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Sea Kayak Controllability

This excerpt from John Winters, The Shape of the Kayak explains why sea kayaks handle like they do.

Sea Kayak Hull Motions

Every force acting upon the kayak causes some form of motion. These are to be divided into six types. Three are linear or act in straight paths and three are rotational and act around an axis. The linear motions are surge, the fore and aft change in acceleration; sway, the side to side motion; and heave, the vertical motion. The rotational movements are yaw, the rotation of the kayak hull around the vertical axis and the natural result of any changes in direction; roll, which occurs around the longitudinal axis, and pitch which is motion around the transverse axis. The causes of these motions can be internal, caused by kayak paddler motion and actions or external and caused by waves, wind, or currents. While all can affect controllability it is yaw, or the horizontal rotation about the center of gravity that constitutes most of what we call controllability.

(Note: In this article the term "kayak" is used for simplicity. Everything said here applies to canoes as well.)

Sea Kayak Controllability

It is common to discuss controllability as if it were two distinct and separate elements: maneuverability and course stability (commonly called tracking). This is a convenient but unrealistic approach for they are closely related in their effects and interaction. Sophisticated mathematical models and empirical testing have been used to predict ship handling characteristics for, unlike kayaks, a designer cannot shrug off a multi-million dollar investment that reduces everything in its path to match sticks as it wanders uncontrollably through a harbor. Kayak designers are more fortunate and kayaks that handle poorly can be abandoned in the backyard with no one (the designer hopes) the wiser.

How A Sea Kayak Turns

Anyone expecting a kayak to turn in the same fashion as an automobile gets a rude shock. The reality is more like a car in a skid than a carefully controlled turn on dry pavements. When a turn is initiated boat's center of gravity (following Newton's first law of motion) attempts to persevere along its original path but the pressures on the hull are no longer balanced and greater pressure near the bow on the outside of the turn causes a turning moment about the center of gravity. AS the boat turns the bow describes a smaller arc than the CG and is angled inside the turning circle. Meanwhile, the stern describes a larger arc and plays crack-the-whip behind. To paddlers the boat appears to rotate about a point lying approximately twenty percent of the length aft of the bow (the precise point varies with hull configuration, heel, trim, etc.). This illusion is caused by the flow of water across the hull and the drift angle. Since the boat rotates about its CG, the moments of the forces causing the turn determine the rate and radius of the turn. If the lateral resistance is increased forward by trimming down by the bow or a control stroke, the turn rate is increased and the radius decreased. If the lateral resistance is increased aft, say by trimming down by the stern, the turn rate is decreased and the radius increased. In a crude sort of way we can see what is needed to provide course stability since it is the opposite of what is needed to cause a turn. Ideally we want the boat to hold course on its own reacting only to turning forces initiated by the paddlers and then responding predictably and settling down on the new course. This ideal is called Class 1 course stability but few kayaks have it. Most have varying degrees of course instability. Although Class 1 stability is most desirable, course instability is not serious unless extreme. The nature of paddling is such that control is integral with power and easily and rapidly applied. Nevertheless, the requirements of differing skill levels impose some designer control over how easily a kayak is controlled.

Factors Affecting Sea Kayak Course Keeping

For most paddlers, simply maintaining a straight course is challenge enough and any kayak that does so with minimal effort is a blessing. The following are general guidelines in improving course stability;

Course stability improves with:

A lower block coefficient (CB)
Increased L/B ratio
Stern-down trim
Increased hull profile aft
Increased L/H ratio (Length to draft)

Course stability is slightly affected by:

Location of LCB
Mid-section shape
Waterline shape
CP within normal limits

Since turning is the result of moments acting about the CG, controlling these moments is the method by which course stability is achieved. This control is a function of hull shape, how it is presented to the water flow, weight movement, and paddle actions. Because a kayak is so easily trimmed fore and aft and athwartships, trim is a valuable tool in improving controllability. Altering the hull shape with heel, trim of both can offset the turning moments caused by off-center power application, wind or waves. This is particularly true in kayaks with fine bows and full sterns whose shape alters significantly with small changes in trim. Such boats are a challenge to the inexperienced and one reason why kayaks with LCB's aft of 55% (particularly when coupled with a deep forefoot) have gained a bad reputation. Extremes in hull shape almost always produce extreme results.

Factors Affecting Sea Kayak Maneuverability

Any characteristic deleterious to course keeping is generally desirable for maneuverability yet the designer cannot apply them carelessly. For, as much as one might desire high maneuverability in whitewater or flatwater slalom boats, the feedback loop must be slow enough to suit the paddler's experience and skills. Kayaks most suitable for skilled paddlers are frequently too sensitive for beginners just as "insensitive" boats may lack the response experts desire. To compensate for a lack in maneuverability an expert can use techniques unavailable to the novice to promote rapid turning. This is the case with marathon racing kayaks that, having been designed for good directional stability, are turned by heeling. Such gymnastics are not always practical. When waves are piling up all around and a few gallons of water are sloshing about it is no time to be contemplating a controlled-heel turn.

Because maneuverability is so important to the whitewater kayaker, it is worthwhile examining how a kayak can best meet their requirements.

The obvious approach - to increase rocker, decrease length, and use rounder sections - has a down side in lost speed and acceleration. Increasing rocker decreases the Cp unless the waterlines are inordinately blunt or "U" shaped. In any case resistance is affected and usually for the worse. A second method is to use a sharp turn of the bilge in such a way as to accentuate the turning moments of water acting on the hull

Sea Kayak Maneuverability is improved by:

Decreasing underwater profile with increasing effect at the ends
Increasing vertical prismatic coefficient (CVP), i.e. more "U" shaped sections
Shortening waterline length
Increasing L/H ratio (reducing draft)
Increasing CB
Increasing sectional coefficient (CX), i.e., using harder bilge midships
Moving LCB aft. (improves forward turning only when heeled and angled relative to the flow)

There is no particular order to this list since conditions and skills play such an important role. Most whitewater play boats have heavy rocker, high Cvp short length, high L/H ratio, higher CB (note that the high CB is attained despite extreme rocker), hard bilges and an LCB aft of the forward waterline ending.

The Sea Kayak Balancing Act

There is no "best" blend of directional stability and maneuverability except in the sense that some combination will satisfy a paddler's preconceived idea of what he needs. "Preconceived" is the key word. Conditions, skills, and objectives are rarely constant and the boat that "feels" best may not be best for the objective. The design problem is to match the boat to both paddler and conditions. The balance between maneuverability and course stability is an important part of what is described as the "feel" of the kayak and every paddler has his own idea of what is right. Fortunately, adapting to a new or different kayak is not difficult and paddlers willing to spend the time can master even the most challenging kayaks.

Sea Kayak Rudders and Skegs

Unlike canoes that rarely have rudders or skegs both are popular among sea kayakers. Traditional sea kayaks rarely used these devices and it isn't clear why they have become so popular today. Nevertheless, they are common and it is worth examining their function.

Sea Kayak Skegs

Wind from any direction except dead ahead and dead aft will exert a turning moment (yaw) on the hull. This moment is determined by the relationship between the wind force and the hydrodynamic forces created. Figure 1 shows these forces. For steady motion and in the absence of a rudder the forces must balance so that:

R sinb = W sin a

If C and L coincide there is no turning moment. If L is forward of C the boat will turn into the wind. This is the normal condition but, on some kayaks with very full waterlines forward and high ends, L will be aft of C and the kayak will turn away from the wind.

The locations of C and L depend upon the wind direction and the yaw angle. C does not always move aft uniformly and the limits imposed by seating do not permit effective adjustment of windage or trim for control purposes.

In some boats the tendency towards weather helm is so strong that an inordinate amount of effort is required to maintain course. One method of correcting this is through the use of a retractable skeg. The skeg shifts the center lateral area aft and balances the turning moments at the bow. The farther aft the skeg is located the smaller it needs to be and the lower the added resistance will be. Thick skegs of airfoil shape and high aspect ratio are not always the best answer. In theory they do develop more lateral resistance through lift but this can be offset by the turbulence in the skeg trunk. One must be careful to balance merits of increased lift over increased drag.

Some skegs are mounted well forward where they act more like centerboards than skegs. In this position their purpose is to resist lateral motion and, in so doing, reduce leeway. This in turn reduces the turning moment at the bow. Centerboards must be somewhat larger than skegs to be effective and they have correspondingly greater drag.

Sea Kayak Rudders

Because a rudder can develop greater lift by increasing the angle of attack relative to the flow it is more effective than a skeg. Of course it can also be used to control direction and for this reason it is popular with less skilled paddlers.

Unfortunately many rudders are poorly designed. Most are simple flat plates that stall at low angles of attack. An airfoil shape, though more expensive to manufacture, is a significant improvement. The NACA 0006 airfoil is commonly used on sailboats and should work quite nicely on kayaks. Another failing in commercially available rudders is that the rudder head systems allow a great deal of wobble and vibration. This increases drag and reduces the response of the rudder. Rudder systems should always be rigidly mounted and free from play in any direction.

Space does not permit a discussion of the theory of airfoils or their design but an excellent source of information is Ross Garret's "The Symmetry of Sailing" listed in the references page of John Winters' web site.

Arguments against rudders and skegs focus on their added cost, complexity, possibility of failure, and that they are used to cover-up poor boat design. Arguments for them are that they allow better control in severe conditions without the need for advanced skills. The debate will not be resolved in our life time.