50 m long. And the drawing examples are from the KD 860 plans.

click to enlarge

Picture 1. PELICAN rudder, length 2,2 m

Start by choosing a suitable rudder board section. In the example the NACA 4412 section was chosen. The rudder cord is 260mm. Because of this small chord only one stringer is used. If the chord is more as 300 mm a second stringer must be used at 2/3 of the chord. To begin, draw the section to scale and determine the angle for the jig flanks. The plywood planking must just touch the leading edge as shown.

click to enlarge

The drawing examples are from the KD 860 plans.

Determine the length of the rudder. This will be the length of the jig. In this case the jig is 1470 mm long. The flanks are spaced 300mm from each other, so you will need 6 flanks. Make them from plywood or particle board. For dimensions see jig drawing upper left. Build the jig base according to the drawing 3. Build it up on a strong base. Again we used particle board.

click to enlarge

Drawing 2: rudder example

The dimensions of the jig corner pieces and inner wood pieces are only examples. But use strong and straight lumber. Cut out the sides of the rudder. Use good quality 4 mm plywood. For dimensions see drawing 2. Scarf the trailing edge as shown in the drawing. Each side must be 1,5 mm thick at the end. This can best be done with an electric planer.

click to enlarge

Drawing 3: Jig

Mark the position of the stringer(s). Chamfer the corners of the stringer. Use good quality Spruce for this part. Mark the position of the stringers on the outside of the panels (center). Drill 2 mm pilot holes. About 200 mm apart. Glue the stringers to one side. Check for correct distance in relation to the leading
edge. Fasten with small nails. Remove the nails after curing. Apply epoxy to what will be the inside of the rudder panels. Stick the two panels, at which will be the leading edge, with heavy plastic tape together. Place the panels in the jig and push them to the sides of the flanges

click to enlarge

Picture 2. Dry run of the side panels. Observe plastic tape on the end and the panel leading edge.

Use the inner pieces of the flanges as wedges. Before applying plastic tape around the corners of the wedges. Close the front and back end of the panels at the leading edge with plastic tape. Mark the height for the epoxy mix. Make a mix of epoxy, micro spheres and fused silica. Place the jig level in both axes. Fill in the epoxy mix (see picture 3). Any spilled epoxy has to be removed. Watch for correct height (15 mm).

click to enlarge

Picture 3. Filling the leading edge with epoxy mix.

Let the epoxy mix properly cure. Prepare 25 x 25 mm lumber as long as the rudder to close the trailing edge. Close the rudder with clamps and the lumber in between (see picture 4 and 5) and check for a good fit. Do this as a dry run. Open the rudder again and correct if necessary. Prepare two UD carbon strips ( 400 gr/m2 ). Wide 50 mm, as long as the rudder. (See picture 4)

Apply epoxy glue to the gluing surfaces leading edge stringer(s) and to the sides that the UD carbon strips have to be glued to. Apply epoxy to the carbon strip. Now partly close the dagger board. Place the carbon strips as shown in the drawing 2. Watch that these are bonded also to the sides. Eventually you have to close the sides a bit more.

When you are sure the carbon strips are in the right position, apply epoxy glue to the trailing edge area and close the dagger board with the lumber and clamps (see pictures 4 and 5).

click to enlarge

Picture 4. Detail of closed rudder.

click to enlarge

Picture 5. Rudder closed.

Screw the stringer(s) down. Use the pilot holes to do so. Let cure properly. Remove the clamps and lumber. Remove the rudder from the jig. Remove the plastic tape on the leading edge. Trim the leading edge. Round off the trailing edge to the correct radius. Remove all screws and nails. Close the lower and upper part of the rudder with a 8 mm plywood piece. Close it on the inside! Fill all nicks, dents and screw holes with epoxy putty. Apply 2 layers of 220 grams/m2 glass cloth. Fill the glass structure with thin epoxy putty and sand down to a smooth surface without sanding in the glass fiber. Apply primer and paint.

Finish the rudder otherwise according to the original drawing. We use this system for building rudders and boards for catamarans form 5,5 m
onwards for more as 22 years. All builders found this way the simplest to build. No customer reported any failure over all the years. The load on lee boards is very high. Use the next thicker section as the designer specified. The boards will be anyway lighter as on the plans.

Bernd Kohler

More Articles about Rudders

& Supplies