这句话的出处为下面阅读理解文章。

The word "brick" did not appear in the English Language before the fifteenth century. Bricks may have been known in Britain before then, but they were no common enough to be mentioned by any writer. We know that they were used in ancient Egypt, in India, and in the Roman Empire; they were used again in parts of Europe; but there is a strange gap in the history of brick making which left Britain, at least, without bricks for building for several hundred years. When they did arrive in Britain, it was in a very strange way. In the fifteenth century Britain’’s wool trade was still flourishing and shiploads of wool were exported to other countries, among them Germany and Holland. These ships would have had to return empty, unless their captains were able to pick up a return cargo, and no captain likes an empty ship. For another thing, it is a great waste of space. Rather than return home with an empty ship the captains would fill up with anything, and at about this time somebody had the idea of filling up with bricks. These were already being used extensively in Holland and Germany, where the brick makers were anxious to sell their surplus stocks. These bricks must have been a godsend to the builders of London and southeast England. There was very little building stone in that part of the country, and stocks of good building timber (especially from oak-trees) were running low. Bricks were first used for town houses and for humble cottages. Before the end of the fifteenth century the most enterprising builders had set up their own brick works wherever there was a good supply of the right kind of clay, and soon bricks were almost the commonest building material in the country. 

Unless的词典释义为：— used to say what will happen, be done, or be true if something else does not happen, is not done, or is not true

而if引导虚拟条件时的释义为：used to discuss the imaginary result or effect of something that did not happen or that is or was not true

由以上unless和if的释义的对比我们就可以知道，unless不用来引导虚拟条件句，而只用来引导真实条件句，即unless引导的条件是可能实现的，因而主句描述的情况也是可能发生的。网友的这个句子中，主句是作者对过去情况的一种推测，由于是对过去情况的推测，所以用情态动词would后接不定式完成体，即这些船不得不空船而归的必要性是有可能的。Unless从句表示过去的条件，这种条件也是有可能发生的。所以这个句子不是虚拟语气，而是陈述语气，只不过主句用了would的推测性用法，对过去的情况进行推测而已。