The lure of a ferry to Nova Scotia is clear: It turns a thousand-mile road trip into a half-day journey by water, saving time and money for commercial and tourist traffic.

So, why has it been difficult to replace the ferry service from Portland to Yarmouth, Nova Scotia, that over the past several decades has been provided by Prince of Fundy Cruises, Scotia Prince Cruises and finally by the high-speed catamaran operated by Bay Ferries Ltd.?

The key is the word “commercial.” The recent recession affected both the frequency of truck traffic to and from the Maritimes and the ability of the Canadian government to provide a subsidy to the service.

Bay Ferries said the subsidy was essential to its profitability, even though the Scotia Prince didn’t require such aid. However, the Prince ceased serving Portland when its owners and city officials couldn’t reach agreement on issues involving terminal facilities in 2005.

Another issue is that Portland isn’t the only possible U.S. terminus for the service. Five companies have expressed interest in providing the service, mentioning Boston, Portsmouth, N.H., and Bar Harbor, which hosted the Cat and before it the Blue Nose, as potential U.S. hosts.

Meanwhile, Scotia Cruises, which is not related to the Scotia Prince firm, has talked up linking Portland to Halifax, Nova Scotia, although it now says service to Yarmouth would be possible as well. It has hired former Scotia Prince CEO Henk Pols as an adviser, and says a meeting in Yarmouth is set for later this month.

But no proposal has yet been acceptable to the Yarmouth Area Industrial Commission, which is trying to get a new Canadian subsidy.

Portland’s attractiveness to cruise ships has provided a steady tourist business, even as truck traffic wends the miles north through New Brunswick and back south to Nova Scotia. Diverting it to a ferry would appear to be the easy part; the question Yarmouth’s commission would like answered is whether there would be enough fares to keep a potential subsidy within affordable limits.

We should see soon. If the numbers come out right, officials say there could be ferry service by the summer of 2012, ending a three-year hiatus and offering yet another reason to visit the Maine coast — and Maine’s largest city.