Kayaks have to be light and fairly rigid, but tough. Toughness implies the ability to flex slightly on impact with a rock.
Commercially-made kayaks are made from a range of plastics. At the cheaper end is polyethylene which is very tough indeed; then there are kayaks made of plastic sandwich material such as Royalex, Duralex, Airalite & Carbonlite. The lightest, fastest kayaks are made of fibre-reinforced plastic. Known as FRP, GRP or composite construction this is a plastic resin reinforced with fibreglass, kevlar, diolen and/or carbon fibre.
Plastics are great for durability. It is possible to build yourself a composite kayak at home, with or without a mould but most people who build their own sea kayak prefer wood or skin-on-frame construction. The results very often equal or beat plastic boats for good looks, and for either high strength or low weight.
• A lighter kayak goes further and faster for the same effort.
• There will be times when you have to get your kayak out of the water, up the beach, or onto the roof of your car, on your own. For an awkward load like a sea kayak, a total weight of 25 kg is about the most that one fit adult male can handle on uneven ground with reasonable safety. See Carrying Your Kayak.
• If a sea kayak has to be emptied out by other kayakers at sea, this will be difficult if it is heavy.
FRP sea kayaks tend to weigh 23 to 25 kilogrammes. Full-size polyethylene sea kayaks tend to weigh 26 kg to 28 kg. A kayak for racing or light use can weigh as little as 13 kg, whether it is manufactured in expensive carbon fibre or built at home from plywood.
Here's a true story. Three kayakers from Plymouth University went round Lands End and found big surf on the beach where they intended to land. They were in one of the original Nordkapps, an Anas Acuta and an ocean racing kayak. The kayaks all looked similar, and they were all professionally constructed by reputable firms using glassfibre in polyester resin. The first two weighed about 24 kg and the ocean racing kayak weighed about 18 kg.
On the way into the beach, each of them did a forward loop in the surf. None of them was terribly happy about that but they could all roll. The Nordkapp and the Anas Acuta were fine, but the ocean race boat broke into three pieces.
Kayak for all uses
If you favour the rough-and-tumble style of sea kayaking, involving encounters with rocks, surf and deep-water rescues, a long kayak has to be quite heavy. How are you going to feel about emptying a friend's kayak at sea if you have a 13 kg racing kayak and 700 gramme racing paddles? A man's FRP sea kayak usually weighs between 23 kg and 25 kg, including deck lines, hatch covers, seat and footrest.
How about this for a test? Put the kayak on water or something soft, and have somebody weighing 90 kg walk around barefoot on the deck. Turn it over and have the tester do the same on the hull. The last time your editor hauled himself out of the water and onto the foredeck of a friend's 18 kg fibreglass sea kayak during a rescue exercise, it made a nasty crunching noise under his elbows and he got off again very quickly.
A kayak must have reasonable strength all over because a rigid kayak is a single monocoque structure, like an egg or a car. However it does not need to be equally strong in all places. The art of the kayak builder is to put strength where it is needed. In the 1970s, kayak manufacturers were still learning how to save weight without making kayaks that were too light to last. For example, they found that you can't make a very light hull skin and stiffen it up by adding bulkheads, transverse ribs and a tracery of carbon fibre. Eventually the skin rips along the dotted line.
The hull needs to be strongest and thickest under the seat and for one metre forward and backwards from there, because that is the main location which will be gouged by rocks when you misjudge the depth of water, and the place that gets most stress when an outgoing wave leaves you balanced on top of a sharp rock. The keel also needs to be reinforced under the stern because it will be abraded when you slide down a beach and into the water for a seal launch. It needs to be strong at the bow, because you may occasionally be taken unawares by a wave and surf into a boulder. If your kayak goes vertical while landing through surf and your bow then strikes the bottom, the whole front half of the kayak needs a fair amount of strength or it will snap off near the footrest.
Turning to the deck, it needs to be strongest around the cockpit. The back of the cockpit must often support your weight as you get in, often in some haste. The foredeck just in front of the cockpit must be strong enough to support the crushing weight of a flooded sea kayak during an X-Rescue.
A tough sea kayak can be made that weighs only 20 kg if the designer abandons the traditional overhanging ends and produces a shorter boat, or the manufacturer uses unusually expensive materials and/or difficult manufacturing techniques.
Every square centimetre of hull and deck weighs something. A woman's kayak can be narrower, perhaps a little shorter and therefore lighter.
A child's kayak, with its smaller dimensions, volume and load will be very light but strong enough if made from 3.5 mm plywood. See Intended Use.
Kayak for light use in sheltered waters
A sea kayak weighing 13 kg can be stiff and rigid, great for racing but won't withstand a great deal of rough use.
Not everybody agrees with that. We admit that in 1970 Geoff Hunter went solo round Britain in a Kayel Angmagssalik plywood kayak which may have weighed as little as 13 kg. Also that Paul Caffyn went solo round Japan in a 13 kg kayak made by Sisson Kayaks in Kevlar, stiffened and reinforced in places with Coremat and carbon fibre. He tells us "it did 4,021 something miles and finished the trip in excellent condition despite trying conditions at times. Would not call that careful leisure use. I still have the kayak in my garage. Not sure what it is with Kiwi boat builders, but they can built very light and very strong kevlar kayaks."
If you want a 13 kg kayak, you can have one custom-built in kevlar and carbon fibre, which will cost you an arm and a leg, or you can easily and cheaply build yourself one in plywood. Some very nice designs are available. See Building In Wood - Plywood.
Good things about fibre-reinforced plastic kayaks:
• They have a very attractive glossy exterior.
• Although not as stiff as wood of similar weight, FRP is quite a stiff material so a kayak hull can be made rigid without being thick and heavy
• Except in very lightweight laminates, an FRP kayak is tough enough to seal launch by sliding it into the sea from a sand, gravel or shingle beach with the kayaker already seated.
• An FRP laminate is not affected by water unless it is kept wet for very long periods. Fibreglass yachts occasionally suffer from osmosis; fibreglass kayaks don't.
• The usual FRP laminates are only slightly affected by sunlight, so FRP kayaks can be stored outdoors for long periods if necessary.
• An FRP kayak is quite easy to repair or modify. See How To Add Accessories To A Kayak. Some prefer to leave repairs to a professional.
• If you have access to a mould, you can easily make an FRP kayak. You don't necessarily need a mould. See Build Your Own Kayak In Fibre-Reinforced Plastic.
• FRP is not all that bad for the environment. True, polyester resin is made from petrochemicals and a lot of VOC solvents are released in the building process. True again, both the resin and the fibreglass reinforcement have a large carbon footprint. On the other hand an FRP kayak will last a very long time with no maintenance. There may now be an effective green alternative to FRP. See Biocomposites. And plywood is not as green as you might think.
The not-quite-so-good things:
• Even basic fibreglass construction is expensive because the materials now cost quite a lot, and it takes hours of work to make each kayak.
• A fibreglass kayak may be damaged by impact. If you hit it with a hammer you would cause local fractures and might even make a hole. Your kayak may be gouged if goes over a sharp submerged rock and may be severely damaged if smashed onto a rock or dropped off the roof of your vehicle. A lightweight FRP kayak may break in half if you loop in big surf and one end of the boat hits the bottom.
• FRP kayaks often have tiny leaks along the seams, round the cockpit or at deck fittings.
• The inside of the hull is usually rough with tiny protruding strands of glass, which are very itchy if your bare knee, calf or elbow rubs against it.
• At the really cheap end of the market, quality control can be a problem. See Imported Kayaks.
Want to get more technical? See Fibre-Reinforced Plastic (FRP).
Also known as PE or polythene, it is amazing stuff. It can be used to make plastic shopping bags, dustbin bags or washing-up bowls. If a UV inhibitor is added it can be used to make a bucket for the garden or transparent sheeting to cover a polytunnel or greenhouse. A variant with ultra-high molecular weight is made in heavy slabs which are slick, slippery and tough enough to make skid plates for swamp boats and sled runner inserts for dog sleds; or produced in thin strands and spun into Spectra or Dyneema low-stretch cord for sailboat halyards, power-kite lines or bow strings. From which you will have realised that it is important to pick the right blend when making kayaks.
The good things about polyethylene sea kayaks:
• Cheaper than fibreglass.
• Almost impossible to damage with a hammer.
• Tough enough to loop in surf or seal launch off a boulder beach.
• Not affected by water or salt.
The not-quite-so-good things:
• They are usually heavy. Polyethylene is a floppier material than fibreglass so it must either be made thicker and heavier to get the required stiffness, or made as an expensive three-layer laminate with a foam core. Polyethylene sea kayaks are often a little shorter than their fibreglass equivalent, to save a few kilos.
• Polyethylene is quite easily scratched by sharp rocks or barnacles so the outside of the hull gets increasingly furry, and harder to paddle because it drags a heavy skin of water around with it ("turbulent flow"). What you really want is a smooth hull which moves easily through the water ("laminar flow").
• The original polyethylene kayaks became brittle when exposed to sunlight over a long period. Kayaks being manufactured today have a much more effective UV screen. Be cautious about buying a second-hand polyethylene kayak if it was made more than five years ago, or if it was made by a small, inexperienced manufacturer. It's still a good idea to store any kayak out of direct sun.
• Polyethylene is a saggy material, and the original polyethylene kayaks sometimes got an ugly concave patch under the seat. If strapped down hard onto a roofrack on a very hot day, a polyethylene kayak may develop permanent slight creasing. Again, kayaks being manufactured today should not have this problem, especially if they are made of a three-layer laminate.
If the hull splits, the crack may carry on getting longer unless you make a small round hole at the tip of the split with a 2 mm (3/32") drill or a piece of hot wire. Splits can usually be welded with a professional hot air gun such as a Leister Triac, but there is a risk of melting the plastic around the split and making the problem worse. Some kayak shops have a repair service.
Plastic sandwich materials have the potential to produce a kayak as light, glossy and durable as fibre-reinforced plastic but for the price of polyethylene. The plastic is made in bulk and sold in sheets to the kayak manufacturer for use with moulds made of polished alloy. The manufacturer cuts the approximate shape of the hull out of one plastic sheet, heats it up and uses gravity and vacuum to suck it into the hull mould. Then the same for the deck.
Sandwich sheets consist of several layers laminated together, each layer having different properties. If the main plastic is tough but rather floppy, a stiff but light hull can be made by using two thin layers of it and laminating a thin layer of foam between them. If the main plastic is tough but dull or easily scratched, a glossy hull can be made by laminating a layer of some harder plastic on the outside.
Royalex is a Uniroyal trade name. It is popular for Canadian canoes which need to bounce off rocks in white-water rivers. It makes a durable and attractive hull with excellent "memory" so that it has been known to spring back into shape almost undamaged after being bent in half round a boulder. It stays smoother than polyethylene and is lighter and has better UV resistance. However it is not as light or glossy as fibre-reinforced plastic. Apparently it is a five-layer sandwich with outer skins of PVC (vinyl) backed by a layer of ABS. The central layer is also ABS but it expands when heated during the manufacturing process, giving the laminate a foam core. A sea kayak was made in Royalex as long ago as 1994. It weighed about as much as a current polyethylene kayak but the technique does not seem to have caught on and we have never seen one.
Duralex, Airalite and Carbonlite are also trade names for plastic sandwich materials. As far as we know they all consist of ABS thermoplastic with an acrylic exterior. Perception Kayaks are making increasing use of Airalite, and Eddyline Kayaks are using Carbonlite for sea kayaks. The appearance, weight and toughness of kayaks moulded in these materials is said to be similar to glassfibre. We have not yet seen one and have no experience of their toughness, durability or ease of repair.
See Folding Kayaks.
• Build your own kayak in fibre-reinforced plastic
• Build your own wood kayak
• Building in wood - plywood
• Building in wood - strip-plank
• Building in wood - cold moulded
• Build your own skin-on-frame kayak
• Skin-on-frame materials in Europe
• Skin-on-frame kayaks & offshore safety
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• Fibre-reinforced plastic (FRP)
• Build your own kayak in fibre-reinforced plastic