An internal combustion engine having a front and a rear and adapted or converted for marine use, including a turbocharger driven when in use by exhaust gas pressure for compressing aspired air, the engine being internally cooled by a first internal closed coolant circuit with flow caused by pump and connected through one or more non-mingling heat exchangers to a flow of raw water caused to flow by pump in a second, open circuit; the engine having at least one bank of combustion chambers each chamber having a corresponding exhaust port brought to a manifold face of an engine head, characterised in that (a) one exhaust manifold for each bank of combustion chambers is formed from a solid thermally conductive mass having a thick wall surrounding a duct capable when in use of carrying hot exhaust gases from the exhaust ports; the wall being sufficiently thick to include a plurality of internal conduits or galleries (215, 216, 217, 218); the exhaust manifold having a closed end and an open end; (b) coolant apertures (101E, 101F, 101G, 101H), comprising a coolant aperature nearest the front and a coolant aperature nearest the rear, are formed through the manifold face of the engine, each aperture creating an opening into an internal coolant gallery surrounding a corresponding combustion chamber; (c) the manifold face along one side of the exhaust manifold includes a series of exhaust channels each aligned with one of the exhaust ports of the bank and leading into duct, and includes a series of coolant channels (101A, 101B, 101C, 101D), comprising a coolant channel nearest the front and a coolant channel nearest the rear, each cooling channel (101A, 101B, 101C, 101D) connecting a corresponding one of coolant apertures (101E, 101F, 101G, 101H) into the plurality of internal conduits or galleries (215, 216, 217, 218) of the exhaust manifold, wherein said coolant channels thereby include the exhaust manifold within the closed coolant circuit and when in use limit a temperature of the exhaust manifold by removing heat into the coolant; (d) the exhaust manifold further including at least one outlet at the open end for coolant and an outlet for exhaust gases; said outlets being aligned with corresponding inlets of an exhaust manifold extension serving to separately carry the coolant as and gas outflow as from the exhaust manifold to at least one turbocharger; characterised in that (e) each coolant channel is provided with a predetermined resistance to flow of coolant therethrough such that the coolant channel nearest the front provides a path of least resistance for coolant flow within the engine and the manifold; said path traversing the interior of the engine from a rear inlet to the front, through the cooling aperature nearest the front and the cooling channel nearest the front and then traversing the length of the exhaust manifold towards the open end thereby when in use limiting the maximum temperature of the exterior of the manifold to an amount compatible with use in a marine inboard engine installation yet maintaining a flow of coolant throughout the engine. (a) one exhaust manifold for each bank of combustion chambers is formed from a solid thermally conductive mass having a thick wall surrounding a duct capable when in use of carrying hot exhaust gases from the exhaust ports; the wall being sufficiently thick to include a plurality of internal conduits or galleries (215, 216, 217, 218); the exhaust manifold having a closed end and an open end; (b) coolant apertures (101E, 101F, 101G, 101H), comprising a coolant aperature nearest the front and a coolant aperature nearest the rear, are formed through the manifold face of the engine, each aperture creating an opening into an internal coolant gallery surrounding a corresponding combustion chamber; (c) the manifold face along one side of the exhaust manifold includes a series of exhaust channels each aligned with one of the exhaust ports of the bank and leading into duct, and includes a series of coolant channels (101A, 101B, 101C, 101D), comprising a coolant channel nearest the front and a coolant channel nearest the rear, each cooling channel (101A, 101B, 101C, 101D) connecting a corresponding one of coolant apertures (101E, 101F, 101G, 101H) into the plurality of internal conduits or galleries (215, 216, 217, 218) of the exhaust manifold, wherein said coolant channels thereby include the exhaust manifold within the closed coolant circuit and when in use limit a temperature of the exhaust manifold by removing heat into the coolant; (d) the exhaust manifold further including at least one outlet at the open end for coolant and an outlet for exhaust gases; said outlets being aligned with corresponding inlets of an exhaust manifold extension serving to separately carry the coolant as and gas outflow as from the exhaust manifold to at least one turbocharger;