Apparatus to control transmission of data, the apparatus comprising: a controller configured to: receive data from at least a first sensor within an azimuth thruster; control storage of the received data in memory; determine whether at least one criterion is satisfied, the at least one criterion varying with the relative positioning of a first antenna mounted on a lower housing of the azimuth thruster and a second antenna mounted on an upper housing of the azimuth thruster, the lower housing being configured to rotate relative to the upper housing; and control transmission of the stored data from the first antenna in response to determining that the at least one criterion is satisfied.