Sailboats are not jet airplanes. This point is simple, obvious. Yet one reason why I’m reading so much about transportation logistics while trying to write my medievalist fantasy is that I read too many passages in too many books that make me mutter the word “Impossible!”

Perhaps the fault lies with me, because my suspension of disbelief only covers the big leaps. I like my horses to only be able to gallop for a realistic amount of time. I like my small cog to function like a real cargo ship, as opposed to a magical bag of gigantic storage capacity.

Piles of loot should be heavy in my way of thinking. Bone should be hard to cut. Distance should be a significant obstacle in certain situations.

Now, I admit that my thinking was heavily shaped by Poul Anderson’s “On Thud and Blunder” essay. I found my copy in a tattered paperback Fantasy anthology years ago. But you can read it at the SFWA website [here]. The essay is worth your time.

I spend a lot of time thinking about plausibility and believability. So I spend a lot of time reading about physical realities.

In one recent book (Asbridge’s The Greatest Knight), for example, I learned about the dimensions of the typical sword used by European knights. The key facts about length of the blade (thirty-four inches) and weight (two-and-a-half pounds) matter to my storytelling in several ways.

From Gillian Hutchinson’s Medieval Ships and Shipping, I learned a lot about the winds, currents, and patterns of a large sea. I also learned that while a ship going from three to five knots could theoretically cover somewhere between ninety-six and 115 miles in twenty-four hours, a cog realistically would average about forty to sixty miles a day because winds are not as predictable as gasoline-powered engines.

Study logistics, and some of your drama creates itself. Will they get there in time? Can they get there in time?