How fast could a 50-person crew row a 12th century ship carrying 20 passengers?

[wolram]

I have tried to work out how fast and how long a 50 man rowing crew could go, i think it not only depends on muscle bulk but also lung capacity, this is in relation to a 12 century ship, i have little idea of the ship design but i thought if i knew the rowing capabilities of the said crew i could work backwards and guess at the ship design.
Added, the boat must be able to carry 20 passengers.

 

[wolram]

The other question is the same but with 300 passengers, i know it is a tall order , but i would be greatful if some one could help out on this problem