SFWG Study Hall

Okay, first, remember, with sail power you have a lot of flexibility. A sailor is partly at the mercy of the wind, waves, and currents, all of which have a hand in the equation.

A really, really good Medieval merchantman (Viking Knorr, which is NOT a longboat!) could do 8 knots (about 10 mph) in the absolute best wind. 3-4kt (4-5 mph) is more likely, with zero always being a possibility. Columbus' ships were no faster. Note, his Caravels (the Nina and Pinta) could sail into the wind about as good as a Knorr, but his Carrack (Santa Maria) couldn't point into the wind at all, so his fleet couldn't use that ability. More on that, below.

Because of this, the pat answer (500 miles divided by 4-ish mph = about five days) isn't necessarily going to be right. It can be, if you want, or if the story wants less or more, here's how you do it:

First, you can use Mother Nature to alter the answer. You can't sail into the wind, for example, requiring tacking. Until modern times, you couldn't sail 'closer' than about 60 degrees, making a trip of a 500 miles actually a zig-zagging journey of three times as far. Before the Vikings and the Chinese figured it out, you couldn't sail into the wind at all, leaving you stuck in an unfavorable wind with rowing, or sailing north or south to find a wind going your way.

This is water speed we're talking about, though. An ocean current is unlikely to go more than a couple mph, but by adding a strong one, you can use it to push these speeds up or down. Remember that it is going to affect the return voyage in the opposite way, though!

(This how the Spaniards used to get across the Pacific from the Philippines to Mexico with absolutely no wind going their way. They just packed a lot of supplies and rode an ocean current going a couple miles per hour. Took months, but worked. They had good winds for the other direction, but the current was against them, stealing some of that back.)

Navigation:
You have a couple issues here. First of all, Stellar Navigation limits you to knowing latitude, but not longitude. The way most people up until the Vikings did it, was to use the stars only to determine where North was. In other words, they were a compass that was useful only about once per day. This is not the method of a people who are willing to leave the shoreline, so you need to assume they are at least as advanced sailors as the Vikings.

The Vikings (in Europe. A few others figured the same thing out elsewhere, like the Polynesians, and the Song Chinese may have learned it from them) figured out how to use the stars to determine Latitude, so they could sail north-south until they were at the right latitude, then sell East or West until they hit the place they wanted. Thus, every trip is a dog-leg. So depending upon geography, your 500 miles might have to be a little longer to create the triangle.

Your magic users may be able to use a more direct approach, using the gifts you mention, which allow them to cut the angle. Or, superstitious sailors may not want to anger the storm gods by cheating, and refuse to do it any but the 'right' way. Obviously, something like this is the author's choice. But never forget, sailors are really, REALLY conservative about stuff like this.

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Regards,
Eric Fretheim
(Eric The Fred)