We call them poly, or sometimes just plastic. Polyethylene is a very popular material for recreational kayaks. There are three variations on the polyethylene theme: rotomolded single-layer, rotomolded multi-layer, and blowmolded.
The rotomolded kayaks start life as a lot of little pellets before they are heated and spun in gigantic ovens. Single-layer plastics are common in small kayaks where the decked hulls remain reasonably rigid. This version of inexpensive manufacturing gets lots of people out on the water. The multi-layer method adds a foam core to the outer layer. Most multi-layer versions cap the foam core with an inner layer. The extra depth adds to the stiffness of the hull and allows the designer to reduce weight (think corrugated cardboard).
Not all poly-based kayaks are the same. The type of plastic used can vary from manufacturer to manufacturer and from model year to model year. Typically, long linear poly chains are better. They are stiffer and stronger, and can be recycled. Crosslink poly (strong yes, but not recyclable) is less common than it used to be, but is still seen. Blowmolding is an uncommon but innovative method of creating a plastic hull. Plastic is pressed through a die under tremendous pressure and then clamped by a mold. The resulting plastic molecules are much longer than with conventional rotomolding. Longer linear chains = better hull stiffness and strength. Prijon calls their blowmolded plastic HTP, which is an abbreviation for a long string of heavy sibilant sounds. I much prefer the 3-letter acronym.
Some manufacturers (Eddyline, Current Designs, Perception/Dagger) have invested a lot of time and effort in a new generation of ABS materials. They use thermoformed ABS (under the trade name of Carbonlite, TCS, or Airalite) because it promises a lot of stiffness and strength for not a lot of weight. They are recyclable too. ABS boats are very beautiful. They have deep lustrous color and shine typical of more expensive laminate boats. From a distance, they look just like their pricier cousins.
Composite construction kayaks use a fiber and a resin to provide a stiff, sleek hull. Composite kayaks can be molded into very complex shapes, and they typically have very low friction coefficients. This translates into sleek, efficient hulls. Differing materials and/or methods of construction will yield different weights and/or strengths.
Fiberglass is the granddaddy of the composite line. It is typically the heaviest and most inexpensive composite material. Its big advantage for today’s paddler is that it’s not hard to maintain. Scratches and heavy dings can be repaired easily.
Kevlar is an Aramid fiber from DuPont. Most folks know it as the primary material in bulletproof vests. A warning: do not shoot your Kevlar kayak. Kevlar kayaks are built to be either very tough or lightweight, but stopping speeding lead is not in their design parameters. Kevlar is tricky and expensive stuff to work with in manufacturing. It is also difficult (though not impossible) to repair. Some manufacturers put one layer of fiberglass on the outside of the hull to aid in fixing dings. This adds weight, so others leave it off.
Carbon or Kevlar/carbon lay-ups promise yet more weight savings. They are typically used in racing boats, or for people with very tender backs. Carbon fiber is extremely rigid and strong for its weight, but the stiffness comes at a cost. Rather than deforming and deflecting on impact, it can break or shatter. It is an impressive material, but should be matched with an appropriate use.
Prepreg (pre-impregnated) fabrics are a new innovation, courtesy of the aircraft industry. Normal composites have the resin added late in the game. Prepregs come with the resin already in place. There are different schemes to cure the resin, but the result is always a very lightweight (and surprisingly strong) hull. One manufacturer is rolling out a line of prepreg boats we think will completely shake up the industry. Imagine a 17’, 22” wide sea kayak that weighs only 29 pounds! Is there a catch? Well, yes, sort of. Prepreg materials are very difficult to work with, and that translates to a formidable price tag.
Lots of people have discovered the joys of wood boats. Wood is very strong, very malleable, quite light, and easy to work with. Some wood boats are built to be skin on frame; others are solid and covered with fiberglass or canvas. Properly maintained, they can last forever. I really like the “feel” of a wood/canvas boat. There is something magical about its combination of stiffness and suppleness while gliding through the water. Wood comes at a price, too, but is often a worthwhile investment in craftsmanship and a lifetime of memories on the water.