  When an ensemble of self-propelled camphor boats move in a one-dimensional
channel, they exhibit a variety of collective behaviors. Under certain
conditions, the boats tend to cluster together and move in a relatively tight
formation. This type of behavior, referred to as clustering or swarming here,
is one of three types recently observed in experiment. Similar clustering
behavior is also reproduced in simulations based on a simple theoretical model.
Here we examine this model to determine the clustering mechanism and the
conditions under which clustering occurs. We also propose a method of
quantifying the behavior that may be used in future experimental work.
