I've noticed lots of people on here wanting boats with interiors. Why not add the cog, which is the ship that very, very slowly replaced the knarr?
https://en.wikipedia.org/wiki/Cog_(ship)
Knarrs have the reputation of being raiders' ships, which isn't really accurate; cogs, by contrast, are regarded as merchant ships. Might be a fun addition. Leave the cargo space the same. Let the cog be slightly slower (max out at 7, say) while having slightly higher soak and hp. Practically, just increase the size of the knarr model slightly and stick a cabin on one end of it. Done.