Blowing in the Wind (How the Wind Impacts Your Boat)

This entry is part 1 of 1 in the series Blowing in the Wind (How the Wind Impacts Your Boat)

If not for the wind and its pressure, it would not be possible to ever sip a free glass of the elixir of the gods –Rum. Wind has its good side, but like most things, it has a mischievous side too. Let’s take a look at both!

Sail

We’ll look at the mischievous side first:

The wind can be quite tricky

The wind, like electricity, always looks for the path of least resistance. So, when the wind encounters a sailboat with its sails raised it tries to find the easiest way to continue on its merry way. The wind has a couple of tricks up its sleeve to make its merry journey easier.

The wind, pressing against the sails, tries to tip the boat onto its side. This tipping force can be seen clearly if one looks at a field of wheat on a windy day. When the wind blows on the grain it pushes the stems over and is able to continue on its intended path. Since the wheat is attached to the ground it remains in place, but the stems, being very flexible, bend to the wind’s force. The root system of the plants, attached to the ground, act as an anchor to hold the plants in place.

The weight of the keel, located on the bottom of the boat, resists the tipping because gravity is pulling down on it. As the wind tips the boat on its side the keel lifts from its natural vertical position and gravity applies a downward force on it. The downward force, pulling the keel with the boat attached, swings the boat back toward vertical. The boat settles in a slightly out of plumb attitude as a result of the two forces, wind and gravity, balancing against each other. That attitude is called the heeling angle. The wind is not able to remove the object that it opposes, the boat, from its path.

Unlike the wheat’s root system holding the stems in place in the field, the boat is not attached to the bottom of the body of water that it’s in. The force of the wind impacts the boat by pushing it sideways through the water. However, the hull of the boat including the rudder and keel stick down into the water. Those things –the keel, rudder, and hull– all have a significant amount of surface area, and as the wind tries to push the boat sideways they present lateral resistance.

Lateral resistance can be demonstrated with a tub full of water and your hand. Place your hand in the tub of water and move it forward as if it were a knife cutting a cake; very little force is needed to move your hand because it presents very little surface area to water (in the direction of its movement). Your hand is creating very little lateral resistance. Now move your hand sideways through the water, as if it were a snowplow. While your hand is the same size, the added surface area in the direction that it is moving causes you to have to apply more force to push it through the water. The increase in surface area causes more lateral resistance, which is the reason for needing to add more force. The hull and its appendages create lateral resistance to the wind the same way your hand does when moving sideways through the tub of water –in the direction the wind is trying to move the boat, laterally (sideways). The large amount of surface area makes it difficult. So, once again the wind is not totally successful removing the obstacle that it opposes, the boat.

Now for the good stuff:

Bernoulli’s principal

In most instances the wind encounters the boat with both a “side-to” component (partially impacting the side of the boat) and a forward or aft component (partially impacting the bow or stern). The exception to this “split” condition of two wind components is if the wind is blowing directly across the beam of the boat, perpendicular to the boat’s centerline.

If the boat is sailing windward, the force of the wind, partially negated by the boat’s defenses described above, can be used to positively impact the boat. In this case –sailing windward– as long as the boat is not positioned in the “No Sail Zone”, the wind’s force has a side-to and forward component. The side-to component serves to blow the boat’s sails leeward. If tension is applied to the leeward jib sheet and mainsheet the wind’s pressure fills the boat’s sails creating a curved shape. At the same time, the forward component of the wind starts flowing around that curved shape. The airflow around the curved sails sets a principal in physics known as Bernoulli’s Principal in play.

Bernoulli’s Principal states (in a very simplified and paraphrased way here) that if you increase the speed of a liquid or gas, as the speed increases the pressure of that substance decreases (please understand that I am NOT a physicist and only vaguely understand Bernoulli’s principal). So, as the air flows around the leeward side of our curved sails it has to speed up –it traveled further around the curve– to meet the air that passes on the inside of the curved sails.   As a result of the air speeding up there must be a drop in air pressure on the leeward side of the sails. After all, that’s what our buddy Bernoulli told us. Mother nature doesn’t like low-pressure areas in the atmosphere and the surrounding air will rush in to fill any such area. So the air on the windward side of the sails pushes the sails to fill that void. The boat, being attached to the sails is drawn into the void with the sails. That is called lift. At the same time that the sails are creating lift, the keel and rudder, also curved from front to back and traveling through the water, are creating lift on the windward side of the boat.

Each of these independent lifts can be shown as a vector. Now there’s an official sounding thing, and I’m a sailor not a science guy; so, I’m gonna try to explain this in terms that even I can understand.

My understanding of vectors, as muddled as it may be, is that they are arrows that are of various lengths and that point in various directions. The length of the arrow represents the speed or amount of the force and the direction the arrow points in the direction that that force is going. If that’s true, and I’m not quite sure it is, then the vector representing the lift generated by the jib would be pointing perpendicular to the angle the jib is set to, and the length of the arrow would be the amount of lift being generated.

Hey! Wake up!

Each of the surfaces that are generating lift –the sails, keel, and rudder– has their own special vector. The really smart guys are able to take all of those vectors and add them up, and the result is another vector that represents all of the added forces combined. Well, that all sounds great!

But, there’s a problem. Lift has a counter-part and its name is drag. When lift is generated, YEAH, so is drag, BOO. Drag works against lift, kind of like the IRS works against the American people, and can be represented by vectors also. But, the news gets worse; there are things, like the hull and propeller, that don’t generate lift but do generate drag. Those really smart guys I was talking about, they do their same vector magic with drag as well. And, in the end, they sum up the final lift vector with the final drag vector and that result is a vector that represents the boat’s speed and direction (assuming the boat is traveling in a straight line).

My father taught me to never rain on someone’s parade and therefore I’m not going to rain on lift’s parade. So, in the interest of parades everywhere ignore all that drag stuff I just told you about. Lift is the wonderful thing that enables a boat to proceed toward the wind. And, THAT’S HUGE! Sailors from all of time should be glad that somebody figured that out. After all, what do sailors want more than anything else? We want free rum, which in my experience, is always located precisely upwind from wherever we are. My friends, if not for lift we would have to pay for our rum. LONG LIVE LIFT!!

Now, go have an uplifting day and sail toward the wind, On the Water…With Captain Frank