Cecil Couldn't Answer: Physics of Sailing

Cecil admitted that he could not answer this question:

When sailing in a small boat one day on a lake, it struck me that I could not figure out how the laws of physics permitted me to sail into the wind. When the wind was directly out of the west, I could sail alternately southwest and northwest. In fact, when I pushed the boom starboard as far as it would go, I could catch the wind from the west and the boat would still go west. How is it that the boat is propelled to move in a direction that is opposite (or nearly opposite) the direction of the force applied? Nobody I know with a physics background could give a straight answer (lots of mumbling about Newton’s third law and the “vectors” of force on the mast). What is the answer?

The force on the sail (assuming it’s perfectly flat) is always at right angles to the surface. If you come up to the wind with the sail alee, the wind’s force will try to send you diagonally, but the boat’s keel doesn’t let you go that way very easily. Your net motion will be close to where the hull points. Of course you’ll make some leeway as the keel slips sideways, but with a well-designed hull you can go somewhat upwind.

I should have said that this is a lot easier to understand with diagrams. I’m sure they’re out there on the Web, or you can sketch your own.

Next lesson: tacking vs. wearing.

You have two forces working your boat when you are sailing towards the direction the wind is coming from. You can only sail approximately 45 degrees into the direction of the wind without the wind stopping you or pushing you back. As the wind fills the sail you get a lateral force that gives the boat steerage way. That means your moving and the rudders working. As the sail moves through the air, the curve of the sail actually creates lift like a wing on an airplane, and that lift gives you added forward momentum. The wind actually pulls the boat forward.

If the sail is curved, it is possible that the air will be forced to move faster on one side of the sail than the other. This results in a pressure differential that can push the boat in one direction. (This is known as the Bernoulli effect, and is also how a plane’s wing gets lift.) I’m not entirely sure if that has anything to do with the phenomenon you experienced, but it does explain how some sailboats can move faster than the wind speed.

[QUOTE=Backwater Under_Duck]
As the sail moves through the air, the curve of the sail actually creates lift like a wing on an airplane, and that lift gives you added forward momentum. The wind actually pulls the boat forward.QUOTE]
Backwater beat me too it :slight_smile:


sigh Messed up tags and an extra “o” – this message board’s lack of an “edit” option is not good for the sanity of perfectionists like me. :mad:

The “lift from a wing-shaped sail” has been the main theory of upwind sail physics for a long time. It’s not universally accepted, though. In recent years, some theorists have worked on the idea that most of the drive comes from simpler, Newtonian force. That is, the sail steers the air off the trailing edge in a direction opposite the direction of the boat’s travel. The air leaving the sail goes southeast, and the boat, in “an equal and opposite reaction,” gets a northwest boost.

Now, before you start shaking your sliderule and throwing red-hot vortices at me, let me tell you that I am not a physicist. I not only cannot technically defend what I just wrote, I barely understand it. (Yes, I’m old enough to remember what a sliderule is.)

Is basically the way force is transmitted from wind to sail to keel.

Sails basically transmit force of the wind perpendicular to the plane of the sail.
Keels basically transmit force of the sail parallel with the keel.

If you hold both the sail at an angle to the wind and the keel at an angle to the sail, you can end up with an angle greater than 90 degrees, and thus be sailing into the wind. The practical limits of this max out at about 45 degrees sail to the wind and about 90 degrees keel to the sail.

Ergo, if you have Wind from the west, the sail can convert that into a force towards the northeast (or southeast) and the keel can convert that into a force to the northwest (or the southwest). And your ship moves with the keel.

All of the above and more.

I always had the best results explaining to my charter passengers how sailing works by this simple analogy: It’s like squeezing a slimy watermelon seed between your fingers, causing the seed to pop out. One finger is the wind, the other is the water. When the two forces are applied opposite each other, the seed takes the path of least resistance and pops out, likewise the boat is pushed forward through the water.

Diagrams and a more technical explanation of the pertinent forces are on this web page:


It seems like with the gazillions of dollars and euros the the America Cup people pour into this issue, what with their fancy computer models and testing tanks, that they would have a pretty firm handle on the specifics of the physics. But just when you think you’ve got something figured out…
Actually, I like the idea of the boat travelling opposite direction of the air flow off the sail. I could never figure out why the beam reach was so powerful. Makes a little sense.

The sail is an airfoil (quite similar to the wing profile of those of the Wright brothers’ Flyer) and develops a “lift” force. As 1010011010 said, the resultant force that drives the boat forward is the interaction of this force and the force of the water on the keel.

The answer is ‘tacking.’

You zig-zag along the wind if you’re going into to it. You frequently shift the boom from one side to the other (lower the boom on someone).

Here’s another cite with info:


I’m sorry - ONE answer is ‘tacking.’ I’m not a sailing guru at all. I didn’t mean to show up as the final resource, just did a bit of it in the navy and in the gulf (of Mexico). I defer to the proper sailboat know-it-alls, and confirm my landlubber status.

I meant to say ‘one,’ not ‘the.’

True. But the question at hand is why does tacking work? Why is there any component of velocity directly into the wind as there must be to make headway up wind.

I’ll ignore any discussion of the Bernoulli effect; see this thread.

It’s time for some verbal diagrams. Say we have a wind from the north. Draw an arrow. Your vessel is headed directly east with the sail amidships. Since the force is always at right angles to the surface of the sail (draw an arrow pointing south), the boat drifts southwards, slowly because of the keel. Let the sail run out to a 45-degree angle, and the force on it is now southeast (draw another arrow). Here’s the trick: you can ‘resolve the vector’ into a pair, one south and one east, chained together so the pair starts and ends where the original southeast one does (draw arrows). Those represent the eastward and southward components of the southeast force. The keel won’t let the vessel slide south very easily, so you go very slowly in that direction, but the hull is designed to move forward with little resistance, so that component of the force moves you quickly, and you’re going almost straight east.

Finally, steer a little upwind, tighten the sail in a bit, and redraw the diagram. Repeat as needed until your boat won’t go any closer to the wind.

Downwind speed is limited because you’re moving with the wind. If you have, say, a 20 mph wind from the north and you’re heading south at , say, 15 mph, you only see the wind passing you at 5 mph, so there’s little force pushing you. If you’re sailing upwind, however, that same north wind will be passing you at something more than 20mph, and you get increased force.

Yeah. It’s important to remember that the relative wind is the vector difference between the wind speed as measured by a stationary observer and the velocity of the boat relative to that same observer. This creates a wind past the sail that is quite a bit greater than the wind speed alone when going up wind and less than the wind speed going down wind.

I always found it a little uncomfortable to sail directly down wind. I much prefer a slight quartering wind so that I know the boom isn’t suddenly going to zip across over (I hope) my head.

I should add that I’m not a very good sailor. A good sailor knows what the boom is going to do but we dubs don’t always.

Of course I should have staed my speeds in knots instead of mph. Bloody landsman!

I don’t see what’s so hard about this.

Let’s say a wind approaches diagonally from the front. The sail changes its direction so that it leaves while going straight out back. Now the component of the wind speed that’s parallel to the boat has gotten bigger. In other words, if you only look at the front-back axis the wind has increased in velocity.

If the wind travels faster backwards then the boat must travel faster forward. Newtons 3rd law.