The Seaworthy Sea Kayak
A few gifted paddlers – not many– can paddle across the Atlantic in a bathtub and shrug it off as a minor accomplishment. The rest of us, mere mortals, must place greater reliance upon our boats to get us out and back. For us the definition of a seaworthy kayak includes being “Forgiving of the most egregious paddling and judgemental errors.” It’s a heavy burden for a boat for seaworthiness is more than remaining upright and dry in large waves. The kayak must be comfortable and able to maintain course and headway under severe conditions. There is little merit in remaining dry only to be crushed upon rocks or swept past a safe landing.
Every aspect of kayak design, individually and collectively, influences performance at sea and it is unlikely that any one feature will either make or break a boat. For the sake of simplicity though we will discuss each hull characteristic individually while keeping in mind that the sum is more important than the parts.
Above the Waterline
Freeboard is a mixed blessing which when it is high enough to keep us dry, is sometimes to high for easy course keeping. The proper balance is not easily determined through experience but studies of ships at sea provide practical guidelines. The US Navy recommends forward freeboard of 8 – 10% but since kayaks are small light, and travel at low speeds, 6% of the length is a reasonable compromise. Of course all the freeboard in the world is of little help if there is inadequate volume forward to keep the boat from plunging deeply into waves. The correct volume is assured by superimposing the extreme wave condition over the hull shape and then designing in enough volume to support the hull as shown in Figure 1. Adequate volume can be achieved by incorporating flare into the topsides (The Navy guides us here and recommends 20 to 25o), building in spray strips and using long overhanging ends. The choice is usually one of aesthetic values although long overhanging ends caise pitching which is counterproductive. One distinct advantage of flare and spray rails is that the direct spreay away from the boat instead of channeling it aft inot your lap.
The boat can be considerably lower aft since one is not likely to be power through waves in reverese. Six percent of the length is a flexiblwe rule of thumb based upon casual observation.
Providing rules for midships freeboard is complicated by the degrees of possible shapes (tumblehome and flare). In the end, it probably takes care of itself since the boat must be deep enough to accomodate your body and stability requirements mandate a minimum amount of vilume above the waterline.
Below the Waterline
It has long been recognised by ship designers that “V’d” sections in the ends produce a more seakindly motion with less pitching and pounding. It should not be surprising then that most Inuit kayaks have sharply “V’d” sections since they would have learned the lesson the hard way.
Pitching (Vertical rotation of the boat around its center of gravity) is As the canoe encounters waves a pitching motion is instigated which drains away energy in the creation of waves. Because the pitching inertia varies as the square of the distance from the center of rotation (Nominally the center of gravity in canoes) tremendous forces are involved and their reduction is advantageous. From a design standpoint, there are two significant paths. The first is to increase waterplane area in the ends which has a damping effect on vertical motion. The second is to design the hull with seperation between the Longitudinal center of bouyancy (LCB) and the longitudinal center of flotation (LCF). Since the hull rotates around the LCB and the LCB has a tendency to shift aft as the bow rises the increasing waterplane aft serves to dampen the motion.
Loading, as mentioned earlier, has a large impact and it is almost always advantageous to concentrate weight in the middle of the canoe. Whether this is done by moving gear or people makes no difference although moving the paddlers has the greatest impact as shown in Figure 9-4.
Midships Section Shape
Earlier in the discusion about stability (Chapter 6), we touched on how hull shape affects roll in waves but it is worth mentioning that the importance of flat versus a rounder bottom shape has been greatly exagerated. Because the local gravity is always normal to the wave surface paddlers traveling across waves which are longer than twice the canoe’s beam do not “sense” so much heel as one would expect . Figure 9-6 illustrates what happens. Nevertheless, canoes with very flat bottoms and a sharp turn at the bilge have a quick roll and tend to be unnerving in short steep seas that match the natural rolling period of the canoe. This is not a significant problem when one has his wits about him but, as fatigue sets in, mental anticipation is low and body responses sluggish. It is not surprising then that such hulls have earned a bad reputation for seaworthyness. Arched bottoms and softer bilges provide a smooth, slow roll requiring a slower response time. Mind you, plenty of flat bottomed Grummans have traversed the the far north so it would be foolish to say they are inherently dangerous.
Section Shape at the Quarters
To achieve the high Prismatic coefficients appropriate for high speed efficiency many designers have resorted to “U” shaped sections aft with very hard turns of the bilge. The increased bouyancy in the quarters lifts the canoe rapidly when a wave passes underneath. Aside from the abrupt motion, the loss in lateral resistance can cause the hull to slide down the wave and broach. A softer turn of the bilge aft and “V’d” sections (associated with low Cvp’s) promote directional stability and are desireable. Compare the two hulls in Figure 9 – 7 and imagine how the wave would affect each as it passes below.
In the bow, the situation is reversed. Rapid maneuvering is best done at the bow whether to avoid rocks in heavy whitewater or to prevent a broach in open water and it is the bow person whose strong draws or prys make the turn. If the canoe is to respond effectively to these strokes, it must be free to move laterally. Therefore, “U’D” sections (as opposed to “V’d” sections) and considerable rocker are desired. It is interesting to note that such sections and rocker may not cause any loss in speed since the waterline length need not be affected. An added bonus is that the rocker reduces wetted surface. The shape that will provide the proper maneuverability without excessive pounding is not easily found.
Canoe design has been greatly influenced by racing canoes and their characteristics applied indiscriminently to all canoes with unfortunate results. Seaworthyness demands the ability to orient the canoe to best encounter waves and should not require heeling or unusual strokes yet many straight keeled, fine ended, obsolete racing canoes have been promoted for tripping. Such boats require exceptional skills under adverse conditions and are unsuitable for the average paddler who is fatigued or possesses rudimentary skills. Futhermore, the deep fine ends bury deeply in waves reducing control in following seas. “U” shaped sections and a rockered bow permit easier course correction and are less influenced by waves. The two types of forward sections are shown in Figure 9 – 8, the more seaworthy shape on the right.
In the final analysis, seaworthiness is a relative quantity strongly dependent upon how the canoe will be used. Quiet ponds and streams are undemanding and impose no need for the same canoe that can duke it out with the big rollers of Lake Superior. Similarly, seaworthiness in a whitewater playboat differs significantly from that required by the wilderness river tripper. Anticipated conditions and priorities dictate hull parameters while the inevitable compromises of canoe retailing dictate the final shape. Striking a reasonable balance between the two is never easy.