Since most of the time as you are sailing, you’re not aiming directly down wind it results in your course not being the same as your heading. It’s particularly more prevalent when you are sailing on a close haul or close reach and can be as much as 20 degrees depending on the wind conditions, water conditions, your sailboat design, your apparent angle with the wind and how your sails are set.
When you are out on the water, the only thing out of that list that you have control over is the set of the sail.
As you begin to learn to sail, leeway is not of great concern because you’re spending more time grasping the concepts of sailing. But as you start to venture out and actually go somewhere, you need to be cognizant of leeway.
Regarding leeway your biggest friend is the keel. Remove the keel on your sailboat and it will probably tip over with the first 2 knot breath. So the biggest reason for the keel is to place weight far below the surface to create a counter weight for the wind pressure aloft. However the keel also has a second function and that is to minimize the sailboat slipping sideways through the water. It’s just like trying to push a piece of plywood side ways through water. It’s hard to do. Thus the keel is acting like that piece of plywood. It reduces the effect of the wind wanting to push the boat sideways through the water.
It’s incredibly noticeable to me when I go out windsurfing. The fin on the board acts as a keel. Because of the speed of the board, it’s quite easy to get the board going so fast with so much sideways pressure on the fin that the water actually detaches from the fin and it cavitates. Under this condition the fin provides no back pressure against side slip and it’s as though it wasn’t even there and the board starts sliding uncontrollably downwind. In fact the first time it happened to me I though that I’d snapped off the fin. As the board slowed down the water reattached and I was able to sail a course. A very weird feeling.
Similarly, the keel on a sailboat provides the sideslip back pressure to allow the boat to sail better towards its desired heading. Yet there is still some sideways movement that we just can’t do anything about except for understand it and account for it. I.E. the heading and course will be different.
Airplanes have the same issue. When flying in a cross wind, the plane moves downwind. The course becomes different from the heading. Before the electronic age, the course correction needed to be calculated. It was relatively simple. You just added the wind vector and your airplane vector to get your required heading vector. And you could even do it on a rotating slide rule which made it even simpler.
The electronic age has deprived us of the need for that old skill. Now a days autopilots are cross connected to the gps and the destination is input. The autopilot takes care of the side slip and puts you automatically at your destination. Regardless of automation, I believe it’s still important to understand the concept.
In day to day cruising type sailing, much of our navigation is done by line of sight with out plotting a course. However, we should still be aware of and account for leeway. Especially during the Friday night beer can race, a little bit of leeway knowledge will help you gain an edge for the rum prize at the finish line.
Here’s a couple of general rules to follow. Leeway is caused by sideway pressure on your sails right? So reduce the sideways pressure by making sure that your sails are never over sheeted (in to far). Make sure your telltales are always flying backwards. As a new sailor begins to learn to sail they almost always tend to have their sails in too tight which adds to leeway.