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Old 04-03-2012
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Why is beam reach (or near to it) the fastest point of sail?

Hi,

I'm new to sailing and I'm trying to understand a little more about sailing theory.

Does anyone here know of a resource that explains exactly why the fastest point of sail for most boats is the beam reach (or close to it)? When I search on google I can find hundreds of site that state it as a fact, but I haven't found any that explain the reasons, preferably with vector diagrams.

I always find things easier to put into practice once I understand why I'm doing them....

Thanks,
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Re: Why is beam reach (or near to it) the fastest point of sail?

Can't draw a diagram here, but I will take a crack.

To start, if you are sailing directly downwind, the faster you go, the lower the apparant wind on your sails. You are out running the wind. So far, so good? The faster you go, the less you have to work with heading downwind.

If you sail upwind, you actually create more wind to work with (increased apparent wind), because you compound the wind as you begin to move into it.

Therefore, it stands to reason that you will get more boat speed sailing into the wind, then away from it. Boy, there are exceptions to that, but let's stay simple.

So why a difference in close hauled and a beam reach? A beam reach remains the lowest point of sail, where you are still being pulled by the lift of the sail, rather than being pushed from behind. Therefore, you may still be creating some increase in apparent wind.

Now the vector part. If you thnk about where the boom is on a close haul vs. a beam reach, this may begin to make sense. Since you are creating a wing with your sail, the boom is much closer to the center of the boat on a close haul, as the wind is coming more directly at our bow. On a beam reach, your boom is eased out so the front of the wing (sail) is facing the wind coming from the side.

For illustrative purposes, imagine that the lift coming off the sail is heading perpendicular to the front of the boom. In a close haul, with the boom in tight, that lift is predominantly pointed off the side of the boat. You don't move sideways, because your keel or centerboard fights the sideways vector, leaving only forward motion. It's like pressing down on marble, but it squirts out from under your thumb. The vector of pressure is toward the table, but the table keeps it from moving in the direction of the pressure and it escapes to the side of least resistance. Your sails are the thumb and your keel is the table.

On a beam reach, the boom is eased out a bit further, so the vector is pointed more forward. The keel fights less, the boat heels less due to less sideward vector and there is more power remaining for forward motion. Sort of like your finger not pressing that marble directly into the table, but at a better angle toward the direction you want it to go.

Hope that wasn't too rudimentary...... or completely confusing. . Thumbs, marbles and tables, what the heck?
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Old 04-03-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Minne is exactly right, but let me simplify it even further-

The beam reach is (usually) the point of sail that generates the most lift in the best direction or point of effort.

That was an intelligent question and I think you're off to a good start.
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Old 04-03-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Simplistic answer. Think of a tidly wink; the boat is stuck between the force of the wind and the mass of the water. Squirting out to the side is fastest, though the exact angle depends on the sails set and the boat.

Full answer. Read a technical book on sailing, such as "Performance Sailing". Study engineering. Google isn't for long answers, and real understanding won't come in a few sentences.

Sorry.
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Re: Why is beam reach (or near to it) the fastest point of sail?

If you are interested in looking deeper, then Magazine Articles is a good source. Although it is 40 years old, it shows that most school text books are even older. It explodes the myths about pressure differentials either side of the sail caused by different flow path lengths that most instructors propagate.

A beam reach is quickest because it is the point where you are still getting lift as well as push (draw yourself a small wind diagram and the vectors on the sail to convince yourself) and reduced resistance from component of the vector for windage from the boat's profile facing the wind in the direction of travel.

Hope that helps. The other answers are also correct and contain an element of simplification (as does my answer) but should all help your understanding of what is going on. Next stop is understanding how sail shape affects this question as well as the point of sail.
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Re: Why is beam reach (or near to it) the fastest point of sail?

Minne,
Thank you for explaining that so clearly!
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Old 04-03-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Minne you might have it but indulge me for a second while I try also.

"If you sail upwind, you actually create more wind to work with (increased apparent wind), because you compound the wind as you begin to move into it."

This is a common way to describe the process but I absolutely hate it. This should make every physics teacher cringe. Yes the boat creates wind when it moves forward but the wind it creates is entirely in the WRONG direction. This 'new wind' can't be harnessed for forward motion any more than pulling on your own bootstraps can be.

The best way to describe the ability of a boat to move faster than the wind is leverage.

Think about this just like doing calculations on prop pitch (inches per revolution). Given a pitch of a prop you can calculate, theoretically how far forward the boat should move for every revolution. So lets do the same exact thing for the sails of the boat except revolutions are replaced by how much the wind moves.

Picture the wind as some stiffer fluid that's determined to move forward and your boat as being friction less and only able to move straight forward back. Lets say the wind is coming from 90 degrees off starboard and your sail is trimmed to 45 degrees. Clearly the ratio is 1:1, if the wind moves 10 ft across your boat you'll move 10ft forward. Now imagine that you pull the sail in a bit, say to 50 degrees (with 90 being all the way in). Now the ratio is 1.2:1. You'll move forward 12ft for every 10 that the wind moves. The pitch of your sails could be said to be 1.2:1.

Theoretically you can pull your sail in to 89.999 degrees at which point for the wind to move 10ft you'll have to move almost infinity! (in our made up environment).

The vector math still works fine here. Lets go back to the 45 degree example with wind at 10knots . If you really did this what would happen is your boat would start moving forward creating wind off the bow. This would add into the wind coming from starboard. As this happens the apparent wind increases, but also changes direction so it's coming more and more from the bow. When it approaches 45 degrees, the angle of your sails, your sails will luff and you'll stop accelerating. In this example, with your sails at 45 degrees this would be exactly 10 knots forward speed (14 knots aparent), which is exactly what the 'leverage' example came to.

The point is that the 'new wind', the wind that you created, only hurt you because obviously this new wind is the wrong direction! Also note that there is no need for the concept of lift here. Personally I find lift to be extra confusing.

More broadly, sails on a boat can leverage only the wind that's coming from the sides. Any wind coming from the bow or the stern simply pushes or drags the boat in a simplistic fashion. But any wind coming from the sides can be leveraged and can theoretically be leveraged to infinity. The reason a broad reach is the best point of sail is because on a broad reach all the wind is coming from the side and all of it can be leveraged by the sails.
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Re: Why is beam reach (or near to it) the fastest point of sail?

asdf: can you explain, without referring to lift, how camber and draft affect the sail's drive?

Why is it that increasing draft powers up a sail? Since increasing draft also decreases the angle of attack, in your model this should result in less leverage and therefore less drive.

Quote:
Theoretically you can pull your sail in to 89.999 degrees at which point for the wind to move 10ft you'll have to move almost infinity! (in our made up environment).
I think this is pretty good evidence that your theory is not a good description of reality. Clearly the limiting forward component of the drive of the sails does not increase without bound as the boom approaches the centerline, and it has nothing to do with friction.

So here's my answer to the OP:

Recall that lift is defined as the component of the force exerted on a body by a fluid flowing around that body, in the direction perpendicular to the direction of distant upstream flow. Drag is the component in the parallel direction.

For example, with the wind a 45° off the port bow, the lift pushes the boat 45° to starboard, and drag pushes the boat 45° aft of the beam. Lift and drag are both contributing to leeway here. If you make your sails more efficient, increasing lift and decreasing drag, your keel is still resisting both of those; if drag was initially greater than lift, you might even be making more work for the keel.

With the wind abeam, the lift vector points straight ahead; the keel is only resisting drag now. If you maximize the lift-to-drag ratio, you're maximizing forward drive and minimizing the work your keel has to do to resist leeway. Also, there is no component of the wind that resists the forward motion of the vessel; drag is a purely lateral force and lift is a purely forward force.

With the wind abaft the beam, lift is reduced, but lift and drag are both contributing to forward acceleration. My understanding is that the fastest point of sail is not a true beam reach on most vessels, but rather a point somewhat aft beam.

This model ignores the keel, of course. The keel also generates lift and drag forces, and as long as the boat is making any leeway, some of the lift will be forward. It's less clear to me where this contribution is maximized, since it's not clear to me how the different factors (drive from the sails? windage? anything else?) contribute to leeway; maybe somebody else can weigh in on this aspect.
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Re: Why is beam reach (or near to it) the fastest point of sail?

Quote:
Originally Posted by AdamLein View Post
asdf: can you explain, without referring to lift, how camber and draft affect the sail's drive?

Why is it that increasing draft powers up a sail? Since increasing draft also decreases the angle of attack, in your model this should result in less leverage and therefore less drive.
No I can't! And that's part of my point. The aerodynamic effects of sail shape and lift are all secondary to what I'm describing just as propeller shape and hydrodynamics are secondary to the simple principle that the blades of the propeller are at an angle to the direction they are spinning! The ratio of that angle defines the basic behavior of the propeller. It's intuitive that while in theory the propeller can drive the boat to infinity the reality os obviously much different.

Quote:
I think this is pretty good evidence that your theory is not a good description of reality. Clearly the limiting forward component of the drive of the sails does not increase without bound as the boom approaches the centerline, and it has nothing to do with friction.
Limiting forward component of the drive of the sails? I don't know what you mean.

But let's agree on one thing. When the apparent wind is in-line with the sails, the sails luff. This is true when you're in irons and it's true on any point of sail if you let it happen. As you increase speed the angle of wind changes to be coming from farther and farther forward. You're only option is to keep drawing in your sails. For any given angle of sail the maximum possible speed you can go is defined by the ratio, just like pitch on the propeller.

This is the be-all-end-all limit for any sailboat speed regardless of lift, sail type or any other factor. Because when this speed is reached the sails will luff and if it's exceeded they will backwind.

But we agree you'll never hit this speed. As your speed increases both the drag from the water and the drag from the wind will increase (because the 'new wind' is in the WRONG direction). At some point this drag equals the forward propulsion you're able to get from your sails and you stop accelerating and reach a steady state.

None of the sail lift or keel lift needs to be discussed to understand the basic dynamics here! The point of the keel is make it so the boat can only move forward or backwards and not sideways. Think about an iceboat with a flat steel sail - no airfoil, no hydrodynamics on the keel. Just a piece of metal to deflect the wind and blades in the ice to keep the boat moving forward or backwards only. It still sails and still sails with the principles of a sailboat.
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Re: Why is beam reach (or near to it) the fastest point of sail?

Quote:
Limiting forward component of the drive of the sails? I don't know what you mean.
I mean that as the boom gets closer and closer to the centerline, the forward drive from the sails does not increase to infinity as your model predicts.

[quote[But let's agree on one thing. When the apparent wind is in-line with the sails, the sails luff.[/quote]

Depending on what you mean by "in line", I'm not sure I agree. But whatever, I agree that there's some point near "in-line" at which the sails luff.

Quote:
As you increase speed the angle of wind changes to be coming from farther and farther forward. You're only option is to keep drawing in your sails. For any given angle of sail the maximum possible speed you can go is defined by the ratio, just like pitch on the propeller.
Can you give a formula for the way in which maximum speed you can go is defined by the ratio? And the ratio of what to what? length of boom / distance from boom to centerline?

Quote:
None of the sail lift or keel lift needs to be discussed to understand the basic dynamics here!
They absolutely need to be discussed. Here's the limits you've mentioned:

1) Ignoring drag, there's no limit. Put your boom on the centerline; your boat will travel at infinite speed. Maximum of speed is based only on the angle the boom makes with the wind.

2) Obviously we can't ignore drag. So drag puts a limit.

All of your dynamical discussions and your analogies to propellers are only interesting in the discussion of point 1. You don't actually state a speed limit due to drag; you just state that there must be point where drag balances forward drive. Your source for this forward drive is the leverage effect which, according to point 1, can be made arbitrarily large.

If you want to talk about drag, you have to talk about lift as well. Lift and drag are two components of the force that the wind applies to the boat.

I get what you're trying to do; you're trying to talk about the kinematics of sailing without talking about the mechanics. That is, you're trying to describe the motions (wind moves across sails, sails move in relation to the wind) without discussing the underlying mechanism that causes that motion. That's a valid thing to do, but your kinematics are completely wrong. Your kinematic statement is "speed is determined by this ratio", which is clearly not true, and then you get around the obvious flaw in the kinematic description by referring to a mechanical one.

My point is not that your understanding of drag is wrong, but that the formula you give for speed in the absence of drag, and the analogy that it's motivates it, are useless. Since the "ratio" rule doesn't describe anything we observe, why use it at all?
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