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  #51  
Old 04-05-2012
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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.
Donna,

This thread has become something else. I'm not going to bother addressing a reply above that said I was wrong and then described my exact point in different words.

I will only address the apparent wind moving forward comment. I meant the angle of apparent wind on your wind indicator will move toward your bow (forward) as the boat begins to move.
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  #52  
Old 04-05-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Here's a graphical representation of the aerodynamic output of a sail in either a closehauled or beamreach orientation.

The "F" is the approx. resultant FORCE output due to the aerodynamics, "X" is the resultant in the direction of the boat and is the SOLE component vector that is responsible for boat speed. The distributed 'arrows' are the vectors of pressure gradient, the F vector is an approximate 'resultant' of all the 'arrows'.

You can see in the 'chart' between the two dwgs. that X1 is smaller than X2, beam reach is faster than closehauled.

The 'pumkin seed effect' AND the slip of the boat's lateral resistance toward leeward is ignored in this illustration, as 'relativistic math' would be quite complex for such a discussion; but however, since the Y vector is greater when closehauled - the pumpkin seed effect is actually GREATER when closehauled because the output in the Y direction (across the beam of the boat) is greater than when at a beam reach.

Simple Speak: just compare the 'length' of the resultant X Vectors in the drawing --- the beam reach produces a LARGER vector (all due to 'trigonometry').

FWIW - lower than beam reach (a high broad reach -- ~125°-135°) is actually faster on most boats because the sail is still or 'can be' in an aerodynamic flow regime and that resultant X vector is even larger than at when on a beam reach, and with less 'slip' , etc.

Last edited by RichH; 04-05-2012 at 12:20 PM.
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  #53  
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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
Donna,

...

I will only address the apparent wind moving forward comment. I meant the angle of apparent wind on your wind indicator will move toward your bow (forward) as the boat begins to move.
Thank you.
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  #54  
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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
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.
This is correct from a boat point of view. In reality of course, energy from the wind is being transferred to the boat and then bled off in hull drag. But, yes, from the sailor's point of view, the wind dies off as you accelerate downwind. There is less push on the sails and rigging. Naturally though, the rudder will still have good authority because you are moving. It's interesting that this is why a jibe is such a trap. You are heading downwind and the apparent wind seems very light, not any cause for worry. But if you veer off then the apparent wind speed picks up and hits your sails with unexpected force. You suddenly have your hands full.

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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.
Well, again, from a boat centered point of view this is correct. The increased wind will push harder on the sails and rigging. There will appear to be more energy available to drive the boat forward.

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Therefore, it stands to reason that you will get more boat speed sailing into the wind, then away from it.
Yes, indeed, IF (and this is a big if) the apparent wind angle stayed the same.

Quote:
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.
I think what you are saying is something like the following. If the boat were stationary and facing perpendicular to the wind then the wind would be coming directly over the side of the boat. And, in this position, 100% of lift from the sail would be used to push the boat forward. Here we have maximum potential (but of course the boat isn't moving yet).

As the boat begins to accelerate forward, the apparent wind will increase and this would theoretically give additional lift to the sail. And, it would if the wind angle stayed the same. However, it doesn't. As the boat accelerates the apparent wind will come more and more from the bow. It will shift forward. And, the apparent wind will shift more forward the faster the boat goes. This is bad because, as the wind shifts forward, it becomes less useful. Why? Because lift is perpendicular to the wind and therefore as it shifts forward an increasing amount of the apparent wind would only generate side force rather than forward propulsion. Also, the increasing side force will have to be countered by the keel and rudder so this will increase drag.

So, why is close hauled worse? As you pull towards the wind the apparent wind vector shifts even more towards the bow, so even though theoretically the apparent wind would increase, there is less and less lift to move the boat forward. More and more of the wind's energy simply goes to pushing the boat sideways which would result in more heel and more leeway but not more forward motion.

I think this is what you were saying and I agree. However, even though I tried to say it as simply as possible I still ended up with three fairly complex paragraphs. And without benefit of diagrams this can still be misinterpreted. I think this thread has included a lot of saying the same thing in different ways.
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Old 04-06-2012
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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 RichH View Post
Here's a graphical representation of the aerodynamic output of a sail in either a closehauled or beamreach orientation.
Thanks, with that it become obvious....
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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 brehm62 View Post
I think this is what you were saying and I agree. However, even though I tried to say it as simply as possible I still ended up with three fairly complex paragraphs. And without benefit of diagrams this can still be misinterpreted. I think this thread has included a lot of saying the same thing in different ways.
Actually I found that very clear. Although maybe that's because I now understand the principal
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Old 04-07-2012
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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
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)?

If you search for boat polars charts:

Click to View Search Results for polar charts boat - Google Search polar charts boat - Google Search


you will find that with sufficient windspeed the fastest course is broad reach. Especially for fast efficient boats or ice boats. It all depends on how your hydrodynamic efficiency compares to your aerodynamic efficiency. You can define a L/D ratio for both parts of the boat (below and above water). Combined they give you the polar charts and the fastest course.
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