The section, called Lower Block 04, is the largest part of the aircraft carrier HMS Prince of Wales.
It is 20 metres high, 80 metres long and comprises a fifth of the overall vessel.
It will take about five days to reach Rosyth in Fife, travelling around the north of Scotland.
The section, which contains the warship's hangar, machinery space, mission systems compartments and accommodation, will arrive at the dockyard in Fife next Friday afternoon.
It began its journey from BAE Systems' Govan shipyard on Sunday morning and passed under the Erskine bridge at about noon.
The section, floating on a sea-going barge, arrived at Greenock at about 15:30 and will be pulled by a sea-going tug, the Carlo Martello.
The Prince of Wales is the second of the new Queen Elizabeth class aircraft carrier to be built for the UK.
Each 65,000 tonne aircraft carrier will provide the armed forces with a four-acre military operating base which can be deployed worldwide.
Even the hull section is bigger than the Type 45 destroyers which were the last vessels to be delivered to the Royal Navy from Glasgow.
The final two smaller sections of HMS Prince of Wales are scheduled for delivery by the end of the year.
The Prince of Wales is due to be completed at Rosyth dockyard in 2017.
A massive 11,200-tonne section of an aircraft carrier built on the Clyde has begun its journey around the coast of Scotland to Rosyth for final assembly.