What is the difference between a "spec" boat and an "unlimited" boat?

Most races have rules that set out limitations on the design of a boat hull in regards to each class.  These classes are sometimes referred to as "Spec" classes due to the fact that the boats used in the classes have to comply with specific design rule requirements.   The boats designed for these classes are sometimes referred to as "spec" boats.  The purpose of having specific hull design requirements for classes is to minimize the boat as a factor that would cause one paddler to win out over another paddler during the race.  The thought process behind creating classes like this is that the emphasis of competition should be centered on the paddler without giving too much of an advantage by allowing a competitor to use a superior designed boat. 

Also, many races have a class that is called the unlimited class.  This class is for paddlers that do not wish to be bound by any rules that limit the hull design or the style of paddles they use in  the boats they race.  For these folks, the design of the canoe is as much a part of the competition as the athlete who paddles the canoe.  Accordingly, the types of boats that are designed with this philosophy in mind are called "unlimited" boats.  The boats are generally designed without the boundries of any specific rules.  Further, both single bladed canoe paddles and double bladed kayak paddles are used to propel unlimited racing canoes.  Often the promoters of a race that includes unlimited canoes will only break down the classes by the number of paddlers in the canoe and/or by the paddler's gender.  

Another thing to note on the above is that sometimes a boat is built to be used with certain rules ("specs") in mind, but a race does not have a class set up for the specs of that boat.  Most of the times when this happens, the boat will be placed in the unlimited class even though the boat was built with certain class specifications in mind.  A good example of this is the ICF sit and switch C2 design.  While this boat was made with certain hull specifications in mind, most races do not have an ICF C2 class.  As a result, ICF C2 most often is included in the unlimited class.