Variable speeds for knarrs based on quality is toxic for spruce caps of all kinds. It can be a death trap to be found in a rowboat or low quality knarr. If the spruce cap doesn’t know to drop his keys into the water and get to shore then the knarr will be stolen. All I see the variable speed based on quality doing is that any developed village will keep other players from leaving their walls and adventuring on the water.
I recommend to change or remove the quality effecting speed. Have all knarrs have the same speed. The quality of knarrs could decrease the amount of damage it takes from deep ocean or whales.
Another idea- could make the materials used on knarr effect the attributes of the knarr. Higher tier metal used to build it could decrease the damage the knarr takes from deep ocean and even whales. The type of clothe used could effect speed but might lead to the same problem. Linen/hemp would be lowest tier. Wool would be mid tier. And silk would be top tier.