Saying a kayak is made of plastic is about as specific as saying a spoon is made of metal. Nice to know, but certainly not enough information to understand how the spoon will perform.
• The spoon is made of hardened stainless steel in which case it will last forever and be very stiff, but will be heavy with it.
• The spoon is made of Aluminum, in which case it will be very light, but could oxidise in salt water and may not last forever due to the material being naturally softer.
• The spoon is made of lead, in which case it will be extremely heavy, very soft at room temperature, and it might poison you whilst you’re using it.
Now depending on your use for said spoon, you may want it to be very durable, you may want it to be very light, you may want some flex in your spoon, or you may want it to be made of lead in the hope it poisons someone.
Ultimately, your end use, your budget, and the compromises you are willing to make will help you choose the right spoon for your needs. There isn’t a one stop answer as to which is right, you just need to look at the material properties and find the best fit!
Kayaks are no different to spoons. The best plastic properties depend on your use, your budget, how heavy you are prepared for your kayak to be, and how long you wish it to last.
So what properties are important to look for?
Looking at plastics can quickly become confusing. With a myriad of differing plastic types, a vast amount of differing testing standards, and a huge variety of figures being quoted it is very hard to know what you’re looking at, or even what you should be looking for.
When we evaluate a new plastic polymer for kayak or canoe production there are numerous things we look at, but there are a few key properties that are relatively easy to measure, and all of which have a very obvious impact on how the end boat will perform, these are:
• UV stability
• Stiffness (flexural modulus)
• Impact resistance
UV stability can vary hugely between different plastic grades. Good UV stability is hugely important, not just from a cosmetic perspective, but from a structural one too. UV damaged plastics lose much of their inherent properties, losing shape, losing colour, but most concerning of all losing impact resistance by becoming very brittle. Below is an example of a two year old kayak made of a low grade plastic. You can see how it has lost it’s colour, lost it’s shape, and become very brittle. Essentially the kayak has become unsafe to use and completely worthless in just a few years of use.
UV stability really is hugely important, because without it the following two properties don’t really matter:
Stiffness is vital for performance in boats. A floppy hull will not only absorb much of your paddling energy, but it will make it harder to catch waves, and will make the boat less predictable through turns. As a rule, stiffer materials also allow a manufacturer to use less plastic – leading to lighter weight kayaks.
Impact resistance is one measure we use to work out how durable a kayak is. There are various testing methods, but the one we prefer involves dropping a heavy weight onto a plaque of plastic that has been cooled to -40 degrees Celsius. (http://www.rotomolding.org/pdf/lowtemp.pdf ) as this essentially exaggerates the action of smashing a boat into a rock/groyne/riverbed repeatedly in very cold conditions.
As a rule a plastic that has a good mix of the above characteristics will make a great boat that will last decades of use.
What about all these different “types” of plastics, what is best?
Quickly scanning through the internet, you can find dozens of different plastic grades being used to produce kayaks.
Sticking to rotationally moulded canoes and kayaks you will find four different categories of Polyethylene being used to make boats – these are:
• LDPE (http://en.wikipedia.org/wiki/Low-density_polyethylene)
• LLDPE ( http://en.wikipedia.org/wiki/Linear_low-density_polyethylene )
• MDPE (http://en.wikipedia.org/wiki/Medium-density_polyethylene )
• HDPE (http://en.wikipedia.org/wiki/High-density_polyethylene )
Often a shop or manufacturer will quote a kayak as being made from one of these four categories of PE, but on it’s own this information won’t tell you a huge amount (it’s much like saying that spoon is made of metal)
Used simplistically the higher the density, the stiffer and more durable the kayak, but as with most things it is not this simple…..
Good quality kayaks and canoes tend to be made with ‘polymer alloys’ which are a compounded mix of the PE types above, with other various additives to improve key properties.
The best alloy grades offer vastly improved combinations of physical properties over any one individual ‘category’ and the alloying process also makes it easy to compound in colour and UV stabilizers ensuring those better properties last longer.
Annoyingly this means you can have a very good MDPE that can outperform a poor HDPE even though the density would indicate otherwise, so we come back to those critical properties of UV stability, stiffness, and impact resistance – all easy to measure and compare as the best way to establish what is best for your needs.
An example comparison:
The majority of cheap kayaks and canoes produced in the far-east are made with Exxonmobil LLDPE – an easily mouldable and very affordable material.
By comparison many UK manufacturers (including Tootega, Silverbirch, Perception, Islander) use a variant of a material manufactured and alloyed by Polymer specialists Matrix Polymers called M-601. It is harder to process than the LLDPE and costs over twice as much as a raw material, but hopefully the key specs below will help to explain why we all use it:
A brand new boat made of Matrix M-601 polymer will have almost three times the stiffness and over twice the impact resistance of a boat made from Exxonmobil LLDPE.
If you add UV stability into the equation, then a kayak made with the Matrix M-601 (or similar grade polymer) will only increase it’s performance advantage over the low grade material with every day that goes past.
Put simply – you would expect a kayak made with the superior material to have virtually the same material properties after five years that it had as new, where as the kayak made from Exxon mobile LLDPE would be fit for nothing but the bin (as demonstrated further up this article)
• Not all plastics are equal – before making a purchase check how the key material properties stack up against a product you are familiar with!
• Stiffer plastic allows for more complicated hull forms to be moulded – giving better performance characteristics
• Stiffer plastic with better impact resistance is harder to break – giving you a more durable product.
• Stiffer plastic makes lighter kayaks – again boosting performance and reducing fatigue, it makes it easier to carry too.
• Low grade plastic can be used to create very affordable products – if resale value, or durability aren’t an issue (for example it’s just for holiday use and you can store it inside) then they could be a good choice.
• Know what you are buying. If the manufacturer can’t or won’t give you answers to your materials questions, then there is probably a problem with the quality of the product you are being sold.