What type of material should the kayak be made of?
To answer this you really need to ask yourself a few questions:
- How much money am I willing to spend?
- Where am I going to use the Kayak?
- What type of paddling will I be doing?
- Will I have someone to help me mount the kayak on my vehicle?
There are a few types of materials available to the kayak manufacturer. They will select the right type of material based on what they will market the kayak for. For instance if they are going to market the kayak for touring, sea expeditions, or playing in the sea (away from rocks and ledges) they will likely select a composite or linear polyethylene construction and some may opt for a cross molecular chain polymer or a thermoform hull. All have various costs, pros and cons depending on where and when you plan on using them.
Common kayak Materials
Figure 1: Polyethylene in bead form (prior to being molded)
"One method of classifying plastics is by the terms thermoplastic and thermosetting. In general thermoplastic materials can be formed repeatedly by heating or molding because their basic chemical structure is unchanged from its initial linear form. Thermosetting plastics, however, undergo some change during forming and result in a structure in which the molecules are cross-linked and form a network of interconnected molecules." (1)
What the heck did that just say? It said that kayak hulls that are of a cross linked (RAM-X, Cross Linked, Poly-X etc) polyethylene design are more difficult to repair with heat. Standard Polyethylene, Linear Polyethylene and super linear polyethylene can be repaired (Scratches and Scuffs) with something as simple as a high heat blow dryer and a putty knife. Each material has their advantage and disadvantage. Comparing hulls of the same thickness, the thermoset hull will be stiffer and more efficient in the water as it should flex less, the trade off is that the a stiffer hull is less impact resistant. Thermoplastic hulls are less stiff but more impact resistant.
Most "Plastic" kayaks are made from Polyethylene. There are a few reasons for this material selection. Polyethylene is one of the most widely used polymers on the planet. It is in good supply and relatively cheap compared to other materials. Polyethylene comes in many different densities and can be colored. Polyethylene has high impact strength due in part to its relative ductility (softness) and elastic like properties. "Poly" kayaks are well suited for a wide range of activities from Recreational Kayaks that get dragged up and down the beach for years to advanced sandwiched material hulls designed for rock hopping, cliff sliding and multi-day expeditions.Some of our higher end Kayaks take advantage of a sandwich design of Linear Polyethylene (PE), a middle layer of High Density Polystyrene and an inner layer (Cockpit side) of Polyethylene. The polystyrene foam (sandwiched between the two PE layers) provides a lightweight yet strong (when reinforced in this manner) hull, in comparison to three layers of Polyethylene. Polyethylene is the low cost alternative for a wide range of kayaks and Canoes and is an excellent material choice for beginners right up to instructors.
Figure 2: P&H Custom Sea Kayaks - Scorpio HV. The hull of this kayak is constructed of Corelite (a polyethylene/polystyrene sandwich).
Polyethylene construction is generally cheaper because the material is abundant and there is little hands on work once the mold is finished. In fact, one of the biggest cost factors of the kayak is creating the mold (apart from any hardware and special sandwich materials). The more lines and smooth intricate angles a kayak has the more expensive the mold becomes (as there is more work required to create it) thus increasing cost.
Molds don't last forever, any material that is repeatedly heated (if it is not cooled in a controlled manner) will harden (Air quench hardening). As the material hardens it becomes more brittle and can chip with impact. The heating and cooling cycle will also slowly distort the mold which in turn distorts the hull. The more kayaks a mold produces before needing to be reworked the lower the cost is for the company for each unit. Companies with skilled workers and a good quality control program will keep their molds longer and should, in theory, have less defects in their hulls.
Part of the reason some companies have "cheaper" kayaks is they produce their kayaks over seas, they use materials with lower quality control oversight and/or they run their molds longer. Bottom line in most cases, you get what you pay for (with some exceptions of course).
|Polyethylene Pros||Polyethylene Cons|
|Durable||Heavy compared with composite materials|
|Impact Resistant||Can distort if improperly stored|
|Relatively Low Cost|
|Can be lightened when sandwiched with other materials|
- Glass Polymer Composites
Figure 3: Fiberglass strands
Composites are nothing more than matrix material composed of two or more materials, usually a reinforcement material held together by a resin or epoxy (vinyl-ester and polyester resins are common). One such composite is fiberglass. Polymer composites are primarily glass reinforced polymer and are commonly known as fiberglass.
Fiberglass is widely used in recreational and commercial vessels from speed boats to fishing boats fiberglass is a tried and tested material. Glass Reinforced polymer strands are normally reinforced with epoxy resin hardened with a catalyst. In this configuration the material is strong and has fair impact strength as it has some flex. Wear resistance is good due to the material being harder than Polyethylene but the material is less receptive to impact loads (impact loads can crack the hull). A harder hull flexes less therefore is typically more efficient in the water. The final Gel Coat finish in all composite hulls can be polished to a mirror finish which also aids in hull efficiency.
Fiberglass hulls are typically lighter than a polyethylene hull. Fiberglass materials are more expensive than polyethylene and fiberglass hulls typically require more hands on work during construction. Fiberglass kayaks typically fill a mid market role, having many characteristics of higher end Graphite/Carbon and Aramid fiber hulls while being lower in cost.
Fiberglass has a lower modulus of elasticity compared to Carbon and Aramid Fiber meaning "tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it". Generally the harder the material, the more brittle it is.
|Polymer Composite Pros||Polymer Composite Cons|
|Lightweight compared to Polyethylene||Less impact strength compared to Polyethylene (more susceptible to fracture on impact loads)|
|Strength to weight ratio is greater than Polyethylene||More costly than Polyethylene|
|Less Costly than Carbon and Kevlar composites yet retains similar properties||More skill required to repair|
|Less susceptible to U.V. damage than Polyethylene||Chemicals used for repair (respirator protection required)|
|Glass is an insulator, therefore GRP has slightly better insulating properties that Carbon Composites|
- Graphite/Carbon Composites
Figure 4: Carbon Fiber Mat
Carbon fiber is similar to both Fiberglass and Kevlar in that it is a polymer reinforced with fibers of Carbon. One material high in carbon is graphite. Graphite strands are often used for carbon fiber. Similar to Glass Polymer Composites, Carbon Fiber usually consists of an epoxy, carbon mat or loose carbon fibers and a hardening catalyst. Carbon Fiber is stronger than fiberglass in compression and tension. Carbon fiber also has a higher overall yield strength and good heat resistance. Carbon fiber shares the high end market with Aramid fiber (Kevlar). It is very lightweight and strong.
Carbon fiber is often used for high end, light weight kayak construction. This composite of carbon fiber mat and resin has a fair amount of flex or elastic modulus but is still much more brittle than polyethylene. It is however very strong and very lightweight. Carbon fiber is an excellent choice for sea and expedition kayaks. because the hull weighs less (but still retains the same wetted area and volume) it can carry more weight in and on the hull.
|Graphite/Carbon Composite Pros||Graphite/Carbon Composite Cons|
|Very light||Less impact strength compared to Polyethylene (more susceptible to fracture on impact loads)|
|Greatest Strength to weight Ratio||More costly than Polyethylene and GRP.|
|Less susceptible to UV damage than Polyethylene and Aramid Composites.||More skill required to repair than Polyethylene|
|Greatest Modulus of Elasticity of composites (most flexible under load without damage)||
Chemicals used for repair (respirator protection required)
|Very resistant to heat||
- Aramid Composites (Kevlar)
Kevlar, most notable for bullet proof vests and armor in armored vehicles, is one of the most sought after materials in kayak construction. Kevlar is strong and light and has great resistance to heat. Kevlar is usually used in mat form with a binding agent similar to Fiberglass and Carbon Fiber. Kevlar is slightly stronger than Carbon Fiber (depending on the grade of Aramid and binding agent used).
Kevlar is used for top end kayak construction. It is the strongest construction material of the three listed here. It is more brittle then polyethylene and carbon fiber but, because of this, it flexes far less making it more efficient in flat water.
|Aramid Composite Pros||Aramid Composite Cons|
|Lightest material (density)||Less impact strength compared to Polyethylene (more susceptible to fracture on impact loads)|
|Strength to weight ratio is greater than Polyethylene and GRP||More costly than Polyethylene and GRP.|
|Less susceptable to U.V. damage than Polyethylene||More skill required to repair than Polyethylene|
|Very resistant to heat (no melting point)||
Chemicals used for repair (respirator protection required)
|Most resistant composite to fatigue and abrasion wear.||
As shown in the graph below, Carbon/Graphite composites are likely your best bet for high end boats that aren't being used in impact areas, otherwise Polyethylene is your option.
|Fibre Tensile Strength MPa||Laminate Tensile Strength MPa||Density of Laminate Grams/CC||Strength to weight Ratio||Modulus Of Elasticity (Youngs)||UV Resistance (Fair Good Excellent)||
Abrasion Resistance (Fair Good Excellent)
Quote 1: Mott, Robert L. Machine Elements in Mechanical Design / Robert L. Mott. -4th ed ISBN 0-13-061885-3 2004, p. 61 12/08/27
Figure 1: Polyethylene in bead form (prior to being molded), Picture: Lluis tgn http://en.wikipedia.org/wiki/File:Polyethylene_balls1.jpg 12/08/25
Figure 2: P&H Custom Sea Kayaks - Scorpio HV. Jeremy Cline. 12/08/25
Figure 3: Roving aus Glasfasern: Selbst fotografier http://en.wikipedia.org/wiki/File:Glasfaser_Roving.jpg
Figure 4: Carbon Fiber Mat: http://webarchive.teslamotors.com/display_data/bweave.jpeg