There is no such thing as the “best” material for kayak hulls.
Just kidding. You have a lot of choices when it comes to the material that kayaks can be made from. Which material you choose depends on what you want to pay, how light the boat needs to be, and what you plan to do with the kayak.
Rotomolded polyethylene is typically your least expensive and heaviest kayak material. It is probably the best choice for recreational kayaks since people don’t usually want to spend a lot for a recreational kayak. Polyethylene is the most flexible material and therefore, the most impact resistant. However, it is also the softest material meaning that it deforms easily when improperly stored, flexes while the boat is being paddled (reducing efficiency in the water), and scratches deeply and easily increasing drag and slowing the kayak as you paddle it. The impact resistant quality makes it the best choice for most whitewater kayaks which are likely to suffer a lot of impacts on fast-moving rivers. Luckily, poly boats don’t usually require much in the way of hull repair because rotomolded polyethylene is difficult to repair. Many people mistakenly refer to polyethylene as indestructable. While it is true that it doesn’t easily crack, it can wear through if you constantly drag the boat around on rough surfaces like gravel and concrete. If you wear a hole in the bottom of a polyethylene kayak, you are mostly out of luck and will need to replace the kayak. The lower price makes it a good choice for a first sea kayak when you have a lot of other accessories that you have to buy to get started in the sport, but serious sea kayakers usually upgrade to fiberglass or kevlar when they can afford it.
Thermoformed or polycarbonate plastics are your next step up in price. These kayaks look shiny like fiberglass and are more scratch resistant than polyethylene. As a result, they have less drag in the water and feel more like a fiberglass kayak when you are paddling them. They are slightly cheaper than a fiberglass kayak and a bit more repairable than polyethylene, but not as repairable as fiberglass. These kayaks are usually lighter than a roto-molded polyethyle kayak of the same length. They serve a niche market for people who must have the lighter weight but can’t afford to upgrade to fiberglass or kevlar. I am not convinced of the long-term durability of this material, despite what the manufacturers would have you believe. There have been some problems with the deck to hull seam not being glued well enough in some boats, so do some research and look thes boats over carefully when you purchase.
Fiberglass is a common choice of material for sea kayaks. It is relatively lightweight and will last a long time (decades). It doesn’t take impacts as well as polyethylene, but it is tough enough to withstand a fair amount of abuse. Sea kayaks don’t usually suffer the kinds of impacts that whitewater kayaks routinely experience. Fiberglass kayaks are typically covered with gelcoat making them more scratch-resistant than polyethylene. The gelcoat can be polished and buffed to remove small surface scratches, and when too much gelcoat has worn off, it can easily be reapplied to the worn areas protecting the glass cloth underneath. Damage to a fiberglass kayak is repairable and can be a do-it-yourself project. Repair materials are easily available at boating stores like Boat U.S. It is also easier to customize fiberglass kayaks since it is easy to epoxy pieces onto the boat. There can be a wide range in the quality of the fiberglass cloth or material used to construct a kayak, so do your homework. Avoid chopped mat glass and look for woven cloth lay-ups.
Kevlar has very similar properties as fiberglass. It is also most often covered in gelcoat. It is, however, lighter than fiberglass and more expensive, usually by about $400-500. Most do-it-yourself repairs to kevlar kayaks are done with fiberglass because kevlar is hard to cut and work with. Kevlar has gotten a bad rap at times. It has been said that kevlar boats are susceptible to gelcoat cracks. That all depends on the construction of the boat. Kevlar is ounce for ounce stronger than steel. As a result, manufacturers can build boats with very thin lay-ups of kevlar making the boats super lightweight. The kevlar will not tear or puncture, but it will flex when it is laid up in very thin layers. Since the gelcoat is stiff and cannot flex with the kevlar, it will crack. Look for kevlar that is stiffened with carbon to reduce flexing, or go with a slightly heavier kevlar layup that resists flexing and you will have no more problems with gelcoat cracks than you would with a fiberglass kayak.
Finally, there are wood kayaks. This is often the choice of the do-it-yourself kayak builder. Wood kayaks can be strong and lightweight, easy to customize, and relatively easy to repair. “Stitch and Glue” construction is easier for most first time builders, but tends to limit the shape of the hull to a hard chine or multi-chine design. “Strip built” kayaks give you unlimited design freedom, but may take a little more wood-working skill. Most wood kayaks today are covered in a layer of fiberglass and epoxy to protect the wood. Several layers of varnish are brushed over the fiberglass after it has cured. Building your own wood kayak will get you a fiberglass or kevlar quality boat at a polyethelene price (or even lower than poly). If you have your wood kayak built for you, you will pay more than the cost of a kevlar kayak. In these cases, you are drawn to the aesthetics of the wood and the design of the kayak and are willing to pay for the craftsmanship that goes into building a kayak like this.
For certain people, folding or inflatable kayaks serve a need for those who have nowhere to store a hard shell kayak, or want a kayak that can be taken on airplanes as luggage. The prices for these kayaks can be all over the map, but generally, you get what you pay for. The really cheap ones won’t last and they probably won’t perform as nicely on the water. If you have the space to store a hard-shell kayak, you will want to go that route. While you may not have to car-top an inflatable or folding kayak, you will have to lay the boat out at home to dry before you store it to prevent mildew, you will have to assemble and disassemble the boat each time you use it, and you will have to repair leaks as the boat gets older.
So which is the best hull material? As you can see, it depends.