does anyone have experience of towing barges through oceans, parcularly by sail, who could perhaps advise me.
For experience with towing, you need to get hold of officers on tugs. I am not aware of any cargo being moved across oceans by tug. I guess the reason is that the tug and barge is much less maneouvrable than a ship carrying the same cargo, and that tug and barge are likely to be in different wave systems, which puts large strains on the tow rope, and threatens to capsize the tug if it turns too far relative to the tow rope when peak loads are high.
Tugs are economical on rivers where there is not enough fetch to build up much of a sea state, and where the tug can leave barges to be loaded and unloaded while to goes off to deal with other barges. Is that why you are interested in barges?
Towing under sail has additional problems. The performance of a sailing vessel, especially its ability to go upwind, depends on the respective lift to drag ratios of everything in the air and everything in the water. If you express each lift to drag ratio as drag angle = arccot(drag/lift), then the course made good can't be any closer to the apparent wind than the sum of the drag angles. Hanging a barge off the back of the boat adds drag and so increases the drag angles. It is worse than carrying cargo, both because barges tend to be less streamlined and therefore have more drag than carrying the same cargo in the sailing vessel's hull, and because cargo carried in the hull at least can add to stability, while cargo carried in the barge does no such thing. When the wind picks up enough, you have to reef. That reduces lift, but does nothing to the aerodynamic drag of the hull and therefore increases the drag angle. Cargo in a barge adds no stability, and therefore the sailing tug has to reef earlier. There are examples of sailing vessels towing stuff with high drag, namely nets, but as far as I know, they only ever towed downwind.
I can think of only two ways around that stability problem problem. One would be a rig that generates no heeling moment, which can be done with a kite. Dave Culp's concept of the kite tug seems like the logical end point of that idea (see AYRS booklet #124 Transport Sailcraft at
Booklets – Amateur Yacht Research Society https://www.ayrs.org/booklets/; note that I am one of the editors of the AYRS journal Catalyst:
Catalysts – Amateur Yacht Research Society https://www.ayrs.org/catalysts/). I don't know whether the kite tug is yet practical, never mind economical.
It sounds like you already have your sailing vessel, and you want to be able to use any arbitrary barge. If you had the option of designing both barge and sailing vessel, you might scale up the Twin Foils Experiment described in Messing About in Boats some years ago. That made a proa out of two hulls, each able to stay upright on its own, connected only with string. The windward hull carries a leeboard and rudders, the leeward hull the rig.
That would let you pile silly amounts of sail onto your sailing vessel, gaining stability from the cargo without carrying the cargo in the sailing vessel itself.
If you don't want purpose-built barges, in theory you could design a paravane that is either attached to the windward side of the barge, or perhaps even between the barge and the sailing vessel. Steer by diagonal lines that shift the paravane and barge fore and aft. I tried it once on a sailing canoe. The paravane worked, steering a deep V hull not so much.
If there is an industry-standard barge for which you would design your vessel, perhaps it would be possible to design a bow and stern section that attach at either end of the barge, and are detached and joined together to sail without a barge. That would be easiest if the barge were brick-shaped. I am still not sure this would be practical even if you started with a blank slate.
P. S. If anyone knows where to find the Twin Foils patent, please let me know. My search has failed so far.
