HMS Queen Elizabeth leaves port today for the first time. It is a moment of national significance. The first of two new aircraft carriers, the second being HMS Prince of Wales, HMS Queen Elizabeth represents the very best of British design, innovation, engineering and manufacturing. As an industrial project, delivered by British skill and enterprise, the aircraft carriers show clearly that Britain can still “make stuff”. A huge number of highly skilled and able designers, engineers and shipbuilders have all played a crucial part in this project. Her arrival however begs two key, if separate questions. First, if we can build two such impressive and sophisticated ships why has non-military British shipbuilding virtually disappeared? The second question is, impressive as they are, what are these two huge aircraft carriers actually for?

As an island nation, dependent on the flow of goods and trade by sea, the near collapse of British shipbuilding is completely absurd. Under successive governments domestic shipbuilding has become more and more dependent on supplying ships to the Royal Navy. That British shipbuilders should be supplying ships to the British navy seems a sensible and desirable thing, but that the navy has become almost the sole client for the industry is deeply unsatisfactory.

The industrialist Sir John Parker has been tasked with producing a report and leading the development of a national shipbuilding strategy. Rebooting the nation’s shipbuilding industry should be a policy priority for a partnership between government and British industry. Both Conservatives and Labour referenced Sir John’s report in their 2017 General Election manifestos – displaying a necessary cross-party support for the aspiration. Orders for ships for the Royal Navy should not be the sole source or even the main source of business for a vibrant national shipbuilding industry.

But as HMS Queen Elizabeth sails it is right to pause and reflect on the purpose and role of these two great carriers. Can Britain afford them? Aren’t they simply white elephants, hugely vulnerable to attack? Aren’t they a vanity project for a nation that has global pretensions beyond its means? These are all reasonable and important questions, and in answering them you find the answer to why the Royal Navy is the nation’s principal Armed Service.

When he became Prime Minister in 1976 Jim Callaghan, a former Royal Navy sailor promoted to office during World War Two, reinstated the habit of the Prime Minister receiving a daily update of where every Royal Navy ship was positioned in the world. He understood the vital importance of Britain’s maritime presence.

The Royal Navy is not a visible service. It doesn’t glossily mount guard on Horse Guards or at Buckingham Palace. It doesn’t do crowd pleasing red white and blue smoke trailing fly pasts, but every day of every week of every year the Royal Navy is engaged in operations at home and around the world. Providing the continuous nuclear deterrent at sea – unbroken for almost 50 years. Patrolling, chasing human traffickers and drug smugglers, keeping the sea-lanes open, providing a visible presence, making port calls supporting UK diplomacy, providing a platform on which British industry can trade around the world, and other work important to the national interest. The new carriers will be the nation’s flagships. They will enhance and strengthen these vital roles.

The carriers represent a huge statement of intent and provide a significant capability for military action. A variety of planes and helicopters can be operated from them. They can transport large numbers of troops, serve as mobile command headquarters, and are at the very cutting edge of military capability. First and foremost this is what they are for – to be the most effective military force that it is possible to be. To keep Britain safe, the sea-lanes open and to ensure our interests are robustly supported around the world, but they also represent a huge increase in Britain’s ability to do several other things as well. In addition the new carriers have the potential to be huge mobile trade platforms, they will be able to provide massive aid and disaster relief capability, they are floating embassies, as well as flexible air bases.

As Chief of the Defence Staff Lord Mountbatten championed increasing integration and co-operation between the Armed Forces. He had learned the effectiveness of positive co-operation during his time as head of Combined Operations during World War Two. He achieved some success in encouraging the Armed Forces to work more closely together. In an era of ever tightening budgets the focussing of scarce resource where it can be most effective, supported by inter-service co-operation, is more important than ever and it is clear the Royal Navy can provide the single biggest “bang for the buck” for the British taxpayer that is available. Short of a full-scale land intervention, the Royal Navy, with the support of embarked Army and the Royal Air Force, can provide the necessary military capability with freedom of manoeuvre from the sea, poised over the horizon, ready to act when called for without  the requirement for land or air bases and the support or approval of other nations, and is uniquely capable of providing a whole range of other services and opportunities as well.

On the sea, under the sea, and in the air the Royal Navy can provide a flexible and effective force anywhere in the world. The Royal Marines provide a hugely effective and highly skilled mobile army – they also provide almost half of Britain’s Special Forces capability too. Alongside diplomacy, trade, aid and information gathering. None of these roles are new of course. The Royal Navy has been doing all of this for over five hundred years.

The USA has a huge aircraft carrier capability and building programme, France and Russia cling to the one carrier they each have. China and India are investing heavily in their carrier programmes. There are good reasons why. Do we in the UK want to be left behind? Presidents and Prime Ministers, it is said, move carriers. Soon our Prime Minister will have that opportunity too.

In recent weeks and months we have as a country been pre-occupied with looking inwards. Ever since the EU-referendum and through the 2017 General Election we have been debating domestic issues. The putting to sea of the first of the carriers is a great opportunity to raise our eyes and look out towards the world, and be confident that we have a part to play, a contribution to make, and something positive to say on the world stage.