Two semi-vessels loaded with different cargoes may be coupled together, provided that there is not too much difference between their respective draughts. Slight differences may be balanced by the water compartments being filled to a greater or smaller extent. The peculiar position of the hatches allows of loading the semi-vessels separately as well as when coupled together. If there is for the time being no necessity for using the vessels in their capacity of separate and duplex barges, any kind of cargo might be loaded that does not require large hatches. The vessels, on account of their more complicated construction, will be somewhat more expensive, but wherever the advantage offered by them outweighs the extra expenditure, they can be used with success. The innovation might be of particular importance where a new canal system is being constructed, since the latter might be subdivided into main canals and branch canals--similarly as in the case of ordinary and narrow gauge railways--the main canal being built of a larger section and with larger locks to suit the duplex barges, while the branch canals could be planned of smaller dimensions calculated to suit the semi-barge. Thus the first cost of such a canal system would be materially reduced as compared with a canal installation of one uniform section throughout. Likewise in mountainous districts with rock soil it would be an important consideration whether a canal had to be blasted out of the solid rock or a tunnel cut, in dimensions suitable for a vessel of 6 or of 14 square meters section below the water line. In this case, even in certain portions of a main canal--where rendered desirable by the rocky nature of the ground--a smaller section might be adopted, which would only be large enough for single semi-barges, so that the duplex vessel would in these instances have to be taken apart in the same way as in a branch canal. The saving to be effected by constructing a canal on this principle, as compared with a canal of one uniform section throughout, must be considerable, and the advantages of the arrangement are apparent. The appended figures will further illustrate the arrangement. Fig. 1 shows two separate semi-barges ready to pursue their journey independently. Fig. 2 shows two semi-barges coupled together ready to be "folded up" by means of ropes and specially constructed windlasses--their lateral water compartments having previously been filled. Fig. 3 shows the duplex vessel after the "folding up" operation just described; and Figs. 4 and 5 show the cross section of two loaded semi-barges as outlined in Figs. 2 and 3. These Figs. 4 and 5 will also serve to illustrate the manner in which sacks and loose produce should be loaded. Fig. 4 also shows the filled water compartments, and the effect of their weight in making the boats lean toward each other.