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56 take the exhaust pipe up and above the waterline and then down again, to discharge underwater.” The seawater supply having cooled the engine, that flow is flushed out with the exhaust. It exits through a silicon tube coaxial with – and surrounding – the (smaller diameter) metal exhaust pipe, so it follows the same arched route. In this manner it advantageously cools the exhaust, helping to minimise temperature in the engine bay. “We don’t want the heat radiated by the exhaust to get inside the sealed engine bay; we have to keep that as cool as possible,” West says. In running the coolant and exhaust coaxially, both exit through the same underwater check flap valve, which October/November 2016 | Unmanned Systems Technology Dossier | HFE H70 marine powertrain Lower crankcase assembly Helical mitre gear drive Mid-crankcase assembly Lower crankcase assembly Mid-crankcase assembly Exhaust discharge