Either by a rectangular single sail, or by oars, or both.
Rivers deep enough for their longboats to sail.
Viking longboats were rowed with oars till they caught wind in a single large square sail.
longboats
Longboats were crucial for the Vikings and other seafaring cultures due to their design, which allowed for both speed and versatility. Their shallow draft enabled navigation in shallow waters and rivers, facilitating trade, exploration, and raids. Additionally, longboats were constructed for endurance, making them effective for long sea voyages. This combination of features made longboats a key element in expanding territories and influence during the Viking Age.
Unlike later ships of the Age of Sail, Viking longboats had many oars, as did the earlier ships of the Roman Empire. (see the drawing in the link below)
Longboats or Longships.
In longboats
longboats
longboats
"Longboats".
In longboats, on horseback, and by foot