If you want quick and simple, the level 5 spell Control Winds is the easiest rubric.
You may choose one of four basic wind patterns to function over the
- A downdraft blows from the center outward in equal strength in all directions.
- An updraft blows from the outer edges in toward the center in equal strength from all directions, veering upward before impinging on the
eye in the center.
- A rotation causes the winds to circle the center in clockwise or counterclockwise fashion.
- A blast simply causes the winds to blow in one direction across the entire area from one side to the other.
For every three caster levels, you can increase or decrease wind
strength by one level. Each round on your turn, a creature in the wind
must make a Fortitude save or suffer the effect of being in the windy
Strong winds (21+ mph) make sailing difficult.
A severe wind (31+ mph) causes minor ship and building damage.
A windstorm (51+ mph) drives most flying creatures from the skies, uproots small trees, knocks down light wooden structures, tears off
roofs, and endangers ships.
Hurricane force winds (75+ mph) destroy wooden buildings, sometimes uproot even large trees, and cause most ships to founder.
A tornado (175+ mph) destroys all nonfortified buildings and often uproots large trees.
There isn't an easy way to chart this out, but if you want a more nuanced suggestion, then try this: Consider the size of the wind when it hits the sail, the size of the sail, and the average of the highest wind speed over that surface.
For example, if you treat the ship like a Huge creature, with a properly sized sail thus (arguably) having an area of 3x6 squares. A standard "Gust of Wind" spell (level 2 Evoc) would have no affect because it doesn't do anything to Large creatures or bigger. Perhaps if two instances of the spell are cast, you can treat it as moving a Medium creature, thus 1d6x5'/turn. And that would be generous, but because sails are designed to work this way and you won't have to factor in the size of the base ship - just the sail. So a sail that doesn't match the ship's size would be a modifier to how quickly it can approach that speed.
Again, it's definitely not RAW, but if you really want to get crunchy with wind mechanics, it's a start.