The Norwegian Oseberg replica capsized during another filming under similar conditions. I counted the oarports. Sure enough, they did. On Viking style vessels crews provide both power and adjustable, obedient ballast. But the power is fragile.
Crews get cold, wet, sick.
Some of your officers hit the wall. Even with a large crew, the loss, through injury or illness, of just one person can effect the efficiency and morale of that watch. The scope of any voyage must be matched to the resources available. I am convinced that the ideal crew is one and a half times the number of oarsmen. Eighteen hands on the Fyrdraca is a little crowded, but it provides full power when needed, and redundancy when something goes wrong or someone has problems.