When sensory feedback is perturbed, accurate movement is restored by implicit error-driven processes, as well as deliberate, explicit re-aiming to strategically compensate for errors. How these processes interact to co-determine behaviour during sensorimotor adaptation is not completely understood. To characterise implicit learning, previous work subtracted reported aiming directions from actual movement directions, or attempted to suppress explicit re-aiming via dual-task conditions or by constraining movement preparation time. Here, by instructing participants to re-aim in the absence of a sensory perturbation, we show that re-aiming is possible even with very short preparation times. Nonetheless, re-aiming is effortful and comes at the cost of increased movement variability, so we also tested whether constraining preparation time is sufficient to suppress strategic re-aiming during adaptation to visuomotor rotation. The rate and extent of error reduction under preparation time constraints were similar to estimates of implicit learning obtained from measures of self-report without time pressure. This suggests that participants chose not to apply a re-aiming strategy to correct visual errors when preparation time was constrained. Surprisingly, participants who reported aiming directions during adaptation showed less implicit learning according to an alternative measure, obtained during trials performed without cursor feedback. This suggests that the process of reporting aiming direction can affect the extent or persistence of implicit learning. In sum, the data indicate that restricting preparation time can suppress explicit re-aiming, and provide an estimate of implicit visuomotor rotation learning that does not require participants to report their aiming directions.