There are many things to take into consideration when purchasing any equipment used for water sports. Am I using this for competitive or recreational use? Will it become obsolete? Will I be using it alone or with someone else?
When choosing a kayak in particular there are many features to consider. One of the first decisions to make is to decide how the kayak will be used. For example, you will not want to use the same kayak in the ocean as you would use in a pond and could even be life threatening.
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If you are using the kayak by yourself you need to know that you can comfortably lift the kayak to the height of the rack you will transport it on as well as put it back in storage. If you are purchasing a two person kayak, the same thing applies. Both of you need to be able to easily move the kayak around as needed.
The next concern is storage. Are you going to store it indoors or outdoors? Do you have enough room for the kayak you want?
The next consideration is what type of kayak you are going to buy. If you are looking for a light kayak just to paddle around in ponds and such a small recreational kayak that is easy to pack will probably do.
If you are looking at a little more aggressive kayaking like all day or even overnight tours. You should consider a light touring kayak with at least two hatches. This kayak will be heavier and will probably require two people to load it.
If you are going to get into really serious kayaking for long distances then you will need a touring kayak with at least two hatches. This type of kayak will be longer than the light touring kayak and have extra safety features such as tow lines and will be much safer for ocean use.
As far as shapes and sizes go, there are many choices for you to consider. In general narrow kayaks with a rounded bottom will paddle faster and maneuver better wider and flatter kayaks will be more stable.
There are many materials to choose from for your kayak such as fiberglass, Kevlar, molded plastic, high impact plastic, and wood. For most people the molded plastic kayak will be the choice due to affordability and durability. These kayaks can take a beating and you can drag them over rough surfaces without damage. If you are a competitive kayaker you may want to go with a high impact plastic or Kevlar kayak.
Keep in mind the lighter kayaks made of Kevlar and fiberglass are more expensive to buy and more expensive to repair.