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post #43 of Old 04-04-2012
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Re: Why is beam reach (or near to it) the fastest point of sail?

Originally Posted by AdamLein View Post
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.
I said the boom/sail. Again the sail is only curved because wind is pushing it more to one side than the other. This is because the boom is at an angle to the wind. If the boom is inline with the wind the sails must luff.

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.

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.
That's not a quantification it's a trivial definition. Of course sum (integral) of the perpendicular forces over the sail tells you the lift. That doesn't help quantify the strength of this force relative to any other. It just defines how we'd find it.

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
Obviously the force is related to wind speed and sail size as well as how efficiently the sail is positioned. The force is not infinite but the mechanics of how the force is generated allow speed to theoretically hit infinity. A friction-less boat could reach infinite speed. Are you saying it can't? I think the "propeller pitch" or wind deflection model is the easiest way to explain this. Lift remains for most an imprecise term that doesn't contain the boundaries for what a sailboat can do.

For example why can't a sailboat sail into the wind? My explaination is that a boat sails when wind hits the sails and get's deflected. Well when it's pointed into the wind there is no wind to deflect and this is totally clear on a simple diagram.

On the other hand lift is less concrete. Why can't the sail generate lift when the wind is flowing straight over it when a boat is in irons someone might ask. Even you had trouble allowing that a sail will luff in this situation. However I'll admit this is my own opinion of the relative simplicity of these two models.

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.
I don't deny this but I don't think it's an effective way of explaining it. Once you allow for the fact that you're sailing off aparent wind, "A sailboat makes it own wind" is a phrase I've seen, you open yourself up to the kinds of mistakes that brehm62 points out. It becomes all to easy to get confused.
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