Kayak design is one of those things that is as much art as it is science; from a performance point of view and also from an aesthetic point of view. I often get asked if I run computer simulations when I design kayaks. I do simulate a number of different elements during the design process, but the final design is still the result of my own understanding, knowledge and experience combined with physics and number crunching. The reason is simply that with the exception of sprint kayaks that operate in a straight line on flat water, most kayaks are designed to perform in a variety of different, ever-changing conditions.
Because the design of a kayak is not something that you can simply put a number on to determine whether it is good or bad, there is often much debate about this subject. The purpose of this article is not to discuss any specific design, but rather to give you the right tools so that next time you’re arguing about a kayak form and function or want to compare two different designs, you will have a better understand of what to consider.
Let’s start with the basics.
What is the difference between a kayak and a canoe?
It is determined by how it is used, not by what it looks like. A kayak is paddled in a seated position, with a double-bladed paddle. A canoe is paddled in a kneeling position, with a single-bladed paddle. That’s it. The method of paddling naturally influencesthe design; there is no mistaking a Canadian canoe for a whitewater kayak, for instance. But often the same craft can be fitted with different outfitting to be paddled either as a kayak or a canoe. In South Africa, 99.9% of paddlers are using kayaks, not canoes.
Before we can discuss how design features affect the performance of a kayak, it is important to get the terminology right.
- Bow: front of the kayak.
- Stern: back of the kayak.
- Splitline: parting line where the two halves of a kayak is joined (in the case of composite and thermoformed kayaks) or where the mould halves meet (in the case of a rotomoulded kayak, which is moulded in one piece).
- Deck: Top of the kayak. Although not technically correct, in the case of kayaks everything above the splitline is generally considered to be the deck.
- Hull: Bottom of the kayak. Again not really technically correct, but generally everything below the splitline is considered to be the hull.
- Sidewall: Side of the kayak, which can include sections above and below the splitline, depending on the design.
- Rocker profile: The profile of the bottom of the hull, when looking from the side.
- Beam: Width of the hull.
- Form: The outline of the kayak when looking from above.
- Hull cross-section profile: The profile of the hull, perpendicular to the length of the kayak. When you see a picture of a cross-section profile, it will generally be the shape of the hull at the widest point of the kayak.
- Rails/Chines: Edges on the hull, generally towards the sides, running along the length of the kayak. Another way of looking at it, the chine is where the bottom of the hull meets with the side of the hull.
- Displacement: The volume of water that is displaced by the hull at a specific load weight.
- Waterline: The waterline of a hull is the line where the surface of the water meets the hull.
- Drag: The resistance to forward motion. Factors like wind, waves and water depth all influence the drag, but as far as the hull itself is concerned, there are two main forms of drag:
- Form drag (also called pressure drag): this is the drag caused by the displacement of water as the hull moves forward through the water, as well as the drag caused behind the hull where the water fills the gap again (commonly called the ‘vacuum’ behind the hull, which is not technically correct, but quite descriptive). The actual shape (form) and size of the hull determines this drag. An efficient shape that cuts through the water will have less form drag.
- Skin friction drag: This is the drag caused by friction of a fluid against the surface of an object that is moving through it. It is directly proportional to the area of the surface in contact with the fluid and increases with the square of the velocity.
- Stability: This is essentially the resistance to rolling side to side.
- Primary stability: The stability of the kayak when it is in upright position. In simple terms, this determines how wobbly the kayak feels when you sit on it.
- Secondary stability: The stability of the kayak when it is on edge. In simple terms, how likely it is to fall over once the kayak has tilted a bit.
- Tracking: The ability of a kayak to keep going in a straight line without steering input.
HULL DESIGN ASPECTS THAT INFLUENCE PERFORMANCE
The most important thing to keep in mind regarding the different aspects of a design, is that none of these aspects work in isolation. It is the unique combination of these different aspects in a design that determines the performance. You can’t just look at the width of a kayak, for instance, and use that as the sole indication of the stability or speed of the kayak. The hull cross-section profile, rocker profile and length all have an influence too.
The longer any specific hull design is, the faster it will be, as the longer shape will have less form drag. However, the length can’t be increased indefinitely, because as the shape gets longer, the skin friction drag increases. There comes a point where the increase in skin friction drag negates the decrease in form drag.
Keep in mind that it is not the full length of the kayak that determines the speed, but the length of the waterline. Some kayak designs, especially whitewater and ocean kayaks, tend to have a large part of the bow and stern out of the water, which means the waterline length is much shorter than the length of the kayak.
Racing kayaks tend to have very little rocker, so the waterline length is almost the same as the kayak length. Some racing kayaks and surfskis now sport inverted bows, which causes the waterline length and boat length to be exactly the same.
In general, wider means more stable, but also slower. However, as I pointed out in a previous article, you are only as fast as you are stable. If you paddle a kayak with a hull that is theoretically faster, but you spend 20% of your energy just on balancing the kayak, you will actually be faster on the water by paddling a slightly wider kayak that is more stable, where you can put 100% of your energy on propelling yourself forward.
When we talk about the width of a kayak, we normally refer to the beam, in other words, the width at the widest point. The width of the hull towards the ends of the kayak, both on the bow and stern side, also has a huge effect on both the speed and the stability of the kayak. This is especially true on rivers and on the ocean, where there are more factors at play than just straight-line speed in stable conditions.
- Cross-section hull profile:
This has a huge effect on stability. Below are the basic types of hull profiles with notes.
- It is the hull shape with the least form drag and also least skin friction drag; in other words, the fastest. It also happens to be the most unstable hull shape.
- V-shaped. This shape tends to have be almost as little drag as rounded, with the added benefit of tracking well. This shape tends to have great secondary stability but not so much primary stability.
- This shape typical has more drag than round and V-shaped, but it has great primary and secondary stability. Flat hulls also tend to be more manoeuvrable.
- The most stable of all, but also the most drag, both in terms of form drag and skin friction.
This doesn’t tell the full story though. For instance, a well designed, narrower flat hull can be as fast but more stable than a wider, round hull.
It is also common for kayak designers to use a combination of these different profiles on a hull. For instance, the hull can start V-shaped in the front, which then blends into a round shape towards the middle, and then goes into a flat towards the back.
- Displacement vs planing hull:
A displacement hull is a hull that literally displaces water as it moves forward. A planing hull climbs on top of the water once it reaches the right speed. This means that just about all kayaks have displacement hulls, with the exception of waveskis, surf kayaks and some freestyle kayaks. Some well-designed sprint kayaks and surfskis do lift out of the water to some degree when they move fast enough, even though they won’t be called true planing hulls.
The advantage of planing on top of the water is that it reduces drag substantially, which enables it to move a lot faster over the water. On a wave, the surfing action combined with gravity produces enough speed to generate lift.
On flat water, it is impossible to create enough speed by paddling to create sustained lift. Also, the same design features that enables a hull to plane at high speed actually increases the drag when the hull is in displacement mode, making it even harder to create enough speed to start planing in the first place. For this reason, the aim of racing kayak designers is to create the most efficient displacement hull, rather than trying to create a true planing hull.
More rocker means more manoeuvrability, but it increases drag, so it makes the hull slower. Less rocker means less drag, but it decreases manoeuvrability. For this reason flatwater kayaks that are intended to go straight most of the time have very little rocker, while kayaks made for whitewater or riding ocean swells have more rocker. This is then also the reason why many racing kayaks (K1s and K2s) that were adapted for river racing in this country by simply adding more volume on the deck tend to be quite horrible in technical rapids where manoeuvrability is more important than speed.
Apart from the amount of rocker, the type of rocker also plays a role. Most kayaks have a strictly continuous rocker, but some have what we call kick-rocker, which means the centre part of the hull can be relatively flat, while the ends are kicked up.
- Bow shape:
A sharp bow pierces through the water, which makes it faster and helps the hull to track better. It also pierces through waves, which can be an advantage in some conditions and a disadvantages in others.
A rounded bow pushes against the water instead of piercing through it, which makes it slower. But the added volume in a rounded bow also rides over waves easier, which can be an advantage in some conditions.
Another common type of bow is the highly rockered flat bow. This is typically used on craft that operate mostly in the surf zone, where this type of bow prevent the nose from diving when surfing on a steep wave.
- Sidewall profile:
- A rounded sidewall is very forgiving when paddling in rapids, as it is less affected by currents. It also makes a kayak easier to eskimo roll.
- Flat or straight sidewalls are a lot more responsive to currents, which can be a downside if you’re paddling at the limit of your capability, but it is also a big benefit in some conditions.
- Flared sidewalls decrease the tendency of waves to crash over the deck, which increases secondary stability. Flared sidewalls also means that the bottom part of the hull tends to be quite a bit narrower than the beam, which means a reduction in primary stability.
- Hard Chines
Hard chines improve the primary stability of a kayak. The also help with tracking on flat water, and for carving in rapids. They are more prone to cause capsizing in choppy conditions and in surf launches and landings though.
- Soft Chines
Soft chines improve the secondary stability of a kayak and it is alsoimproves speed. This is ideal for ocean swells and choppier conditions, and therefore common on sea kayaks and surfskis.
In recent years, more kayaks with multi-chines have been designed, especially on the recreational side of paddling. This is an attempt to increase both the primary and secondary stability of a kayak. However, the increase in both form and skin fraction drag is substantial. In my opinion, all multi-chine hulls are too compromised and have very bad performance. There are more efficient ways to create the same benefit in terms of stability, without compromising the performance of the hull.
- Length-to-beam ratio:
The ratio of length-to-beam has often been used as a measure of the potential speed of a kayak, so I mention it here for completeness. However, while it does give an indication of the sleekness of a design, there are so many other factors at play that I rarely use this ratio myself for any practical purposes.
- Swede Form: Widest behind the cockpit, this form has a longer and more slender entry, giving efficient cruising speeds. Kayaks with this form tend to have great acceleration and track well. The narrow bow will cut or spear through the swell and waves, rather than climbing over them.
- Fish Form: Widest ahead of the cockpit, this form has a more blunt entry. The bow typically has more buoyancy, which has advantages in whitewater as well as in the open ocean.
- Symmetrical Form: Widest in the middle, while the shape toward the bow and stern are pretty similar. This shape increases the kayak’s manoeuvrability, and generally has fairly even volume distribution between front and back, making it a good all-rounder shape.
DECK DESIGN ASPECTS THAT INFLUENCE PERFORMANCE
When I design a kayak, I always start with the hull, as the hull has the largest influence on the performance of a kayak. The deck has a substantial influence in terms of the amount of volume and where the volume is positioned, the shape, and the height. The deck also has a massive influence on the aesthetics of kayak, which matters!
The volume of the deck determines how stable the kayak will be when loaded to the max. It also determines how quickly the boat will resurface when going through waves or holes. But, more volume is not always a good thing though.
More volume means more of the kayak is exposed to wind, which can be a major factor when racing, and it also affects handling on open water. When surfing on waves, the volume of the back deck has a big influence on the angle of the kayak, and can determine whether the bow will get buried at the bottom of the wave or allow it to ride up.
- Peaked decks vs flat decks:
Peaked decks resurface faster when going through waves than flat decks. But again, there needs to be a balance. If it is too peaked and too buoyant, the kayak will start bouncing when you paddle through waves, making it difficult to control. Peaked decks are also affected more by wind. For this reason, flatwater racing kayaks tend to have low volume, flat decks.
- Height of the front deck vs back deck:
If the front of the deck is substantially higher or lower than the back deck, wind will push harder on the high part of the deck, which can keep blowing you off course, especially on big open water.
- Cockpit height:
A higher cockpit generally means better protection from the elements. It also means less chance of water spilling over the deck into the cockpit, in the case of sit-on-tops or when paddling a sit-in kayak without a spraydeck on.
A higher cockpit can get in your way though. If it is too high for your body proportion, you will feel swamped with the high cockpit around you, and you might hit your elbows on every stroke. A high cockpit also makes it more difficult to remount the kayak if you take a swim.
There are many more subtle design aspects that influence the performance of a kayak, some of which every designer would like to keep secret. There are also other factors that affect performance in the long run, like outfitting, choice of materials and quality of workmanship.
Kayak design is a fascinating process. Over the past 20 years, I have designed over 50 different kayaks, most of which made it to production, while others stayed in the prototype phase. I’m still learning, and every new design brings new challenges. I find it most rewarding to be able to combine elements from seemingly unrelated concepts and designs, to create a new whole that has a unique application and spot-on performance.
This article written by Celliers Kruger was originally published in the August/September 2019 issue of The Paddle Mag. This digital publication is free to view and download. Try the ISSUU app on your mobile phone.