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  #11  
Old 04-03-2012
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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 tarmes View Post
?...I'm new to sailing and I'm trying to understand a little more ......,
It seems the OP's stage in their training is being lost in the discussion.
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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
It seems the OP's stage in their training is being lost in the discussion.
Such is often the OP's fate. That said, it's good to consider what lessons the OP can take from the discussion. After all, he's asking "why?", not, "What's the fastest point of sail?" If he's asking why, then maybe he's in a position to benefit from some understanding of the mechanics of sail trim.
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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 AdamLein View Post
Such is often the OP's fate. That said, it's good to consider what lessons the OP can take from the discussion. After all, he's asking "why?", not, "What's the fastest point of sail?" If he's asking why, then maybe he's in a position to benefit from some understanding of the mechanics of sail trim.
Agreed. Good discussion.

I was really referring to the suggestion that the conventional way of describing what was going on would "make a physics teacher cringe". Not many physics teachers make good sailing instructors, especially entry level. Some exceptions, I'm sure.
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Re: Why is beam reach (or near to it) the fastest point of sail?

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

It puzzled me at first, and still does.

I understand that it is possible to drop more pressure across the sail surface by accelerating the airflow across the leeward side than by (downwind) using the sail as a barn door and so decelerating the air past the sail barrier rather than accelerating it across a curved surface.

It is facinating to know that my ship is fastest when the apparent wind is slightly ahead of me. It can get a bit wet sometimes in deep water as those waves tend to be wet when they hit. Downwind, broad reaching, things are easier.
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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 Rockter View Post
I understand that it is possible to drop more pressure across the sail surface by accelerating the airflow across the leeward side
That's what I had always understood as well. However, do you know about Arvel Gentry's ideas? He says that the slot effect actually decelerates airflow on the forward lee side of the main, and that this reduces the pressure gradient and thereby reduces separation of flow.

He bases this on actually measuring flow rather than the guessing that he accuses previous researchers of doing. Hm!

See Magazine Articles, especially "Another look at the slot effect".
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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 AdamLein View Post
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.


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.
Inline means that if the wind is moving paralel to the sail then the sail must luff and the boat can't accelerate.

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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?
Just like pitch on a propeller, look more closely at how that's defined and you may understand what I'm saying. Technically it's the Tangent of the angle of the sail given that 90 is close hauled (tan=infinity), and 0 when the sails are let all the way out (hitting the spreaders perpendicular to the centerline).

Perhaps it wasn't useful to bring this up but this forms the upper bound on speed. The point is that only wind coming off the side of the boat can be leveraged in this fashion and on a beam reach (or close to it) the coponenet of wind in this direction is maximized. This is more technical than necessary.
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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?
[/quote]

No. 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). 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).
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Re: Why is beam reach (or near to it) the fastest point of sail?

Wow, my question generated some great responses, thanks everyone.

I wish I could have contributed, but I was suddenly invited out sailing, so I took that option instead

In any case, between all your answers I certainly understand better what's happening now. I also happened to stumble across another reference to "Performance Sailing" so I've ordered a copy for my amusement.

mortyd - I fully agree with what your saying. No amout of theory alone will make me a better sailer, for that I need to spend time in a boat. Nevertheless, I find the theory interesting and having a better understanding of the physics of what's happening can't do me any harm. It's the engineer in me.
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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 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.
No, but none of your degree needs to be applied, in my oppinion, to explain why a sailboat moves forward. For racers trying to maximize speed and tweak sail shape? You need a PHD in fluid dynamics for that.
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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.
I say this all the time. One of the things I love about sailing is that there is always something else to learn. Some people enjoy broadening their minds and want to learn the theory behind certain things. Personally, I've reached my threshold somewhere around "wind helps make it go" but others want to know.

I hope your graduate program teaches you how to be a little more helpful with all that knowledge you're paying to learn about. Otherwise, I hope you'll be a researcher and not an educator.
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