A few weeks ago, I had the opportunity to visit Schleswig. One of the pleasures of the trip was seeing the famous Nydam boat, a thirty-oared galley sunk in a lake around 400 CE. It had apparently been captured in war and was sunk as a sacrifice to the gods in a lake which received sacrifices for several centuries. A Greek would have called it a triakonter, although Germanic ships in the fourth century CE differed from Greek ones in the fifth century BCE in many details.
The ship used to be a point of contention between Germany and Denmark, since it was being excavated during the Schleswig War and ended up in German territory.
I haven’t taken time to seriously research this ship, and my course on nautical archaeology was a long time ago, but here are some webpages about the boat by German museum, an American professor, and a Danish society.
Shipwrights who have looked at the ship think that it would have been unstable in rough seas, and historians sometimes compare it unfavourably with Viking age ships with their tall masts and greater stability. One interpretation is that it was designed to travel the inland waters of Denmark rather than to venture into the open sea. It is impressive that most of the long side planks (strakes) consist of a single piece of oak and not several boards joined together. Europe once had old-growth forests too.