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 How To Build High a Performance Rudder
(or Centerboard)

By Jay Moran

(Look to the Nav Bar to the left for specs on some rudders or keels.
We only have a few but we'll post them as we get them!
If you can help by measuring your Rudder or Centerboard supplying dimension
it will of great assistance to a fellow Paceshipper!)

Many of you may have seen my story on how I broke my rudder.  Although I replaced my rudder because my original broke, it turns out there are other reasons why one might consider replacing their rudder (or centerboard).  This article specifically goes over the steps of designing and building a rudder, though the same steps and theory apply to most centerboard designs.

 As you will see, rudder design can have a significant impact on your boats overall performance.  A properly designed rudder can result in improved rudder authority which and reduce weatherhelm.  A good replacement rudder design can also actually increase the net speed of your boat through the reduction of drag and as a result of more lift with less drag, indirectly increase windward performance.

In this article, I will cover:

The Theory of Rudders (lifting surfaces)

Rudder Foil Design

Rudder Design Configuration

Construction Methods/Materials

Modifying Your Existing Rudder

Construction steps for a replacement rudder (or CB)

Note: You can click on any photo on this page to see a larger version!

Construction of a new rudder is really straight forward technically although there is a bit of manual labor and craftsmanship required for the process. Nevertheless, one should be able to complete their new rudder in about 20-30 hours (not including waiting time for epoxy and paint curing!) with a few hand and power tools.

 Proper design is critical and requires some mental exercise to assure maximum performance.  Interestingly, although one might think a rudder specifically designed for a boat MUST be the RIGHT design, my (relatively unscientific) sampling of rudders on boats in local boatyards indicates few rudders seem to be designed for optimum performance!

Rudder Lift Theory

Many intuitively think a rudder does its work by deflecting water to one side thereby exerting a force on the rudder that causes it to push the stern of the boat in a direction opposite the deflection. This is at best an over simplification and really, it is lift that (the difference in pressure between one side of the rudder and the other) that does the work. Lift is generated by the slowing of the flow of the fluid in a region very close to an air foil's surface called the boundary layer and it is this "drag" caused by the water's viscosity that creates the lift.

In the diagram to the right (Figure 1), the same amount of fluid is flowing between each set of streamlines. As a result, the speed of the fluid flow increases where the flow lines narrow and decreases where they widen. A fundamental of lift is predicted by Bernoulli's equation which mathematically shows that where a fluid flow velocity increases, the pressure drops (and vice versa) and it is this differing pressures that causes the force we call lift.
Figure 1

As the angle of attack (AOA - angle of rudder centerline to the flow of water) is increased, the lift increases as does drag to a point. This point where the lifting surface stops lifting is called the "stall angle".  The stall angle is a critical point where lift often decreases to near zero almost instantaneously and the lifting surface offers only drag. 

This is caused by the separation of the boundary layer from the lifting foil creating turbulence which increases pressure reducing or eliminating lift. Additionally, the turbulence increases DRAG significantly.

As we know, the "flat plate" rudder will produce lift (many of the smaller Paceship boats have aluminum flat plate rudders in fact), however, the flat plate rudder is relatively inefficient for all but the smallest angles of attack because the boundary layer begins to separate at relatively small angles of attack.

Figure 2

We can improve rudder performance by shaping it in such a way as to reduce boundary layer separation. We do this by providing a shape that more closely follows the natural streamlines (as shown on the top of the plate in Figure 1 above) reducing abrupt changes in direction for the flow.. 

Such a design is familiar to us and as seen on airplanes and sails (as in in Figure 3.)

Figure 3

Some other points:
When a rudder is running in neutral position, the lift of the symmetrical surfaces is canceled out and there is no directional control. One can also imagine holding a rudder so that its centerline is perpendicular to the centerline of the boat.  In this case the rudder acts only as a brake offering no directional control (lift) and all drag. Somewhere in between is the stall angle where drag begins to significantly exceed lift.  This point is typically at between 25 and 45 degrees.

In designing a good rudder, we also must be concerned about the foil's leading edge. The leading edge should be designed to deflect the fluid smoothly to the maximum chord as rapidly as possible while maintaining good aerodynamic flow (meaning to transition quickly while maintaining and attached laminar flow along the lifting surface). A long trailing edge allows the fluid to merge back with the (flat side) flow smoothly with minimum turbulence.  This attention to minimizing turbulence is critical to create an efficient foil (turbulence creates drag and kills lift).

How does a rudder with TWO curved surfaces create lift?  Suffice it to say, it is the movement of the rudder through the water toward the low pressure side the helps to "straighten out" the flow of  the higher pressure side creating a virtual straight path for the higher pressure side. In short, think of it as another example of relative wind that we are used to thinking about when trimming sails. To Menu

Rudder Foil Design

In order to achieve maximum lift, one must utilize a fluid foil design that is appropriate for the size as well as speed range of the boat.

Foil designs have been well studied, most particularly for use in aircraft.  However, the same principles used in foils made to slip efficiently through air at high speeds also apply directly to  a foil running through water at relatively slow speeds.  It turns out water actually has virtually identical physics (density, viscosity etc.) at slow speeds as air at high speeds.

The repository for foil design standards rests with NACA who catalogs foil shapes which have been carefully analyzed and tested.  After studying various foil shapes, I chose the NACA 0012 foil.  This is a 12% foil shape meaning the maximum cord depth (rudder thickness) is 12 % of chord length (distance from the most forward point of the rudder to the most aft point). 

The 12% foil was selected because it is the most efficient design in the speed range of 3 to 6 kts...the normal sailing speed range of most medium sized ballasted hull sailboats.  A boat that ran faster would benefit from a lower % (thinner)  foil (such as the 0010 or even 0008 foil) while a slower boat might best be served best with a larger % foil (such as the 0013 or 0014 foils).

To the right is a chart of the NACA 0012 foil profile stated as a percentage of chord length.  To determine the chord thickness for any point along the chord length just multiply the rudder length by the chord width number found on the chart and divide by 100.  EXAMPLE:  to determine the chord thickness 30% from the leading edge of a 20 inch rudder, you multiply the rudder length of 20 times 12 and divide by 100.  In this case the chord at that point would be 2.4 inches thick. 

Note also the leading edge of the rudder should be shaped into a relatively blunt elliptical shape.  The ellipse need not be perfect to perform well. Those who wish to form an exact elliptical shape into their rudder can contact me for further details.

Getting the foil shape and leading edge imposed onto the basic rudder shape is the most critical task and some effort should be made to duplicate the profile as closely as possible.  

NACA 0012 Foil

Position along rudder length (Chord Length)
Chord thickness
% of
Chord width
Chord Length

(% of rudder length)

0 0
5 7.1
10 9.4
15 10.7
20 11.5
25 11.9
30 12.0   (See EXAMPLE)
35 11.9
40 11.6
45 11.2
50 10.6
55 9.9
60 9.1
65 8.3
70 7.3
75 6.3
80 5.2
85 4.1
90 2.9
95 1.6
100 .6

That said however, one need not work with tolerances any closer than say 1/16 to 3/32 of an inch.  Under any circumstances, the most important part of shaping is to make sure that the wetted surface is "fair" (smooth with no radical hollows or high spots in the shape transition). 

A good way to check for this is to make a profile gauge out of heavy cardboard (a "female" foil shape if you will) that allows you to check the profile during shaping. To Menu

Rudder Design Configuration

With the foil designed, one must then go about the task of designing the basic rudder configuration.  One can engineer a rudder from scratch, however since you likely already have a rudder or know its dimensions from some source, I won't cover that issue here. 

If you are going to build a new rudder it is best to consider the characteristics of the old one and improve upon it.  The main questions to ask yourself are:

1.  Do you feel weatherhelm angle is excessive? (more than 15 degrees or so?)

2. Do you feel the tiller required excessive force to control the boat?

3. Do you feel the rudder had inadequate low speed authority?

If you are sure you have rimmed your boat out properly (balanced trim) and the answer to questions 1 & 3 is YES!!!, then you need more lift through additional lifting surface (rudder area) as well as a the additional lift that a well designed foil can offer.  You might add from 10 to up to 20% additional rudder area by making it a little wider (and deeper as long as it doesn't extend beyond your keel).  Longer is better where possible as a rudder that extends deeper into "clean" water will be most efficient.  A rudder that is bigger than you need won't hurt you much unless chord length is so long that it requires excessive tiller force.  The additional drag will be minimal.
If the answer to question 2 is yes, you might think of designing the rudder so that a portion of its wetted area is forward of the hinge point (or more than is already present on the current design).   This will put some of the lift forward of the hinge counteracting the lifting force aft of the hinge.  Kind of like adding "power steering" to the boat!  However, don't put more than about 25% of the rudder forward of the hinge point or it will make the rudder unstable (it won't stay more or less where you put it for brief moments when you are busy with other things). I designed my rudder with a chord length of 23 inches with 5 inches of that extended forward of the gudgeon pins (under the transom). To the right is a profile of the finished shape. rud-finshape.jpg (47783 bytes)
Outside of the above, the rudder design configuration is pretty simple and you need only consider your mechanical connections to your boat and tiller when determining the rest of the rudder shape. To Menu

Construction Methods/Materials

There are many construction methods that might be employed successfully to a rudder design.  I won't cover them all in detail, but here are some commonly used methods:

1. Styrofoam core with several layers of fiberglass -It is quick to shape and relatively maintenance free.  Strength is dependent on the thickness of the fiberglass.  Obviously care must be taken not to crush the rudder when attaching pintles and rudder straps.

2. Laminated marine plywood with a layer or two of fiberglass is a good alternative and also fairly simple to manufacture. It has added strength compared to foam cores and will not crush as easily. An added feature to this method is that when shaping, the layers of wood in the plywood become exposed in strips making fairing easier (keep the stripes even and the shape will be very accurate. However, half the grain in plywood is oriented wrong for shear strength, as plywood is manufactured alternating the grains of the layers 90 degrees.

3. Laminated solid wood is the strongest practical alternative. It takes the most work to build but will stand up to a lot of punishment (pound for pound, wood is stronger than steel!). Additionally, although it should be covered with several layers of epoxy, fiberglass tape need not be utilized for strength.

After having broken a plywood core rudder, I opted for the laminated solid wood for my new rudder! To Menu

Modifying Your Existing Rudder

You can modify your existing rudder foil rather than building a new one, by adding material to it to form the foil shape. Since the rudder has no additional structural  requirements, you could glue on Styrofoam or plywood and plane it to a finished foil shape.  You could also make size adjustments to the rudder if required.  A couple of applications of fiberglass mesh and epoxy resin would make the finished shape strong and fair. To Menu

Construction steps for a replacement rudder (or centerboard)

You can build a rudder with relatively few tools.  Here at my condo in Florida, I don't have the benefit of my normal home woodworking shop!  So, I had to do the job with a short list of hand and small power tools.

The tools I used included:

1. Electric drill with a square head screw driver bit and miscellaneous drill bits
2. Power planner
3. Electric sander
4. Router (or a sharp chisel)
5. Saber Saw
6. Putty knife
7. Hex screw driver handle (to receive the square head screw driver bit)

The first step is to make a full scale cross-sectional layout of the foil on paper.  This is necessary to determine the widths of each of the laminate pieces.  It is wise to do as much as possible on the individual pieces to get them into the approximate shape of the foil. This avoids a lot of planing work later.  This paper layout can also be used to make the planing gauge after all the sizes are determined.  Make each piece approximately 1/8 inch wider than the final indicated dimension to allow plenty of additional material for final planing and sanding. 

Keep in mind the laminate pieces have to be wide enough in the area that will extend vertically to fit your tiller and tiller strap widths.

Once each laminate piece width is determined, you must determine the length of each piece to approximate the shape of the rudder.  I used "2 by" construction grade spruce so each piece was 1-1/2 inch thick.  Each piece was hand selected to make sure there were no large knots (more than 3/8 of an inch) in any piece and no knots close to the edge to maximize strength and to minimize filling.
Since I don't have my table saw here at our Florida place, I went to a lumber yard that offered millwork services to crosscut and rip each piece to the proper length and width. The picture to the right shows the individual pieces stacked against one another for a final check of the layout before proceeding.

I checked the individual pieces for width and length and also did a rough sketch on the boards of the final overall rudder shape. 

Once complete I was ready for the next step, which was to drill and pin the individual piece, and do a dry fit. Pinning each board allows the messy gluing process to go quickly and easily because each board has been pre-located to the next.

rud-blanks.jpg (27636 bytes)
rud-pinning.jpg (32789 bytes) The process of pinning involves centering each board on the next then drilling through. At first I tried to measure the centers of each board and drill a hole in each.  However, this was a tedious process that was less than precise with my hand tools. I found that centering each board on the next by eye and drilling offered the quickest and most accurate result. I placed a 5/16 inch dowel pin every 12 inches or so in each board.  It is important to mark and number each board on the same side to make sure they go together in the correct orientation during glue-up.

After the pinning and dry fit was checked, it was time to do the glue up. I used West Systems Epoxy.  I got a box of 2 1/2 inch stainless steel wood screws that box head screws (they have a square slot that is very effective for driving with a portable drill).  I used the stainless steel screws to pull the glue joints together and to provide a little extra structural strength.

A boat builder friend told me that if you have the where with all, he prefers to drill through all of the pinned pieces and insert several brass threaded rods with nuts on either end to help pull the sections together as well as add shear strength to the rudder.  My take on that is that although it might add some structural integrity, it requires that you work very quickly to glue up all the pieces before the epoxy sets.  I think one would have to be very practiced to get it all together in time!

Make sure you do your glue-up in a well ventilated area (outside is best!) and use eye protection as well as an inexpensive mask designed for this purpose.  Use latex gloves while gluing up as well.  Epoxy is a hazardous material that deserves paying attention to safety!  Follow the manufactures directions and safety warnings carefully.

The gluing process involves two steps with the epoxy. First you must "wet" each piece with the two-part epoxy so that the resin penetrates the wood surface.  Make sure you add glue to the pin holes during this process. Epoxy thickened with the recommended filler fibers to make a peanut butter like paste is then added to one of the boards and the pieces are assembled onto their pins. You should put this paste on fairly thick to make sure that contact is 100% between the boards. 

West Systems offers two different hardeners and I used the slower version for this process. I applied the epoxy using disposable paint brushes, although I know some prefer other applicators such as rollers and notched trowels etc. 

rud-glueing.jpg (34703 bytes)

Finally, the stainless steel screws are added to pull the pieces together and hold them while the epoxy cures.  I predrilled holes for the screws. You know you have a good bond when glue flows out along the joint consistently all along its length.  I used screws every 8 inches or so along the boards. When you apply the resin to the boards, they begin to bow away from the wetted surface as that surface swells and expands.  Start with the screws toward the center and work your way to the ends to pull the boards together. Scrape off any epoxy that spills out of the joint as best you can to avoid having to shave the hardened material later!

The boards are glued and screwed one "lift" at a time until the entire assembly is complete.  It is wise to let it cure overnight before proceeding with the work.

The Next step is to cut out the rough shape to the finished shape of the rudder. A skill saw is handy for the straight lines but not necessary if you have a steady hand (or guide) with the Saber Saw!
rud-shaping.jpg (52328 bytes)

The Next Step is to plane the rough foil into a finished shape.  This is best accomplished with a power planer (a WONDERFUL boat builder's tool that is a must in your tool repertoire!).  If you don't have one, one can be acquired starting at about $60.  I highly recommend you buy one!.  If you are courageous and have a lot of tenacity a hand plane will also work!

The job goes quickly with a power planner (it took me about 2 hours).  Once you have shaped the rudder to it's final dimensions finish the work with a good sanding to get a smooth surface.

Note that the trailing edge should be about 3/8" inch thick and finished in a radius.  Any thinner than this would make the trailing edge too fragile unless you were to fiberglass it.  I didn't bother on my rudder and stuck with the thicker trailing edge.

Once you have gotten the shape cutout, planed and sanded, you can cut in any necessary slots or other modifications to the basic shape. The picture to the right shows one of the slots I routered into the rudder for the pintles.  This was required since the rudder was almost 3 inches thick at this cross section and my Heavy Duty "J-24 style" Schaefer Pintles were 1-1/2 inches wide (you won't likely find any wider!).  By the way, the Schaefer Gudgeons and Pintles are bullet proof and I highly recommend them.

Since my rudder extended forward under the boat, I also had to shape that portion below the waterline but above the forward extension in a "Bow shape" to give it an aerodynamic shape (to reduce drag).  The bow shape cutout is shown to the right of the pintle slot in the picture.  I did this easily with a sharp chisel and finished it with sanding.

rud-pintle.jpg (37606 bytes)
rud-filling.jpg (25647 bytes)

Once you are satisfied with shape of the rudder and have cut in any modifications necessary, it is time to fill any voids and cracks.  You should use epoxy for this as well.  I used West Systems Epoxy mixed with a "fairing filler" additive (which is made up of resin "micro-balloons").  

Add enough filler to make a peanut butter like consistency paste. Use a putty knife to apply it.  For this and all the rest of the steps, I used the fast cure hardener to significantly cut down the time between applications of filler and epoxy coatings.  Add enough material to make sure the voids are completely filled as there will be some small amount of shrinkage as the resin cures.

Once the filler cures, carefully sand the rudder to a nice smooth finish.

Now you are ready to coat the rudder with several applications of epoxy to aid in keeping water away from the wood!  You could apply fiberglass tape to any areas you deem necessary for extra strength.  I decided it unnecessary for this stout appendage.  

Apply the epoxy with no additives (it goes on pretty thickly) and sand between coats.

rud-epoxy2.jpg (27900 bytes)
rud-epoxy.jpg (53961 bytes)

After several coats of epoxy and sanding, the rudder will take on a deep sheen.  

The rudder was painted (black) to provide maximum UV resistance for the epoxy although any color would be fine.  A few coats of anti fouling paint was applied to the area below the waterline.  The pintles and tiller straps were fitted to the finished rudder and attached with new stainless steel hex head screws. The rudder was ready to go!

Since I was careful to get the dimensions close on the pintle placement, the rudder dropped right into place on the stern.

Of course, the rudder had a little buoyancy so I had to actually hold it down as I dropped the pins in but outside of that the operation was a cinch!

The new foil is AMAZING in it's performance improvement. Low speed authority is much improved and weatherhelm deflection much reduced. I believe the boat can go to weather a lot better as well. At all speeds, the flow of water over the rudder remains laminar and no turbulence can be seen along any point on the rudder which means drag is minimized.  The "power steering" built into the rudder has reduced tiller force tremendously without sacrificing stability.

I strongly recommend you take a look at YOUR existing rudder to see that it is a proper foil.  The exact foil shape is not as critical as long as the chord width is about 12% of the chord length and the widest part of the foil is placed about 30% of the distance back from the leading edge. If it is not, you would really benefit from a new or modified rudder shape!

The same steps apply for a centerboard design and construction albeit with slightly differing dimensions!  Also, note a centerboard must be WEIGHTED or else it won't sink! This might be accomplished by drilling holes into the centerboard filling them with molten lead or even making a lead casting for a tip piece.  If you are unsure as to how much lead is needed, let me know and I will calculate the amount of material needed to assure the it will go and stay down!

I intend to replace my centerboard next time the boat is out of the water.  I am excited about the improved performance I KNOW I can inspire into my boat with a modified design!  I already know the original design is less than optimal.

When I do that, look for an addendum to this article! 

If there are any questions or comments on this article, or I can be of any help to you with your rudder or centerboard project, please feel free to email the webmaster!

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