They were for tunny fishing. I think (I'm no expert) that each had a row of shortish lines on it, each line with a baited hook. The poles were lowered to horizontal (you can see the tackles for doing this) so that the bait dangled in the water - presumably the 19th cent equivalent of the pole fishing which is extolled on tins of tuna we buy in Sainsburys etc. What I don't know is how fast they aimed to sail when they were fishing like this, and I wonder why they were such big powerful craft. It may have had to do with how far they had to go to the fishing grounds, and the need for speed getting the catch back to shore. On holiday in northern Brittany in the 70s we saw rows of them pulled up above high tide and left to rot - very sad, but inevitable I suppose. Very striking were their beeootiful underwater lines. It's good to know that at least one of them is alive and well.
Michael