Kayarchy - the sea kayaker's online handbook and reference

 

Sea kayak design

Guillemot sea kayak in strip plank

Guillemot design by Nick Schade, built in cedar strip plank by Glyn Edwards Kayaks of Whitstable

 

 

We've already mentioned some aspects of sea kayak design - rocker, volume, volume distribution, cockpit size & shape, the seat, knee braces, and deck shape.

Why a long, narrow kayak?

"A narrow kayak is faster and much better in a choppy sea because it cuts through the waves, while a wide one bumps like a box".
F Spencer Chapman, Watkins' Last Expedition, 1934

A classic sea kayak has a fast hull shape, and not too much hull area in contact with the water ("wetted area"), but it also has reasonable stability.

Plan view of sea kayak

The dimensions of a sea kayak seldom stray outside this range:

For a man 180 cm tall, a length of 4.8 metres to 5.5 metres long, and a width of 48 cm to 60 cm, which is getting a bit bulky to roll easily.
For a woman 165 cm tall, a length of 4.5 metres to 5.25 metres long, and a width of 48 cm to 58 cm.

This is why:

1) Long boats can go much further than short boats for the same effort. Long boats have a higher cruising speed than short boats. This is immediately obvious if you look at a sea kayak and a short river kayak going along side by side. Even at walking pace, the back of the short kayak is sucked down, the front rises up, and the kayaker's energy goes into making waves rather than going faster. With a sea kayak, this effect is not noticeable except in very shallow water or at sprint speeds.

2) Short boats have to be fat in order to support a kayaker's weight, and being fat makes them even slower. If a sea kayak and a short river kayak start from the same point and are paddled in an identical way, the short kayak will go half as fast. Also it will wallop along in clouds of spray so the kayaker gets wet.

3) Long slim boats have excellent longitudinal stability - they like to go in a straight line and will continue to go straight for a long way when you stop paddling. A short fat boat constantly tries to turn to right or left, and when you stop paddling it will often turn right round within seconds.

4) Most kayaks 51-59 cm wide are easy to roll, wide enough to stay comfortably upright in ordinary conditions, but not so wide that they lean over every time a wave comes at them from the side.

5) Every square centimetre of underwater surface creates drag. Paddling a boat which has too much wetted area is like driving a vehicle with the handbrake half on. The underwater cross-section with the least drag is a semi-circle. That, with a partly-flattened bottom for stability, is the underwater shape of a fast sea kayak.

6) A long boat is fast, but a very long boat is not necessarily faster. A slim ocean racing ski might be 6.5 metres long. It would have a considerably higher cruising speed than a sea kayak, but only if paddled by a gorilla who has the power to overcome the drag created by all that wetted surface.

7) Other problems with the very long kayak or ski are that it will either be heavy, making it hard for you to get it onto your vehicle, or too fragile to survive contact with rocks; on a roofrack it overhangs the ends of your vehicle so far that it is a hazard in traffic; and it may be too long to fit in your garage.

Kayak stability

Lines of soft chine sea kayak"I think the stability of a [Greenland] kayak is regulated by the angle of the sides rather than the depth or width, for this kayak was two inches narrower than my last one and much lower in the water [but]... so steady... that I felt a hundred times safer than I had ever felt before ".
F Spencer Chapman, Watkins' Last Expedition, 1934

A kayak has two kinds of lateral stability, initial and secondary.

High initial stability means that a boat feels solid and stable when it is sitting upright. A wide kayak with a flat bottom has very high initial stability and feels great to beginners but in waves the kayak will constantly try to conform to the surface of the water. So it will lurch from side to side every time a wave goes under it from the side. Also, a kayak with very high initial stability is a very slow kayak.

Very low initial stability, as in a racing kayak, means that the kayaker can never relax. When stationary, (s)he will have to use the paddle constantly just to stay upright. See Sculling For Support.

High secondary stability means that the kayaker feels secure when leaning over to one side. Low secondary stability in a kayak means that if you don't keep the boat perfectly upright, if you lose your balance for a moment, you must be very quick to do a support stroke or you will capsize. Kayaks are usually slightly wider on deck than at the waterline, to give them secondary stability.

The American magazine Sea Kayaker often publishes detailed reviews of kayaks including the subjective opinions of several kayakers and a computer-generated stability table. www.seakayakermag.com.

Stability depends on two main factors:

1) The width of the kayak. A kayak much less than 53 cm wide will be too unstable for an adult male beginner. A kayak much more than 59 cm wide will feel clumsy and will lurch from side to side in rough water. These are total widths, the width where the hull meets the deck. Most kayaks are slightly narrower at the waterline. A sea kayak which is unusually narrow at the waterline is probably suitable only for racing but if it has ordinary total width will become more stable when carrying a heavy load of expedition equipment.

2) The shape of the kayak's underwater cross-section. Imagine a kayak which has the cross-section of a simple box, so it has a completely flat bottom and vertical sides. It will have high initial stability but when the kayaker leans sideways, at a certain point it will suddenly flip upside down. If the bottom is flat towards the ends, the kayak will also be noisy, bumpy and wet except on completely flat water.

Then imagine a kayak with a cross-section which is a perfect semi-circle underwater. It will be very fast but its low initial stability means the kayaker will find it difficult and exhausting to keep upright.

The solution is a compromise. The cross-section of most kayak hulls is rounded but with a flattened patch in the middle, say 20 cm wide and 2 metres long, to give the boat something to sit on.

The bottom of a Greenland kayak is not flat but a flat V. This has low initial stability because the kayak always wants to sit on one side or other of the V, but high secondary stability. So it feels tippy, but it's fine when you get used to it.

Stability also depends to some extent on further factors:

3) The height and weight of the kayaker. The average man has considerable weight in his shoulders and upper body and needs quite a stable kayak. Unisex kayaks are designed to fit men. The average woman is not only shorter but also has a lower centre of gravity relative to her height, so she may feel comfortable in a narrower kayak. See Sea Kayaks To Fit Women.

4) The height of the seat. Racing kayaks and wave skis have seats 1 cm or more above those of touring sea kayaks. This makes them unstable by raising the paddler's centre of gravity. A high seat also makes a kayak hard to roll.

5) The length of the boat. Physics says that a longer boat will be more stable than a short one of the same width and cross-section. Amongst other things, that means that a child's sea kayak can't be as lean and mean as one for an adult. See Child-Size Kayaks.

6) The amount of rocker. In other words, whether the bottom of the kayak, seen from the side, is straight from one end to the other or is more banana-shaped.

7) The volume of the boat. A very narrow kayak sits deeper in the water and may be less unstable than you would expect.

8) Whether the seat exactly fits the kayaker. If the kayaker is securely connected to the kayak by firm contact with the seat, knee braces and foot rest (s)he will have much better control than if the seat is too wide, and will probably perceive the boat as being more stable.

9) Whether the kayak is carrying a heavy load. Most small boats are more stable if carrying heavy ballast low down in the hull.

Radical designs

Sea kayaks based on traditional designs are beautiful, functional and elegant, and their owners love them. However they have certain flaws.

• Because a traditional sea kayak is heavy it can be hard to lift onto a roof rack and because it is long it may be hard to find a place to store it indoors.

• The sharp overhanging ends of a sea kayak look nice. However in high winds they create a wind vortex at each end of the kayak, making it harder work to paddle. See Leeway.

• The potential cruising speed of any non-planing boat is determined by its length at the waterline, not its total length. A waterline length of 4.5 metres is long enough for a sea kayak. A traditional sea kayak may be that long at the waterline but have an additional 50 to 70 centimetres which is almost always out of the water.

• A traditional sea kayak is awkward in big steep surf. It is dangerous for other people in surf because its pointed ends can kebab a swimmer; it is impossible to steer reliably on a steep breaking wave; it often goes towards the beach out of control and sideways; and its front end often digs deep into the water in a "pearl dive". This is because the top of the front deck is more or less flat and horizontal, the front end of the hull is axe-shaped and bites into the water, and the hull often has very little volume at the front.

Any large, steep wave approaching a traditional sea kayak from behind lifts up the back of the kayak and tries to force the front underwater. The front of the kayak then acts as a brake, slowing the boat and making it feel very unstable for a few seconds until the wave overtakes and goes past. If the wave is really steep, it may push the front of the kayak so deep underwater that the kayak becomes more or less vertical. In shallow water, the front of the kayak may hit the bottom and be damaged. In slightly deeper water, the kayak may loop end-over-end ("pitchpoling"). This can be fun, but only if you are experienced, close to a nice soft beach, and your kayak is unladen.

One possible solution, which has been applied across one manufacturer's range of composite sea kayaks, is to flatten the underwater shape of the hull, especially at the front. It gives the kayak something of the shape of a snow ski. This makes for great performance in surf because the hull lifts out of the water and planes towards the beach at high speed. The flat bow skims over the surface instead of biting into the water, which means that the kayaker retains full control and can make a radical change of direction every couple of seconds if (s)he wants to. There is a price, though. A flattened underwater shape at the bow can give you a noisy, splashy, wet ride on flat water unless the sea is dead calm.

Like sea kayak designers, the designers of racing catamarans want to minimise pitchpoling and the effects of wind on the hull, but they have no interest in tradition. Whether they are sketching a 6 metre day sailor or a 25 metre round-the-world racing machine, they don't do overhangs. Well, OK, some of Dick Newick's gorgeous trimarans have overhangs but aren't they more cruiser-racers? www.dicknewick.com

Seen from the side, racing catamaran hulls have ends which are either near-vertical or rounded like a torpedo. Seen from the top, they resemble a torpedo more than a knife because they have plenty of volume near the front of the hull. The top of the front deck has a steep ridge which helps it slice upward through the water if it gets buried in a wave.

Bow of NACRA catamaran

You will find the same shape at the front end of the outrigger canoes and racing skis that are used for big-money ocean racing in the Pacific. Manufacturers such as Huki (www.huki.com) emphasize that the front end of their OC-1 paddle racing canoes are designed to reduce "pearling". See Pacific Outrigger Canoes.

It is often said that the overhanging ends of a sea kayak are there to enable it to cleave through a wave and rise quickly to the surface again. No kayak needs that ability more than a downriver racing kayak, which must have sparkling performance in heavy water. So why do they never, ever have overhanging ends? Under the International Canoe Federation's racing regulations, the maximum length for a downriver race boat is 4.5m. For maximum speed, none of that can be wasted in overhangs. Consider Prijon's Bala and Sesia (www.ms-composite.cz) and Vajda's Gapa and No Limits (www.vajdagroup.com). Their ends are very similar to those of racing catamarans and ocean racing canoes/skis. By kind permission of MS Composites, this is the Sesia:

Downriver race kayak

At sea, downriver race boats catch the wind and weathercock quite badly, but that doesn't mean the design has nothing to offer. For a light and obedient sea kayak, take a downriver race boat. Cut off the "wings", which are there only to comply with the ICF regulation that a competition kayak must be 60 cm wide. Keep the total length at 4.5 metres. Increase its stability by splitting it vertically from one end to the other and inserting a narrow flat panel to increase its real overall width to 58 centimetres. Split it horizontally to make its profile a few centimetres lower. You don't want the bow to be so high it catches the wind or in strong side winds the kayak would constantly try to turn downwind. Add a drop skeg. You now have a boat which weighs much less than a traditional sea kayak, goes faster, is no more affected by crosswinds, will surf on a shallow ocean swell, and in big surf is likely to get all the way to the beach with minimum excitement. At one time quite a lot of general purpose kayaks were like this. From time to time a manufacturer tries a sea kayak of this sort, but kayakers always seem to prefer the traditional shape.

 

 

 

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