The battle begins unexpectedly because King Olaf did not expect his wife' s brother' s fleet would attack so he sailed peacefully past the Danish fleet. The Danes allowed most of the Norwegian ships to pass because they were looking for Olaf' s Long Serpent in particular to attack. Jarl Eirik actually tells the Swedes and Danes not to attack because he is familiar with the ship that King Olaf sails on.
When King Olaf realized that his brother - in - laws fleet meant to attack him Olaf Tryggvason turned his fleet around and decided to meet his new enemy in battle instead of running away. King Olaf has little respect for the Danes, who he refers to as " Forest Goats ", and has even less respect for the Pagan Swedes who he calls " Horse Eaters. " Olaf does, according to Heimskringla, feel a little apprehensive when he sees the ships of Jarl Hakon' s son Eirik because, " They are Norwegians like us ". He was of course fully aware that Eirik would fight hard to avenge his father' s death.
Olaf next did what all Vikings of the time did during sea battles. He tied his ships together, with his ship in the center as a command post. The Long Serpent was both longer and taller than all the other ships and was therefore a great platform with which to direct the flow of the battle. From his defensive position it made it hard for the Danes and their allies to bring their superior numbers to bear. The saga tells us that the Danes and the Swedes attacked from the front, but were pushed back with heavy losses.
Heimskringla gives most of the credit to Eirik and his Norwegians who it is said attacked from the rear and cleared Olaf' s ships one by one. The Danish account of the battle by Saxo Grammaticus tells us that King Olaf jumped overboard in full battle gear as Eirik and his men began to fight their way onto King Olaf' s ship. Instead of risking capture he states that KIng Olaf committed suicide instead of risking capture. The Norwegian / Icelandic accounts tell that nothing was known of his death and that he was seen still fighting before his disappearance. One mythical account even has Olaf surviving by swimming ashore where he is rescued by a Wendish ship.
It is my opinion that he either committed suicide by jumping overboard or was knocked overboard in the fighting. I do not in any way shape or form accept that he was able to swim ashore in full battle gear. If he had survived he would have reorganized his men and tried to reestablish his control over Norway, which after his death was split up by the victors. It remained that way until that most hated of Christians King ( later Saint ) Olaf Haraldsson reunited, after the Battle of Nesjar, Norway into a single Christian Nation. King Olaf Haraldsson would meet his end at the Battle of Stiklestad.
Go with Odin's wisdom, Freyja's love, and Thor's protection !