My sworn ambition is to outrace the average sailing dinghy with my dirt-cheap, homemade canoe sail rig. I am not talking about the high-performance types: the Lasers, M-Scows, or Flying Scots of the world. They will forever remain beyond my competitive reach. I have my sights on the typical, common daysailers out there, like X-Boats, Sunfish, or Capris.

Initially, I had assumed that the realization of my ultimate triumph would entail an increase in sail area over my 42-square-foot rig, and perhaps the addition of an outrigger. But subsequent refinements in the rig, in leeboards and in rudder design, have yielded such performance gains that I may be able to achieve my life goal even without making the contemplated changes. If so, my rudder will certainly prove to be a critical piece in the puzzle.

Prior to the development of this rudder, I had relied on a paddle for course control. Although my Viking forbears (having spent my formative years in Minnesota, I feel entitled to claim kinship) would have approved of such a primitive mechanism, its use had a number of drawbacks. I was trapped in or near the stern of the canoe, where my live-ballast was least advantageous. I had to handle the steering paddle with both hands while tending the
mainsheet, so I was unable to give full or proper attention to either. If I drew the paddle out of the water even momentarily, the canoe would immediately luff up and the rig would depower. While these made for an exciting, engrossing sailing experience, I necessarily paid a price both in convenience of operation and in speed.

The most critical variable of a rudder is its size. Rudder area is usually tabulated as a function of a boat's lateral plane. However, the type of boat under consideration and its operational speed are also factored in. Faster boats need smaller foils (proportionally) than slower ones. A sailing dinghy typically has a rudder representing 15 percent its of lateral plane; displacement yachts have rudders closer to ten percent. Philip Bolger designed a series of
dinghies with deliberately oversized rudders. His intention was to have the rudder pick up part of the centerboard's function. The centerboard could then be smaller, creating less of an obstruction in the cockpit and the boat's draft could then be reduced modestly. Bolger reported general success with this experiment but with some reservation. A larger rudder, in order not to become too unmanageable, must be a balanced rudder, carrying part of its area forward of its pivot point. But a balanced rudder comes with a number of complications; installation and removal procedures being among them.

My version of the Radio Flyer rudder is quite a bit smaller than the
guidelines call for. My Old Town camper canoe, when loaded with me and 20 pounds of sailing gear, has a lateral plane of slightly over one-and-three-quarters square feet. When the canoe is heeling, just under one square foot of the leeboard is submerged. The total lateral plane area,
then, is approximately two and three-quarters square feet; about 400 square inches. The area of my Radio Flyer rudder is 28 square inches; a mere seven percent of the lateral plane, or less than half of what the guidelines call for in a sailing dinghy.

It may be irrelevant to the rest of the world, but I also find it informative to compare foil size to sail area. A twenty-eight square-inch rudder represents slightly less than one half of one percent of a 42 square foot sail. By way of comparison, this is one-third the relative size of my Catalina 22's rudder. Her submerged rudder area is just over three square feet and her sail area is rated at 212 square feet.

By any measure, my canoe rudder is undersized. Naturally, I pay a price for this responsiveness. If wind and wave prescribe otherwise my canoe does
not follow her skipper's commands. The up side of this, however, is speed. If less effective, an undersized rudder necessarily creates less drag. In addition, an undersized rudder is not subject to the same pressures as a large one. As a consequence, its construction can be light, as well as its weight.


Yet a rudder's size is not the sole determinant of its effectiveness. Designers apply the concept of "rudder volume"* which also factors in the length of the lever arm. A rudder's volume is the product of its lateral area times its distance from the boat's hydrodynamic center. In theory, if you mounted your rudder on an arm that extended far astern of your transom, a little, tiny rudder would suffice - at least when it wasn't dangling in the air.

Depending on a number of circumstances, a foil's aspect ratio will give some indication of its effectiveness. Square foot per square foot, a tall, narrow (fore to aft) foil is more effective than a low aspect ratio foil. Until it stalls, that is. And then the low aspect foil has the advantage. A boat that tends to operate at high speeds can get away with a rudder of smaller area, provided the rudder is of high aspect ratio. The aspect ratio of my C-22's rudder is 3:1; just over three feet in height (or depth) by one foot. This appears typical for the boat trying to strike a compromise between racing performance and cruising.

Somewhat counter-intuitively, a thicker foil is less prone to stalling than a thin one (speaking now of the dimension athwart). For the same reason, a blunt leading edge is preferable to one that is razor sharp. In addition, if the rudder's maximum thickness can be kept well aft, stalling can be further delayed.
This is not to suggest that quarter inch metal plate would not serve as rudder material. It most surely would. However, the low-pressure side of a flat, sharp-edged rudder would frequently be stalled; creating turbulence and drag rather than the lift a well-shaped, streamlined foil generates. It would work, but not as well as its streamlined counterpart.

Does this mean that if you're striving to create a go-fast sailing craft that you want a nice, thick, high aspect ratio rudder? Well, not really. Because then you've increased the cross-sectional area of the object you're trying to move through the water. Your rudder may indeed be very effective in its function, but it's still slowing you down. I have experienced this phenomenon with my C-22; by winching up the swing keel in light breezes, she'll go noticeably faster. In sectional width, too, a compromise is struck between rudder efficiency and speed potential. A thick section improves a rudder's function, but increases its resistance.

Finally, in a discussion of foil efficiency, there is the issue of power loss at the foil's extremities. Obviously, any wind that is deflected over or under your sail is wasted, providing no thrust. This is equally true of underwater foils. To prevent water from being deflected downward along the bottom edge of a rudder, Philip Bolger (Boats With an Open Mind) has recommended the addition of a small, horizontal plate. He claims that this minor refinement dramatically boosts the rudder's effectiveness. Wings on keels, likewise, purport to have the same effect on keel efficacy.

Power loss downwards is only half of the story, however, since a power loss can also occur at the top of the rudder. Rudders that pierce the water's surface are particularly vulnerable to power loss. At certain speeds and angles of incidence, "ventilation"* occurs. Air is drawn in and down the rudder along the low-pressure side. The suction side of the rudder starts, in effect, to suck air. Naturally, the portion of the rudder functioning in air is exerting no force. It is serving no useful purpose. The way to avoid this is to mount your rudder (or keel, centerboard, or leeboard, for that matter)underneath your hull, where it's never exposed to air.

Given the preceding, does my Radio Flyer rudder reflect a careful, methodical analysis of design features, or an inspired synthesis of same? In a word: No. My literature search was conducted largely after the fact. Instead, the rudder design is more a product of seat-of-the-pants engineering dictated by the materials I happened to have on hand. This, in turn, meant that it was destined to a be low-tech and inexpensive device. The single constraint I
imposed on the design was that the submerged area of the rudder be as small as I could get away with. I was willing to sacrifice responsiveness for speed. But having no notion of what the minimum surface area might be, I built in the possibility of enlarging or reducing the area. The rudder fins extend up over the cross-member. I can adjust the rudder area simply removing a half dozen screws, sliding the fins up or down, and remounting it in the desired
location.

The rudder blades or fins are half-inch plywood, two and three quarters inches wide. The trailing edge of the blade is ripped at a 45 degree angle to minimize the elbow grease necessary to achieve the final shape. The thwart is a one-by-four, ripped to the same two-and-three quarter inches as the rudder blades. The corner brackets or reinforcements are two-by-four, ripped and mitered. These items are assembled as shown in the isometric. I use a toilet tank bolt as a pivot. Fortunately, for those of us who are rudder makers, toilet manufacturers have thoughtfully provided us with solid brass bolts. They have even shown the foresight to include wing nuts. (rudder isometric diagram)

Since the rudder has two fins which extend into the water, a balance of pressures occurs, and helm pressure is very light. In fact, were it not for the shock cords, the rudder would remain perpetually at whatever angle of attack it was set. With this rudder, you never have to wrestle to stay on course; you have a low-tech equivalent of power steering. A rudder which automatically wants to maintain its angle of incidence is conducive to course stability. I
have found I can set my canoe's course, release my hold on the rudder line(s), and manipulate her course by trimming the sail, or by shifting my weight; either athwart, or fore and aft.

Further, since rudder area is small to begin with and divided into two parts, each fin is only half as large and half as deep as a single rudder of equal area. As a result, the rudder's draft is minimal. This is a distinct advantage to the shallow water cruiser. Since the rudder fins extend a mere three or four inches below the canoe's keel, a kick-up mechanism strikes me as unnecessary. However, such a device would be easy to incorporate, given the light
pressures involved in the normal operation of the rudder.

I have mounted this rudder on both the stern and bow. It works in either location. When located in the bow, it is closer to the canoe's hydrodynamic center, hence rudder volume is necessarily reduced. If I intended to keep it there, I might choose to enlarge the fins, somewhat, to compensate. Or, alternatively, I could mount it on a bowsprit at the appropriate distance from the hydrodynamic center of the canoe.

The leeward blade or fin of the rudder will invariably experience more pressure than the windward one. The boat's heeling immerses the lee fin while drawing the windward one out of the water. Further, since the boat is necessarily sliding leeward, the leeward fin will be "upstream" of both the hull and its windward counterpart. The American Flyer rudder takes advantage of this situation by keeping the flat surface of the lee fin facing leeward.
(diagram-rudder sections) Reverting to our airplane analogy, this means that the flat side of our "wing" (or our most effective wing) is facing down (or the equivalent thereof). The individual who is skeptical that much advantage is imparted might consider how well an airplane would fly if its wing were upside down. Marchaj (Sailing Theory and Practice) reports that there has been some attempt to exploit this principle by incorporating paired centerboards into a sailboat's design. In this arrangement, the port board - with its flat surface to port - is lowered whenever the boat embarks on a starboard tack. It is a
feature that obviously has yet to capture the public's imagination, but its theoretical appeal is impeccable.

On the other hand, it might not prove difficult to build in a weather helm and to create a mechanism that encourages the boat to weathercock. The hydrodynamic center could be moved forward and/or the center of effort aft. Or, perhaps, simply ballasting the boat to trim by the bow would achieve the desired effect. ( See Small Craft Advisor, May/June '01, for a discussion of helm balance). A third option might be to re-tension the rudder cables upon
coming about; the weather one tightened, the lee line slacked. Or, perhaps, the sail could be sheeted to the weather rudder cable in a way that if everything were let go, the tension on the sheet would make the boat heave to.