How do choices in design of a paddle-craft influence its handling characteristics?

Question

If you imagine cutting a canoe or kayak along each axial plane, you get three shapes. If you cut along the waterline and look down from above, you get the plan. Cut thwart-wise at the widest point and view from head-on, that's the cross-section. The cross-section may or may not include tumblehome, a narrowing of the sides toward the gunwales. Finally, cut lengthwise and view from the side, that's the rocker. Designers have a variety of shapes to choose from for each of these, as the sketches below illustrate.

(This discussion ignores things like external keels and tracking channels.)

As a paddler, you will need to learn how your craft handles, and will observe these characteristics:

1. Stability - high resistance to capsizing
2. Speed - low resistance to moving forward through the water
3. Maneuverability - low resistance to changes in direction
4. Tracking - high resistance to wobble (aka jitterbug) while moving forward

My question is how the various choices in plan, cross, and rocker translate into improvements or sacrifices in stability, speed, maneuverability, and tracking. I already have some broad intuitive notions, as well as a sense that what improves one of these traits generally degrades another. I am wondering if someone can help with the gritty details, possibly even get into the physics and mathematics of it (i.e. give us a primer on fluid dynamics as it relates to paddle sports.)

Just as an example, I have a kayak with a flat rocker, mid-range plan, and gently rounded cross-section with tumblehome. This kayak is very stable, has good tracking, is reasonably maneuverable, although it is not fast. If it were narrower, my guess is it would be faster but less stable. If the cross-section was more round, it would be more maneuverable but at the expense of its good tracking. Are these ideas correct? Why exactly?

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Note: The image captions are, in order: long narrow plan, short wide plan, flat cross-section, round cross-section with tumble home, vee or hard-chine cross-section, flat rocker, moderate rocker, round rocker.

Answer 1

Ay-yi-yi! This is a very broad question, a true answer to which would probably require a PhD in hydrodynamics or years of relevant engineering expertise. Note that the interplay between these design characteristics can be very complex and depend greatly on the exact conditions (flat water / waves / moving water / rapids / ..., skill of paddler, travel speed, ...). Furthermore, there are many, many, many other design characteristics that could be considered (freeboard / keel / bow shape / hydro-planing / ergonomics / ...), as well as considering that the hull shape varies over the length of the boat.

All that being said, here's a very rough guideline.

Reverse Engineering

Much can be learned by comparing images of modern boats and considering the intended design. In comparing the general hull shapes, let's try to reverse engineer the design.

• A crewing scull is built to travel as fast as possible in a straight line over absolutely flat water. Stability is of minimal concern; all that matters is speed. Consider what can happen at even the Olympic level with a little bit of chop (or purportedly a blade snagging a piece of floating garbage). While there can be some very complex hull designs, a scull will generally have: a long narrow plan, a chined cross section, and very little rocker.
• Sea kayaks are generally built for covering long distances on reasonably flat water, generally with few course corrections. The paddler has some ability to provide active stabilization in a well fitting cockpit. A sea kayak generally has: a long narrow plan, a chined cross section, and a slight rocker.
• Canoes are also generally built for covering long distances on reasonably flat water, generally with few course corrections (note that there are whitewater canoes, which look very different).Passengers are located higher off the water in a canoe than in a sea kayak. Thus, a canoe has a higher center of gravity and lateral stability is more of a concern---passengers have less ability to provide active stabilization by shifting their weight. A canoe generally has: a long narrow plan (although wider than a sea kayak), a flat cross section, and some rocker.
• Whitewater kayaks. Whitewater kayaking requires frequent changes in direction, with much of the stability coming from the paddler actively balancing and maneuvering. Most of the forward movement comes from the current---actively paddling downstream is generally inefficient. Many designs are also built around surfing waves / playboating / whitewater rodeo. A whitewater kayak generally has: a short wide plan (often limited by leg length), either a flat bottom with hard chines or a gradually rounded bottom, and a lot of rocker.

Now that we have a handful of actual boat shapes, let's look at each of this hull characteristics in turn. Assume that we are tweaking each of these parameters in isolation (i.e., all else being equal), along with many other disclaimers.

Plan:

A longer, narrower plan will generally result in less hydrodynamic drag (particularly dependent on the bow shape). It will also generally track straighter, but at the cost of reduced lateral stability and more difficulty turning. In comparison, a short wide plan would easier to turn and more stable, but give more drag and be slower.

Cross Section:

A flat bottom will provide the most lateral stability (and usable interior space), but will have a harder time tracking straight. It will also "skid" or "pivot" turns, unless actively tilted on edge.

A rounded bottom will be easier to tip from side to side, which can be beneficial in a kayak. It will behave somewhere in between a flat bottom and a hard chine, depending on how rounded it is.

The "vee or hard chine" cross section will generally track straight, as it has somewhat of a keel effect. This shape also has a secondary balance point when the boat is tipped on its side, allowing it to "carve" turns (this is how you turn a sea kayak, for example). On the flip side, this shape is difficult to turn in place.

Rocker

For rocker, we need to consider how much of the boat is in contact with the water (freeboard vs. beam, etc.). A more pronounced rocker will

• Be easier to turn (starts to emulate a short, wide plan if we look only at the portion of the hull in contact with the water).
• Be able to ride up and over waves as opposed to punching through them.
• Be more prone to "planing"--rising up out of the water with increased speed. Think of a speedboat rising up out of the water as it hits high speeds.
• Be somewhat less fore/aft stable.

Answer 2

Since there is a great general answer already, I'll add one from my experience with white water kayaks.

Length: Longer boats have a higher hull speed. When moving at more than the hull speed relative to the water you'll start creating waves, wasting energy. Shorter boats turn faster, and have less problems "boofing", going over drops without diving in nose first. Boats designed with bigger rivers in mind will often be a bit longer, the slightly longer turning time is easily offset by the higher top speed. Boats made with small and steep creeks in mind are often a little shorter. White water kayaks in general are short by flat water boat standards, as they need to be nimble. Really short boats are used for freestyle kayaking, performing tricks like cartwheels and loops.

Rocker: More rocker also helps in boofing, and a pronounced rocker is on the scale of white water kayaks more important in this aspect than a short length, as well as in smoothly crashing through waves and holes. Any kayak designed for the really serious runs will have a lot of rocker. Boats designed with weaker currents in mind, or those designed for surfing, often have less rocker, or only on the front, to make sure the water doesn't flow on top of the boat and push the nose under. Boats with less rocker sit shallower in the water and hit less rocks, and it helps in creating a big flat surface for the effect discussed below.

Cross section: A round profile is used to create a forgiving displacement hull on serious river and creek running boats. An unexpected stream or push won't take the boat off course or tip it over. Flat profiles are used for control, for instance on boats designed with surfing in mind. A large flat surface with square corners gives the boater a lot more control over the craft and more steering power, even while planing, but it also gives any unexpected currents more control over the boat. A flat boat can be a joy to pilot, it's very reactive and takes little power to paddle if you're using the currents right, a more round boat makes you feel like you can take on the world. A lot of round big river kayaks these days do have rails running along the sides of the boat under or around the water line, they give back some control while not making the boat significantly more punishing. White water kayaks (with the exception of downriver boats, but that's its very own specialized thing) do not have anything remotely resembling an edge, rail or keel in the middle of the boat.

That's roughly it. Speed = long, forgiving = round, boofing = rocker, surfing = flat, freestyle = short, control = flat or rails. For a better feel of what this means, find some good pictures on manufacturers sites or comparison sites like the Playak buyer guide.