Inland waterway tanker for transportation of liquid product over inland waterways, e.g. crude oil, said tanker comprising - a hull having an outer hull shell; - one or more liquid product cargo tanks arranged within said hull, spaced from the outer hull shell to provide a double hull configuration, - a tanker propulsion engine installation, said installation comprising: a). a combustible fuel powered propulsion engine; b). an exhaust system for exhaust gas from said engine, c). a cooling water circuit for cooling said engine, said cooling water circuit including a cooling water pump, - a cargo tank heating system for heating liquid product within said one or more cargo tanks, said heating system comprising: i. a closed thermal oil circuit filled with thermal oil, said thermal oil circuit comprising heat exchanger ducts extending along and/or within the one or more cargo tanks to heat the liquid product therein, ii. a high-temperature thermal oil heating device having capacity to heat the thermal oil in said circuit to a temperature above 100 °C, characterized in that the cargo tank heating system comprises a thermal oil circuit heat exchanger having a cooling water flow path forming part of the cooling water circuit and a thermal oil flow path forming part of the closed thermal oil circuit and being separated from said cooling water flow path, said thermal oil circuit heat exchanger being adapted to exchange heat between the cooling water and the thermal oil in order to allow for heating of the liquid product in the one or more cargo tanks by heat recovered from the cooling water, preferably in a low-temperature circulation mode wherein the thermal oil temperature is below the cooling water temperature of cooling water emerging from the propulsion engine, and in that the water cooling circuit comprises a bypass duct adapted to allow for bypassing the cooling water about the cooling water flow path in the thermal oil circuit heat exchanger, and in that the heating system comprises one or more valves (13,14) allowing to selectively open or close the cooling water flow path and the bypass duct to selectively pass the cooling water through the thermal oil circuit heat exchanger or to bypass the thermal oil circuit heat exchanger. - a hull having an outer hull shell; - one or more liquid product cargo tanks arranged within said hull, spaced from the outer hull shell to provide a double hull configuration, - a tanker propulsion engine installation, said installation comprising: a). a combustible fuel powered propulsion engine; b). an exhaust system for exhaust gas from said engine, c). a cooling water circuit for cooling said engine, said cooling water circuit including a cooling water pump, a). a combustible fuel powered propulsion engine; b). an exhaust system for exhaust gas from said engine, c). a cooling water circuit for cooling said engine, said cooling water circuit including a cooling water pump, - a cargo tank heating system for heating liquid product within said one or more cargo tanks, said heating system comprising: i. a closed thermal oil circuit filled with thermal oil, said thermal oil circuit comprising heat exchanger ducts extending along and/or within the one or more cargo tanks to heat the liquid product therein, ii. a high-temperature thermal oil heating device having capacity to heat the thermal oil in said circuit to a temperature above 100 °C, i. a closed thermal oil circuit filled with thermal oil, said thermal oil circuit comprising heat exchanger ducts extending along and/or within the one or more cargo tanks to heat the liquid product therein, ii. a high-temperature thermal oil heating device having capacity to heat the thermal oil in said circuit to a temperature above 100 °C,