Recently, four cognitive support concepts were developed, that should help to handle high-y demand situations. These concepts are based on a cognitive task load model and can be realised with a specific method for cognitive task load analysis. The model and method describe task load in terms of three behavioural factors: the percentage time occupied, the level of information processing and the number of task-set switches. This paper shows the design and evaluation of a prototype user interface providing the proposed support concepts. Application of the design method proved to be possible for the chosen maritime domain (the bridge of an icebreaker) and resulted in an interface that contains an information handler, a task scheduler, a diagnosis guide and a rule provider. The resulting prototype user interface was evaluated with navy cadets, to study the effects of cognitive support, under high and low task load, on task performance, mental effort and out of the loop effects. The use of support functions leaded to a substantial increase in performance, especially at high task load. Costs on out of the loop effects, like the not reacting on an implemented wrong advice and a decrease in understanding of performed actions, could not be found.