Inevitably replica vessels travel at a far slower average speed than predicted, and this plays hob with twentieth century scheduling. You can run a diesel 24 hours a day, day after day, without a complaint or pulled muscle, blistered palm or sore butt. You can't really do that with a crew. It is one thing to run with the storm, quite another to bull your way through head seas. I can count at least four replica longships lost over the last 30 years.
Pessimism is the best planning policy.
The average speed of the Fyrdraca on the Chesapeake and its estuaries, under sail or oars, is one to two knots. I used to feel pretty bad about this until a history professor in Salt Lake City analyzed the Viking raids up the rivers of France, observing which monasteries and towns were raided when. Maybe after a month or so, given good living conditions, the crew would harden up, but I suspect that even if we grabbed a boatload of Marines from Quantico, we would see a dip in performance after several days. Crew Size Crew size is critical on historic vessels, especially if they are oared. A Gokstad replica was lost with all hands in the early Fifties.