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
The Theory of
Rudders (lifting surfaces)
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.
As the angle of
attack (AOA - angle of rudder centerline to the flow of
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.
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.)
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
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
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
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
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
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.
Position along rudder length
of rudder length)
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
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.
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:
Do you feel weatherhelm angle is excessive? (more
than 15 degrees or so?)
Do you feel the tiller required excessive force to control the boat?
Do you feel the rudder had inadequate low speed authority?
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.
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.
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.
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:
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.
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.
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
having broken a plywood core rudder, I opted for the laminated solid wood
for my new rudder!
Modifying Your Existing
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
Construction steps for a
replacement rudder (or centerboard)
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
tools I used included:
Electric drill with a square head screw driver bit and miscellaneous
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
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.
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
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
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
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
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
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
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.
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.
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.
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!
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!
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
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.
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
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.
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.
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").
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
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
the epoxy with no additives (it goes on pretty thickly) and sand between coats.
several coats of epoxy and sanding, the rudder will take on a deep
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
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!
I was careful to get the dimensions close on the pintle placement,
the rudder dropped right into place on the stern.