design has its specific advantage, including performance, manoeuvrability, stability and paddling style. Kayaks can be made of metal, fibreglass, wood, plastic, fabrics, and inflatable fabrics such as PVC or rubber, and more recently expensive but feather light carbon fiber. Each material also has its specific advantage, including strength, durability, portability, flexibility, resistance to ultraviolet and storage requirements. For example, wooden kayaks can be created from kits or built by hand. Stitch and glue, plywood kayaks can be lighter than any other material except skin-on frame. Inflatable kayaks, made from lightweight fabric, can be deflated and easily transported and stored, and are considered to be remarkably tough and durable compared to some hard-sided boats. Equipment There are many types of kayaks used in flat water and whitewater kayaking. The sizes and shapes vary drastically depending on what type of water to be paddled on and also what the paddler would like to do. The second set of essentials for kayaking is an off-set paddle where the paddle blades are tilted to help reduce wind resistance while the other blade is being used in the water. These vary in length and also shape depending on the intended use, height of the paddler, and the paddler's preference. Kayaks should be equipped with one or more buoyancy aid (also called flotation) which creates air space that helps prevent a kayak from sinking when filled with water. A life jacket should be worn at all times (also called a personal flotation device or PFD), and a helmet is also often required for most kayaking and is mandatory for white water kayaking. Various other pieces of safety gear include a whistle for signaling for help; throwing ropes to help rescue other kayakers; and, a diving knife and appropriate water shoes should be used depending upon the risks the water and terrain pose. Proper clothing such as a dry suit, wetsuit or spray top also help protect kayakers from cold water or air temperatures. Types of kayaks "Sit on top" kayaks place the paddler in an open, shallowly-concave deck above the water level. This style is usually used for non-white water activities as most find it harder to stay inside the kayak while also preventing them from "rolling" which allows the user to upright themselves if they flip over. There are some benefits to sit on tops such as the ability for a "dry hatch" these are