As I'm sure you know, there are two sagas describing the discovery of Vinland, the "Graenlendinga Saga" and "Eirik's Saga."

If I use my old copy of the text (Soga um Eirik Raude - translated by S. Eskeland, Det Norske samlaget, 1907),and translate it into english, the translation would go something like this:

"When they clashed there was [a] fierce fighting/battle and a hail/a large amount of missiles/thrown things came flying over/towards them, for the Skraelings were using catapults/throwing machines. Karlsefni and Snorri saw them hoist a large sphere on a pole; it was dark blue [ie, meaning black] in colour. It came flying over the heads of Karlsefnis and his men and made an ugly sound/din when it struck the ground."

Now, what to make of this? The footnotes mention that in one copy of the original text the sphere was described as being about the size of a sheep's stomach or even similar to a sheep's stomach.

As far as I know (and please correct me if this information is incorrect!) there are no records of Native Americans in this area using boats larger than canoes, so whatever weapon it was it must have been was a relatively small weapon or device.

The vikings were not unfamilar with staff slings or trebuchets, and other throwing weapons, according to Konungs skuggsjá, which describe in great detail the use of various weapons, including those. They were also experienced with weaponry and warfare from many different places in Europe. So according to S. Eskeland, whatever it was that frightened the men so, it was not a weapon they were familiar with.

Colonial era (ie, post-Columbian) Indians used no such weapon (that I'm aware of, but I'd love to be proven wrong!). According to one source that you'll find repeated a lot online is that H. R. Schoolcraft, Indian Tribes of the United States, I (1851) wrote that "Algonquin tradition has preserved the memory of a formidable ballista which was made of a boulder sewn up tight in a skin and slung at the end of a long rod." I've not access to the book, so I am somewhat unsure as to the veracity of the quote from the book.

Please don't take this as verified fact, it is a possible explanation and nothing more. Of course, this hasn't stopped popular science magazines and articles (in Scandinavia, Ill. Vitenskap has written about this more than once) from forming the conclusion that the vikings encountered Algonquin native americans, who used some form of staff slings at the end of a pole which was used to sling sacks of skin or leather filled with stones or boulders.

Imagine taking a boulder or stone, and sewing it into a piece of leather, skin or stomach from an animal, then flinging it from a pole (like you'd fling a sock with an orange in it, to use one easily understood comparison) from a large pole that's being waved around. Or it could've been some sort of small, two man ballista or something resembling an jai alai bat bat.

Edit: I should add that of the two sagas only one mentions the missiles being thrown. The other saga does not mention it, but instead talks about "war-rattling from poles". Which is some type of mechanism attached to a pole that makes noise when swung around so it turns. If the natives were peaceful they would swing them clockwise, when they were attacking they swung them counter-clockwise. It's been suggested by some authors that the sagas have intertwined or mixed up the memory of the skirmish.

Edit 2: Obviously, there are lots of fantastical elements in the sagas, so unless an expert on native americans can shed some light on the possibility of a weapon resembling what the sagas describe being used at this location and era the whole story might not be rooted in truth.

I highly doubt such a weapon was ever used. As I hasten to point out whenever such questions come up, the sagas are very problematic as historical sources. The same saga you've got there has a woman, Auður, witnessing her own doppelganger appear before her with inhumanly large eyes, who when asked her name replies "I am Auður" before vanishing with a crash. Elsewhere in otherwise entirely sober and seeming historical family sagas, people encounter terrifying manifestations of the undead, ghostly seals appearing out of solid ground, monsters with eyes the size of the moon, etc. The Norse metaphysical conception of the world can be loosely compared to a sort of nested ring of concentric circles: at the center you have a well-settled agricultural Scandinavian landscape, and as you move further and further away you encounter weirder stuff. The fields and forests around your homestead are already a sort of liminal space, where you might begin to encounter a variety of supernatural beasties; as you move further out, say to Iceland, you're moving further into the territory of the supernatural and the "Other," and the landscape becomes more supernatural - the boundaries between worlds are blurred. By the time you reach, say, Africa you might encounter unwholesome "blue men," or when you reach Vinland you might find natives hurling shrieking blue orbs the size of a sheep's guts. (To be very clear, my description of the Norse' worldview is a scholarly effort to make sense of a variety of literature, rather than anything directly stated in our primary sources. John Lindow's article "Supernatural Others and Ethnic Others: A Millennium of Worldview" in Scandinavian Studies 67 touches on this, as do a great number of other folklorists' work.) Ultimately, I think this work is better considered a work of imaginative literature inspired by history; attempting to directly relate the natives' supernatural weaponry to history is both speculative and somewhat missing the point.

Is it in anyway clear in the text whether it must be the knob which is the size of a sheep's belly, or could the pole perhaps be the length of a belly (a fundamentally strange choice of measurement)?