The UK's two aircraft carriers are now costing £6.2 billion, so just over £3 billion each.

To put this into context, at £1.45 billion the new Forth Road Bridge, which is the largest transport infrastructure project in Scotland, is costing less than half the price of a single aircraft carrier.

Despite their unbelievable cost, in terms of ship size the carriers are not really that big. Their length of just over 280 metres is considerably less than a mega-container ship of 400 metres, or the near 400 metre long mega-cruise ships currently being built at yards in Germany, Italy and France.

Each aircraft carrier is 65,000 tonnes. Sounds big, maybe, but the largest cruise ships are over 220,000 tonnes and each mega-container ship can load over 200,000 tonnes of cargo.

In fact the 'shipbuilding' cost element of each aircraft carrier is probably less than 10% of the overall contract cost, with 90% or more of the funding related to paying for the sophisticated systems on board. Indeed the basic steel hull of a 300 metre ship does not even cost close to £100 million, as a fully completed mega container ship comes in at that price.

So the carrier contract is rather less about shipbuilding (or shipbuilding jobs) and much, much more about expensive systems, likewise other naval vessels such as destroyers.

What we know for sure is that there are no plans for the UK to build more aircraft carriers, which is why within the union we are about to see a big fall in the number of shipbuilding jobs in Scotland. Westminster plans for future warship builds are not sufficient to maintain the current number of shipyards or shipbuilding jobs, which is why we are also about to see the consolidation of the two Clyde Shipyards into one.

So we need to think about a more sustainable future for Scotland's shipbuilding industry and to assess whether this is more likely to come with a Yes (putting economic policy for Scotland into our own hands) or a No (leaving decisions about our jobs and future in the hands of Westminster).

Scotland needs around 100 new ferries over the next 10-20 years to serve our many island routes. So this suggests that if we really want to build ships we should be building ferries.

Norway has 300 ferries, Demark 200, Greece 400 and many other EU states have ferries, most of which are needing replaced, which means the European market is large, and on our doorstep.

After a Yes vote in September, an independent Scottish Government will also need to build a new coastal defence fleet. This is to 'replace' the current Scottish coastal defence fleet that, well, doesn't exist; we should recall the two relatively recent, I am happy to say friendly visits of a Russian battle fleet to 'Scottish' waters, albeit with not a British Naval ship in sight.

A quick analysis of what other similar sized nations to Scotland have done suggests we may need up to 20 new small and medium-sized coastal defence craft. These are likely to be quite different from current British Navy ships, most of which are designed for supposedly British 'defence' activity in far off places.

So Scotland needs to build its own ships for coastal defence, as well as ferries, and a substantial number of both types, never mind the opportunities in other markets such as offshore and cruise shipping. With independence we should be ensuring the capacity is in place for these vessels to be built in Scotland and, given our skills, we should be confident of our ability to succeed in this market. This sort of approach would ensure shipyard employment for the foreseeable future. With Westminster's hands remaining on the economic tiller, however, such orders and opportunities will simply never exist.

Alf Baird is professor of maritime business at Edinburgh Napier University and a member of Academics for Yes