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  #21  
Old 04-03-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Quote:
Originally Posted by asdf38 View Post
Inline means that if the wind is moving paralel to the sail then the sail must luff and the boat can't accelerate.
The reason I hesitated is because the sail is curved, so that when one part is parallel to the flow, other parts might not be.

Quote:
I'm trying to describe the simplest way to understand the mechanics for why a boat moves forward when wind hits it (the sail is at an angle to the wind, the wind hits it and deflects backwards, the keel prevents the boat from moving sideways and the boat has to go forward).
I think you're trying to do more than that. You're trying to say that because the process works like such-and-such, such-and-such is the best way to configure the system to optimize some aspect of the system. But the process you describe leaves out too much and makes too many unrealistic predictions.

Furthermore, your model talks about the angle between the sail and the wind, while trying to make claims about the angle between the wind and the boat.

Does the propeller model describe why a deep reach is not faster than a beam reach? I can still make my boom perpendicular to the wind on a deep reach. Why can't I use your leverage affect to travel just as fast as I would on a beam reach? Now, not only do I have the leverage you describe, but I also get the forward impetus of the wind behind me.

It's much easier to describe the problem using the language of lift and drag than using the language of pitch. In the deep-reach example, the angle of attack is too high, so the flow is stalled, producing significantly less lift, but just as much drag. Fortunately the drag is in the direction I want to go. The beam reach is the furthest-aft point of sail I can get to and still generate lift with the mainsail. Why is that? Because the sail is curved, but the luff of the main can't go forward of perpendicular to the centerline; i.e. the angle of attack can't be lowered enough to prevent the sail from stalling. Is that the whole picture? No, but good luck discussing the other factors without talking about lift and drag.

Quote:
You ignored my example of the iceboat with the steel sail which strips the mechanics of sailing down to the basics, removes lift (in the sense that there are no foils, perhaps you will still use term), and yet we still have a vessel that we both agree will move (I think).
I didn't ignore it, I just didn't see how it's relevant. I totally get your description of a sailboat in water, so I don't think this example adds anything.
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  #22  
Old 04-03-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

I just thought it was because on a beam reach, you have both the "pull" of upwind work, and the "push" of downwind work.

So therefore better than either one by itself. Plus you're parallel to the seas.


The discussion reminds me of the story of the young kid, who asks, "mom, where did I come from?" Mom gives him "the talk", in all its scientific detail.

Kid says, "Oh. Billy says he came from Pittsburgh, I wanted to know where I came from".


It's fastest 'cause it's fastest. At least for displacement hulls. If you can get planing, then a broad reach. Please don't ask why ;-)
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  #23  
Old 04-03-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

It doesn't look cool though. The cool kids are all close hauled and hanging off the side.
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  #24  
Old 04-03-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Quote:
Originally Posted by mortyd View Post
the wright brothers are spinning in their graves.

as a graduate aerodymanicist it seems many sailors think my education took half an hour. no two things can complicate aerodynamics more than low speed and three dimesionality, and sailng has both. leave the junior high school diagrams alone and learn the wind and your sails. the reason s boat sails fatest on a broad reach are many and very complex - just believe and enjoy. i never think about aerodynamics when i sail or fly.
Though not and aeodynamicist (chemical engineer), I had enough course work in fluid dynamics to chose the above as my favorite answer!

Why would I want to think so hard on the weekend, when a hueristic understand is all I can mange real-time anyway?
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  #25  
Old 04-03-2012
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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
The reason I hesitated is because the sail is curved, so that when one part is parallel to the flow, other parts might not be.
The sail is only curved when wind is hitting one side more than the other. If wind is inline with the sail/boom, i.e. paralel too it, the sail will luff.

Quote:
I think you're trying to do more than that. You're trying to say that because the process works like such-and-such, such-and-such is the best way to configure the system to optimize some aspect of the system. But the process you describe leaves out too much and makes too many unrealistic predictions.

Furthermore, your model talks about the angle between the sail and the wind, while trying to make claims about the angle between the wind and the boat.

Does the propeller model describe why a deep reach is not faster than a beam reach? I can still make my boom perpendicular to the wind on a deep reach. Why can't I use your leverage affect to travel just as fast as I would on a beam reach? Now, not only do I have the leverage you describe, but I also get the forward impetus of the wind behind me.

It's much easier to describe the problem using the language of lift and drag than using the language of pitch. In the deep-reach example, the angle of attack is too high, so the flow is stalled, producing significantly less lift, but just as much drag. Fortunately the drag is in the direction I want to go. The beam reach is the furthest-aft point of sail I can get to and still generate lift with the mainsail. Why is that? Because the sail is curved, but the luff of the main can't go forward of perpendicular to the centerline; i.e. the angle of attack can't be lowered enough to prevent the sail from stalling. Is that the whole picture? No, but good luck discussing the other factors without talking about lift and drag.

I didn't ignore it, I just didn't see how it's relevant. I totally get your description of a sailboat in water, so I don't think this example adds anything.

Ok first I'll admit that what I'm saying most directly addresses the question of why a boat can sail faster than the wind, and that answer arrives at a broad reach. Let's review that for a moment.

The reason a boat can go faster than the wind is because the sail of the boat can be positioned so that when the wind hits the sail at say, 10 knot it will deflect the boat forward rate greater than 10 knot. This comes down to a simple ratio and is the same calculation used on propellers to determine the maximum theoretical speed. Just as in propoellers (slip) drag naturally cuts the actual speed to below this theoretical value.

The point here is that only the component of the wind that is coming across the boat can be leveraged in this fashion. The component moving with the boat, or against it simply adds or detracts from the boat's speed in a simplistic fashion. Going dead down wind the boat can only approach the speed of the wind, and going upwind the boat must always be at some angle to the wind so there is a component of wind crossing the boat that the sails can leverage.

Withought getting into vectors you can see that the component of wind crossing the boat is maximized on a reach when the wind is crossing the boat at 90 degrees. Thus for a boat that can go faster than the wind this is when it has the best chance to do it. Any turn twards or away from the wind and you're trading the sideways component which you can leverage for an aft or forward component that you can't.

Again part of the point here is that lift has nothing to do with this equation. The iceboat with the metal sail (no airfoil) can (and would) go faster than the wind with no airfoil lift.

That said, I don't have all the math on hand to weigh this out for the usual case of a boat moving slower than the wind. I.E. I don't know how to prove that a broad reach produces more forward force than going dead down wind other than to point out the corelaries to my faster than the wind argument.

But I don't think you have the answers either. You can claim that lift is the answer for the normal boat but you have to allow that I've pointed out how a broad reach acheived the maximum theoretical speed for a boat with no airfoil lift.

Also consider a boat with a boom that can swing out more than 90 degrees from the centerline. What you've basically said is that lift is the key and the deepest point where air is flowing properly over the sail would be the fastest (this is why you say a broad reach is faster than a deep reach) but what if the sail were able to achieve this flow on a deep reach by actually allowing the boom to swing forward.

I haven't seen you to provide a better way to quantify lift than I have to quantify leverage. But I can prove exactly why leverage allows a boat to go faster than the wind, when this is maximized and what bounds it. I'm not sure you can do the same. Also, I think using drag in a way other than to describe an impediment to forward motion is confusing.

Last edited by asdf38; 04-03-2012 at 11:05 PM.
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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?

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?

Well, it seems to me that it is both.

Lift is simply a force applied perpendicular to the wing. The chicken and egg isn't relevant. Is it the pressure being applied below or the relative lower pressure above? Is it equal and opposite reaction of an applied force? It's all still defined as lift.

Regardless of how you explain why it happens, it's ultimately the differential in pressure that causes it to move. On the flat blade ice boat (trivial point.... all ice boats I've seen had a real sail), the pressure must be applied to the underside. However, that doesn't change what's happening on the upper side, it's just inefficient. Add sail/wing shape and improve efficiency. This seems to have as much to do with improving laminar flow over the top of the wing, as it does capturing pressure/wind underneath.

You get the most efficiency when the tell tales on both sides of our sails/wings are flowing straight back. Lift is both.
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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?

Gosh, isn't it fun when someone oversimplifies an issue in an attempt to illustrate a point, then gets called out on their ovsimplification and spends pages and pages trying either to defend their original oversimplification or to prove that they do really know what they are talking about but that they were only trying to make it understandable to the Newb.........

May I ask a slightly different question but along similar lines to the OP?

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

AFAIK, apparent wind always moves forward on all boats.

A stab at your question (read guess) are the variety of differences in center of effort vs center of lateral resistance (keel design, mast placement, etc). There comes a point where, even if a given boat can make more power with her sails, that center of effort may not be well aligned with her center of LR and the compensating rudder slows you back down. I assume that sweet spot can't be maintained across every point of sail, so some boats find it in a slightly different place.

Well outside my bounds here, but curious if others agree.
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  #29  
Old 04-04-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Quote:
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?

Well, it seems to me that it is both.

Lift is simply a force applied perpendicular to the wing. The chicken and egg isn't relevant. Is it the pressure being applied below or the relative lower pressure above? Is it equal and opposite reaction of an applied force? It's all still defined as lift.

Regardless of how you explain why it happens, it's ultimately the differential in pressure that causes it to move. On the flat blade ice boat (trivial point.... all ice boats I've seen had a real sail), the pressure must be applied to the underside. However, that doesn't change what's happening on the upper side, it's just inefficient. Add sail/wing shape and improve efficiency. This seems to have as much to do with improving laminar flow over the top of the wing, as it does capturing pressure/wind underneath.

You get the most efficiency when the tell tales on both sides of our sails/wings are flowing straight back. Lift is both.
Yeah that's the basics of an airfoil as I understand them as well. I agree with all of this. Yes you can describe any hydrodynamic or aerodynamic force as being due to a pressure difference and that's one way of describing a simple metal sail at an angle to the wind. I just wouldn't chose to describe that with the term lift, just as I wouldn't use lift to describe a propeller or a fan blade which work on similar principles. They hit the air/water at an angle and push it forward. Every kid whose stuck their hand out the window on the highway and 'flew' it by tilting it forward and back to move up and down understands this principle as well. Hence the fact that I think it's an easier principle to explain and can go further towards producing an understanding than "lift".

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. It would fly by creating an angle between it's wings and the ground such that as it flew forward it was deflecting air down just like a fan blade. This is how many simple balsa wood model planes fly, and you can see them flying with the back end lower than the front as they drag through the air.

Quote:
Gosh, isn't it fun when someone oversimplifies an issue in an attempt to illustrate a point, then gets called out on their ovsimplification and spends pages and pages trying either to defend their original oversimplification or to prove that they do really know what they are talking about but that they were only trying to make it understandable to the Newb.........
Honestly I think part of the reason lift works as an explanation is because people vaguely know what it is but don't understand it well enough to ask any more follow up questions. And saying "a component of the forward propulsion of a boat is lift. Lift is maximized on a reach, therefore the boat goes fastest on a reach" is still entirely lacking quantification of the force of the 'lift'. Compared to going downwind why is the lift component gained on a reach more powerful than the simple "push from behind" component that exists running down wind. No one here has answered that including myself except I was able to explain one thing with my model that lift, without being quantified cannot - why a boat could travel faster than the wind.

Last edited by asdf38; 04-04-2012 at 09:11 AM.
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Re: Why is beam reach (or near to it) the fastest point of sail?

asdf38 :

Compared to going downwind why is the lift component gained on a reach more powerful than the simple "push from behind" component that exists running down wind. No one here has answered that including myself except I was able to explain one thing with my model that lift, without being quantified cannot - why a boat could travel faster than the wind.

You have got more forward propulsive force on a reach because you drop more pressure by accelerating the air around the leeward side of that curved sail aerofoil.

When running downwind, you don't drop as much pressure. The air spills round the sail and eddies on its leeward side, and far more of the pressure is recovered on the leeward side. The net result is that there is less of a pressure difference across the sail when running downwind.

Also, when going upwind, your relative wind velocity is improving with boat speed (the relative wind angle will shift forward too). Of the wind, the reverse arguments apply.

Someting like that, anyway.
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