I think much of the reason for Spray’s balance and ability to track well, even downwind, is based on the fact that she is a typical “cod’s head and mackerel’s tail” design, dating back to early in the 19th century. Remember, Slocum rebuilt her from and old, almost wreck, adding more freeboard and also a deck (which I am not sure if she had to begin with).
The main frame on these old designs sits far forward, so they will not tend to round up (to starboard) if suddenly heeled over (to port). Most modern boats have the main frame (too far) aft of the middle. These are faster to windward, but need much more attention to keep them on course downwind. They broach frequently, but with fixed ballast, and being fully decked, they mostly recover without harm.
Just about all open, traditional workboats in Norway were and are built with moderate cod’s head and mackerel’s tail, with maximum beam almost at the fore thwart, like this one: Being open, they depended on good tracking and resistance against broaching to survive. I have sailed such færings quite a bit, and they surely track well, even with quite some swell on the quarter.
Under this link is a photo show of an Ofot-færing being built, and then sailed.
http://www.fosen.fhs.no/videoer/165-a-bygge-en-faering
Cheers,
Arne