Horned Helmet by Henry Treece was first published in 1963.
This is the story of Beorn (Bjorn) a young Icelandic boy who has had a rough time as his mother has been kidnapped by Viking raiders and enslaved, and then his father jumped into the sea rather than face a fight with Glam whose barn he had burned down. The upshot of his suicide is that Beorn now belongs to Glam, but Beorn decides he would rather do anything than be Glam’s slave. He manages to escape and is taken on board a Jomsviking ship when he befriends Starkad. The Jomsvikings are a notorious band of mercenaries and at times Beorn regrets getting on board, the ship isn’t in the best of condition and it looks like they’ll all drown.
This one is written in the style of a Viking saga, so it seems a bit stilted to modern ears, or eyes at times, but I enjoyed the adventure anyway.