How was it difficult for the three friends in the boat to cover the boat with canvas?

From the text:

That canvas wanted more putting up than I think any of us had bargained for. It looked so simple in the abstract. You took five iron arches, like gigantic croquet hoops, and fitted them up over the boat, and then stretched the canvas over them, and fastened it down: it would take quite ten minutes, we thought.

That was an under-estimate.

We took up the hoops, and began to drop them into the sockets placed for them. You would not imagine this to be dangerous work; but, looking back now, the wonder to me is that any of us are alive to tell the tale. They were not hoops, they were demons. First they would not fit into their sockets at all, and we had to jump on them, and kick them, and hammer at them with the boat-hook; and, when they were in, it turned out that they were the wrong hoops for those particular sockets, and they had to come out again. ch 10