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  #31  
Old 04-04-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

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Originally Posted by Minnewaska View Post
AFAIK, apparent wind always moves forward on all boats.
Does the apparent wind move forward or does it help propel the boat forward?
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  #32  
Old 04-04-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

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Originally Posted by DRFerron View Post
Does the apparent wind move forward or does it help propel the boat forward?
If there is no wind but you're walking down the sidewalk at 5mph you feel 5mph of apparent wind, but naturally it's in your face and it's creating drag on your walking. The same is true for the sailboat, any wind created from the boat's motion will impede that motion.

Here is a new point - think about how neat it would be if you could sail off apparent wind. You could head out on a day with 10kt, get moving along pretty well so that the apparent wind was 15kt and then the 10kt of true wind could die off and you could keep sailing on your extra 5kt of wind!. Or you could use your engine to create some apparent wind and sail with no wind! Alas all the propulsive force pushing a boat forward is from only the true wind. As the boat moves the aparent wind will always move so its coming from more and more forward, and impeding the boat more and more.
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  #33  
Old 04-04-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

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Originally Posted by asdf38 View Post
If there is no wind but you're walking down the sidewalk at 5mph you feel 5mph of apparent wind, but naturally it's in your face and it's creating drag on your walking. The same is true for the sailboat, any wind created from the boat's motion will impede that motion.

Here is a new point - think about how neat it would be if you could sail off apparent wind. You could head out on a day with 10kt, get moving along pretty well so that the apparent wind was 15kt and then the 10kt of true wind could die off and you could keep sailing on your extra 5kt of wind!. Or you could use your engine to create some apparent wind and sail with no wind! Alas all the propulsive force pushing a boat forward is from only the true wind. As the boat moves the aparent wind will always move so its coming from more and more forward, and impeding the boat more and more.
I have to think about this, thank you. I'm still not seeing how the apparent wind is blowing in the same direction the boat is moving (how I interpreted "apparent wind [moving] forward"). When I see the diagrams of true vs. apparent wind, the apparent wind is always at an angle slightly offset from true, but still not moving forward. Same with when I stick my hand out of the window of a moving car. I understand what apparent wind is, I just don't know the deeper theory behind it. I perhaps am not understanding your definition of "forward." But that's OK. I'll figure it out at some point.
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Last edited by DRFerron; 04-04-2012 at 10:45 AM.
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  #34  
Old 04-04-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

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Originally Posted by DRFerron View Post
I have to think about this, thank you. I'm still not seeing how the apparent wind is blowing in the same direction the boat is moving (how I interpreted "apparent wind [moving] forward"). When I see the diagrams of true vs. apparent wind, the apparent wind is always at an angle slightly offset from true, but still not moving forward. Same with when I stick my hand out of the window of a moving car. I understand what apparent wind is, I just don't know the deeper theory behind it. I perhaps am not understanding your definition of "forward." But that's OK. I'll figure it out at some point.
It's vector math technically but it's really intuitive if you draw it on a piece of paper. To represent wind draw a line twards where the wind is coming from and make the length in inches equal to speed in mph or knots.

So for the walking example draw you, then draw a line 5 inches straight forward to represent your headwind when walking at 5mph. That's the wind you feel. So what if there was 5mph true wind coming directly from your right? Start where you pencil left off and draw a new 5" line to the right. This new point represents your aparant wind (note it doesn't matter whether you by drawing true wind or headwind, you'll come to the same spot anyway). Draw a line from you to this new point and that's the aparent wind. As you'd expect it's coming from in front of you and from the right and it's a little longer, about 7" meaning it's about 7mph. It's exactly 45 degrees because I set the two speeds to the same value.

Now for any arbitrary angles of wind draw the boat. Then starting at the boat draw the line twards where true wind is coming from (inches=knots or mph). Now consider the boat is moving forward. This creates headwind so draw a line from where you left off with true wind in the direction that the boat is traveling. This new point is aparent wind, draw a line from the boat to it and you have the direction and speed of the aparent wind. This works for any angles. The point is that headwind is always coming from the direction the boat is traveling.

As a verification that this works do the simple case of running dead down wind. Draw a true wind line from the boat backwards at say 10kt, or 10" (again, draw the line twards where the wind is coming from). Then lets say the boat is moving at 6mph which would create 6mph of headwind. So from where you left off (10" behind the boat) draw a line 6" forward. You end up coming to a point 4" behind the boat meaning there is 4mph of aparent wind coming from behind the boat. You knew this already because in this case it's just 10mph-6mph=4mph. But it shows how this works.
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Old 04-04-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Wow, I should have checked in an hour ago.

Okay, so, brief comments on a variety of responses so far:

Luffing when wind "in line": I'm pretty sure the wind is "in line" with the luff of my sail when the telltales stream evenly aft on both sides. The sail isn't luffing because it's curved. The curve is maintained by the angled pull of the sheet.

Lift: perpendicular to freestream flow, not to the foil. Minor point; you could break down the force of the wind into any components you like, but only one is referred to as "lift" in the textbooks.

Acceleration on the lee side: my understanding is that faster flow separates sooner, which greatly increases drag, and that therefore you actually want to decelerate flow on the leeward side (while still keeping it faster than the windward flow), and that this is in fact how the slot effect works.

Quote:
Originally Posted by asdf38
(paraphrased) The lift model lacks in quantification
No, that's patently false. Lift is given by a integral of forces over the surface of the sail. It's not especially useful for internet forum discussions, but it's there.

So yes, your "propeller pitch" model has an easy quantification --- which, as I keep complaining, is horribly wrong, because it predicts infinite speeds. Oh, except that you add drag, which you don't quantify. So your model either predicts infinite speeds, or is not quantitative, depending on which features you include :P

Quote:
Here is a new point - think about how neat it would be if you could sail off apparent wind.
But, you do sail off apparent wind. If the apparent wind is in a direction that permits sailing, that is. Same goes for true wind.
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  #36  
Old 04-04-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

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Originally Posted by Minnewaska View Post
I've witnessed similar debates over the reason an aircraft wing works. Is it the pressure being applied to the underside of the wing, or the reduced pressure on the upper side that is causing lift?
It's actually neither. An airplane wing gives lift because it deflects the wind stream downwards. While an aircraft is traveling, it constantly accelerates a certain mass of air downwards each second. Acceleration x mass = force. This is why the equations for lift include the mass of the air.
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Re: Why is beam reach (or near to it) the fastest point of sail?

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Originally Posted by asdf38 View Post
Also, just to be clear, planes don't need the airfoil to fly. As you say with the flat metal sail, a plane wing could skip the airfoil and it could still fly.
No, that is still an airfoil. An airfoil can be flat (like a butterfly's wing), hollow curved (like many fan blades) or three dimensional.
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Re: Why is beam reach (or near to it) the fastest point of sail?

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Originally Posted by Telesail View Post
What are the major factors that cause the differences in the polars for different boats? Sme seem to have maximum speed closer than a beam reach and others seem to have maximum slightly broader than a beam reach.

I am assuming that all polars are presented for the true, rather than the apparent, wind. Is that right and would that account for part of the difference - because a high performance boat will pull the apparent wind further back when on a beam reach than a more sluggish performer?
No, this isn't related to relative wind. It's related to keel efficiency. A less efficient keel will reach maximum hull speed on a slight broad reach rather than a beam reach (because there is a bit less load on the keel).
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Re: Why is beam reach (or near to it) the fastest point of sail?

asdf38 and Adamlein. I keep wondering if you two are trying to say the same thing in a different way.

In an aerodynamic sense the sail only sees the relative wind. That is, everything that the sail does is related to relative wind, not true wind. This seems to be what Adam is saying.

However, in a motion sense, you can only get propulsive energy from the true wind, not the relative wind. In other words, the amount of energy available will remain constant if the wind speed remains constant. There is no change in the amount of energy available regardless of motion (although a naive assumption might be that as relative wind increases so too would the available wind energy) This seems to be what asf is saying.
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Re: Why is beam reach (or near to it) the fastest point of sail?

Quote:
Originally Posted by brehm62 View Post
However, in a motion sense, you can only get propulsive energy from the true wind, not the relative wind. In other words, the amount of energy available will remain constant if the wind speed remains constant. There is no change in the amount of energy available regardless of motion (although a naive assumption might be that as relative wind increases so too would the available wind energy) This seems to be what asf is saying.
I know I'm being a little hair-triggery and a little pedantic here. Actually I think it's an interesting question. I'd suggest that you can accelerate (gain kinetic energy) whenever the apparent wind is suitable (i.e. coming from a certain range of directions, and of a sufficient speed). If there's a 5 kt current, for example, setting due west, and your boat is pointed north and at rest relative to the current, she will experience a 5 kt beam wind that she can sail on. So, there's no true wind, but acceleration is still possible.

True wind is important for considering sailing in places your boat isn't in. It's also important for considering the implications of wind against current. But I'm confident that it is irrelevant for figuring out how much energy your boat can get from the wind (irrelevant in the sense that apparent wind tells you everything that you need to know, whereas true wind doesn't).

Quote:
It's actually neither [low pressure above nor high pressure below]. An airplane wing gives lift because it deflects the wind stream downwards. While an aircraft is traveling, it constantly accelerates a certain mass of air downwards each second. Acceleration x mass = force.
And force x area = pressure. There absolutely is a pressure gradient across the wing and it absolutely does generate lift.

You are just discussing lift from the microscopic viewpoint, i.e., there are some particles with momenta and they transfer momentum from one to another when they interact. You're absolutely right, that is *all* that is going on, and there are no other effects that need to be considered.

However, it turns out that that's way more information than is really necessary or practical for actually calculating lift under a given set of conditions. If it were just air molecules bouncing of the airfoil, it wouldn't be okay, but it isn't... it's air molecules bouncing off the airfoil and off other air molecules. The airfoil gets momentum transferred to it from air that's still pretty far away. So instead of trying to work that into the equations, we take a macroscopic view of things and talk in terms of pressure. You can solve for the pressure field in a steady fluid flow around an airfoil, and then add up the pressures on the airfoil surface, to get lift.

So while your momentum-only description is valid, its best use is in deriving equations that tell us pressure at every point in the fluid.
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