How are we to parse the phrase "A ship was a long time turning"?

Solution 1:

It was a long time comming is not an idiom and there are plenty of examples that have a long time as an adjunct that comes between a form of be and the gerund-participle (-ing form)

A sample from the Corpus of Historical American English

Pretty Betsy Canning was a long time finding out that Husband Alec was not the man she thought she married. (Time Magazine: 1937/04/05)

Preach was a long time recovering from this 1946 injury. (Saturday Evening Post: 2/3/1951, Vol. 223 Issue 32, p30-67, 5p)

She was a long time milking the cows; her hands were so tired she had often to stop and rest them, while the tears fell unheeded into the pail. (Other Main-Travelled Roads; Garland, Hamlin, 1860-1940)

Brin was a long time responding. (Wishsong of Shannara , Terry Brooks, 1985)

Ella was a long time coaxing Rancie to be friends, because the child was wild and shy. (Harpers Magazine (1935-04) pages: 525-532)

We understand most of these either as late or for a long time.

Similar constructions with noun phrases as adjuncts of duration exist as well:

They were five days finding parts to rebuild the engine. (The Hotel New Hampshire, John Irving, 1981)

An English family by the name of Bancroft, who had been seven weeks crossing the ocean in a sailing-vessel, had located and started a real greenhouse. (Song of Years , Bess Streeter Aldrich, 1939)

Do not let us assume that this is the bill for a long, active operation; we were two days fighting and previously we had been one week bombing (House of Commons, army_supplementary_estimate_1956)

Although it may not be common, this construction has been in use for some time.