Round vs Square Hull Cross Section
Kayakers will fully appreciate how their kayak handles once they've owned it for a time and have paddled the boat in a variety of conditions. It's also helpful to learn about the general concepts of boat hull design and how these can affect a kayak's paddling characteristics. Doing so can steer you towards a kayak that best fits your needs. This post is limited to the subject of cross-sectional hull shape – a parameter that has an effect on catching waves in following seas.
For a complete run-down of the many factors that contribute to a sea kayak's overall handling characteristics, a good place to start is Nick Shade’s book on strip-built kayak construction The Strip Built Sea Kayak (1997). The first chapter of Nick's book has a comprehensive discussion on the subject of hull design with clear definitions of technical terms as prismatic coefficient, center of buoyancy, etc.
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| Figure 1: Round vs Square (boxy) Kayak Hull Cross Section |
The cross-sectional shape of a sea kayak hull is by no means the main factor determining how a kayak will paddle – width, length, buoyancy, rocker, etc., will all have more significant effects on boat behavior. However, once you’re in the realm of the “typical” sea kayak size, that is 16-18 feet long and 20-22 inches wide, the cross sectional shape of the hull becomes an important element to consider.
Round-shaped hulls provide the most efficient shape, with the minimum wetted surface and therefore the least friction (drag) through the water. All things being equal, a kayak hull with a rounded cross sectional shape will be a bit faster than one with a square-shaped hull. This is true for all “displacement” boat hulls and is why racing kayaks and surf skis, whose principle purpose is to go fast, are all essentially round in cross section. Think of the Epic kayaks as one example. In a kayak with the proper width and buoyancy, round-hulled kayaks can also be very stable and confidence-inspiring in confused seas. One of my favorite kayaks, the Valley Pintail, is in this category as is the Nordkapp, Avocet, and many others.
By contrast, square-shaped hulls improve a sea kayak’s ability to plane on a wave or “surf” at the sacrifice of some speed. The flat bottom and boxy sides of this hull type allow the boat to catch waves more readily and will keep the boat on the wave longer (with proper paddler input) than a rounded hull kayak of similar length/width/rocker. This is especially true in following seas where you can sometimes see the square-shaped kayaks catch a wave and burst ahead of the others. When a boat starts planing, it exceeds its non-planing displacement-hull speed for a short period until it can no longer keep up with the wave it’s riding on (due to hull drag or by surfing down into the wave in front which slows the boat). In following seas, after the wave passes beneath the boat, you can keep paddling to catch the next one. In short order you’ve covered a larger distance than paddlers who are not catching waves. It’s also great fun to feel the sea take you for a ride faster than you’re able to paddle under your own power. At shore breaking surf zones the same surfing/planing advantages of the square-shaped hull apply.
A perfectly flat hull would surf great (see Figure 2) but would be ill suited to the varied duties of a sea kayak – which must also paddle in a straight line, must handle wind/waves from all directions, and shouldn’t be too hard to turn when needed. So even sea kayaks with comparatively flat hulls will have a “shallow-v” shape to some extent to help with tracking and to minimize leeway (being blown sideways downwind).
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| Figure 2: yes a FLAT surface for surfing |
“What about surf skis?,” you may ask, “they’ve got rounded hulls and yet surf great.” Of course that’s true – any kayak will plane on a wave given the right combination of paddling speed, wave speed, period, and steepness. But surf skis maintain their position on a wave with the help of a rudder and surf best on longer wavelength swell due to their longer length. By contrast, the typical skeg-equipped sea kayak will not maintain its position on the sweet spot of a wave for long unless its boxy shape keeps it on the wave face. In following seas or shore breaking waves, a round hull will veer off the wave more quickly. Sure a round-hull kayak can catch a wave but it will be more of a struggle to stay on and the result will be a shorter ride and missed chances to catch waves. (By the way, I’m talking about moving waves here, not standing waves. A round-hull boat like the Valley Avocet or Sterling Reflection can sit nicely in the saucer-shaped pocket of a perfect tidal race standing wave. But catching moving waves over a straight course in following seas is not either boat's forte.)
In my experience, the price you pay for a kayak whose boxy shaped hull assists catching waves is a boat that is a bit of a bore to paddle on flat water. They’re less “slippery” and may be less playful in calm conditions than round-shaped hulls. Boxy hulls are a bit slower and sometimes a drag – literally, some feel like they’re plowing through the water with greater resistance. To remedy this problem, manufacturers are experimenting with combination hull shapes – combining a roundish hull in the front part of the kayak to make for smooth entry through the water, joined with a square-shaped hull from the cockpit backwards to add a more solid edging feel and to improve surfing in following seas. As of this writing (2019) P&H and Current Designs have some sea kayaks with “combination” hulls so-to-speak, which look intriguing.
Anybody can look at a kayak and see for themselves the general shape of its hull in cross-section. Flip the boat over, look at and feel the hull – is it round or boxy? Also, look inside the boat at the front and rear bulkheads. What shape are they? Square-ish or round-ish? (See Figure 3).
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| Figure 3: Look at the bulkheads |
Some new “sea kayaks” are being designed with atypical dimensions for improved surfing ability. These include boats with increased rocker (the up-sweep of the hull’s keel line from bow to stern) and increased width and volume massed closer to the cockpit. While helpful for surfing shore-breaking waves, these modifications come at the expense of a boat’s general sea kayaking ability. Increased rocker hurts directional stability (paddling in a straight line) and may reduce a boat's speed. Drastic changes in the distribution of a boat’s volume can limit its speed as well. A kayak that doesn’t extend a good portion of its volume out to the bow and stern will not be as fast as one with more evenly distributed volume (this has to do with "prismatic coefficient" and "effective waterline length" terms best described elsewhere, check Nick's book). Also, lack of volume in the bow may contribute to the problem of "pearling", when the bow gets buried in the wave in front, another factor that can limit a kayak's surfing ability in following seas. Sea kayaks with shorter sterns also handle surf in following seas a bit better in my experience - less boat volume is buried in the wave which seems to help keep the kayak going straight and allows it to be more responsive to rudder strokes. (Separate link to post on that subject HERE.)
Buoyancy is a factor in this discussion, as is paddler weight, regardless of hull shape – a more buoyant kayak (or a lighter paddler) will be more playful, will push-back more when edged, will surf more readily, and will feel more lively overall regardless of hull shape (round or square). That’ll have to be a topic for a separate post.
That’s it for now.
