I was thinking more along the lines of long distance travel rather than short distance pursuit, but both would be interesting to hear about. Any info appreciated!
hi! I don't believe these will answer your question directly, but you may be interested in these posts
Reconstructions show that it's much faster to travel by sail than to use oars, although longboats could cover long distances with crews operating half on, half off. There's some good footage of it in this BBC documentary, (replica at top speed).
Regarding sailing speeds, I'd recommend looking at the Viking Ship Museum website. It's a great resource for the five ships excavated from a single Viking-Age site in Denmark. These ships were all scuttled to block a channel, so they give us a snapshot of a Viking-Age harbor (probably around 1070). In quick review:
So around 1000, which is when longships were at their finest, an average ship would sail at about 5-10 kts (6-12 mph, 9-18 kph), whereas longships could travel at up to 17 kts (20 mph, 31 kph). In terms of long distance sailing, this means that a ship based out of Norse Dublin could reach the Atlantic Coast of Scotland or round into the English Channel in about two days. Given favorable weather, it could probably circuit the North Sea in a matter of 2-3 weeks. It's average speed would be similar to most other ocean-going vessels of the period, but it's top speed could get much faster.
To put the top speed of a longship in perspective, a monk standing on shore would see the horizon at about 3 miles/5 km away. Given Skuldelev 2's shallow draft (and the beating evident to her keel), her crew could have sailed her straight onto the shore and had boots on the ground within 8 minutes of being sighted. Not a good day to be a monk.
My history professor said approximately 10 knots, which is I believe 12 mph; fast for the time.