Up until the advent of steam power, wind drove ships. Ask any sailing ship captain and he’ll tell you that wind direction and speed often changes. Without modern instruments gauging wind strength and direction is art and science tempered by experience.
Anemometer – the instrument that measures wind speed – design has changed little since the Italian Renaissance artist Leon Battista Alberti designed the first one in the 15th Century. It was four wooden paddles that rotated around a stick. The faster they rotated, the stronger the wind. All well and good, but calibration varied from one unit to another and it wasn’t until the middle of the 19th Century when the Brit John Robinson came up with the accurate three-cup anemometer commonly used today.
Locating the anemometer is critical because it had to be mounted out of the turbulence caused by the sails and rigging. Reading it remotely wasn’t possible until the late 19th Century so they were often mounted on the railing at the aft end of the quarterdeck so the captain could easily walk over to it.
Weather forecasting as we know it today, didn’t exist during the Age of Sail or to the middle of the 20th Century. A ship captain’s world was what he could see. It also explains why many reduced sail at night.
Gauging wind velocity was critical to both ship’s speed but also survival. Hang too much sail in a strong, or variable winds such as those that accompany squall lines and the captain could demast the ship. Not enough sail and the ship didn’t sail at its best speed. How much sail to carry at any given time was a matter of experience learned the hard way.
Vanes mounted on the top of the masts told the captain the direction of the wind. There, they were above the turbulence from the flow of air around the sails and easily seen from the quarterdeck.
Wind direction and velocity dictated not only which sails to set but also how they were trimmed. The solution is still used today. Rows of thin string called ‘telltales’ were sewn into the sails in rows. Main, top, top gallants and royals had at least one row. The bigger sails had three. Staysails and jibs also had their telltales attached to both sides of the sail.
The position of the telltale told the captain how the sail was trimmed. When staysails and jibs were sheeted home for maximum efficiency, the telltales were horizonal. Slacken a staysail or jib and then the telltale would angle down. If they fluttered, then the sail was luffing. The location of the telltale told the captain how to trim the sail.
Telltales on the square sails hung straight down when the wind was coming from over the stern. If the ship was on either a port or starboard tack, the ends of the telltales pointed down wind. Looking at their alignment, the captain could quickly tell if the sail was trimmed for the most push.
For most Age of Sail captains, experience enabled them to deal with wind and weather. The one challenge that confounded most were the ocean’s currents. These rivers in the ocean often moved at three to five knots and affected the ship’s speed over the bottom and the time needed to get to its destination.