['Seafaring is known to be a demanding occupation but the implications of ageing in seafarers are poorly understood.', 'To investigate task demands and work ability in merchant seamen at sea and to identify factors predicting work ability.', 'This was a cross-sectional study carried out on a single vessel during a summer deployment. Instantaneous heart rate (HR) was recorded at 5-s intervals during representative 8-h shifts in 41 merchant seamen. Participants completed the work ability index and also rated their daily task demands using the National Aeronautics and Space Administration Task Load Index (NASA-TLX). Body mass index (BMI), waist circumference and demographic details were recorded.', "Work demands were found to be moderate according to both HR data and TLX scores. The mean BMI was 27.5 kg/m(2) (standard deviation 3.3) and the mean age was 47 years. The majority of participants rated their work ability as 'good' to 'excellent'. The best predictor of work ability was the interaction between BMI and age, whereas there was a statistically significant negative correlation between HR and age."] Do age and BMI interact to determine work ability in seafarers?