The theory of the problem of collision at sea has been formulated by many authors and the results of the theory have been used, notably by Morell, Hollingdale and Calvert, to establish practical manoeuvring rules to avoid collision. Any treatment which attempts to take into account the uncertainties in the information available to the navigator is bound to be technically difficult. It is arguable that a theory in which no account has been paid to the possibility of equipment errors is incomplete.