It is widely known that tugboats play very important roles for harbour manoeuvres of large ships. There have, however, been few researches on abilities of such tugboats. In the present report, an attempt is done to establish the procedure for estimating the abilities of tugboats running with large ships. Results of captive model tests of tugboats are first discussed with special emphasis on interfering effects induced by large ships near them. The procedure to estimate their pushing abilities is secondly discussed. It is finally pointed out that hydrodynamic forces induced by their manoeuvring motions are taken into consideration for estimating such abilities of tugboats.