But you have to eat meat, and lots of it, if you're going to row. Besides, rowing was for the slaves. Who is going to volunteer to play the role of slave? Those who like to get whipped and those, well...you dot the lines...

The climate in the Medieval Warm Period may also have helped the Vikings a little. The North Sea's weather is a mite more hostile than it was in, say, 950 A.D.

To Mrs. P.'s point, I'm not sure that most Viking boats were slave-powered -- if I'm not mistaken, most Viking longboats were crewed by fighters and the bands on board were multi-functional -- the ships were light enough that a band of strong men could portage the damn things around rapids, falls, etc. which meant that no river town in Europe, no matter how far inland, was safe from their depradations. Slaves might be tempted to jump ship at that point; but fighters under their liege lords would want to get back to Norway with literally all the portable wealth they could carry.

But I'm no expert on the Norse, so I'd want to research that point and am willing to accept that their bigger boats may have been slave-powered. Consider that to some of their far flung colonies (Iceland, Greenland, Vineland, Nova Scotia) they brought cattle, women, tools, and families, so there must have been some pretty big boats in those groups.

I imagine that in a long period of being becalmed, the rowers would get pretty sick of salted fish and stale beer; and like the Norwegian blue parrot, they must have indeed been pining for the fjords, slave or no. That being said, I'm sure there were levels of toughness in those S.O.B.'s that we can barely contemplate.

"A furore normannorum libera nos domine" as the ancient prayer has it.

http://en.wikipedia.org/wiki/Norsemen