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I've been watching a video on learning to sail, https://www.youtube.com/watch?v=k6DEToYI-7w&ab_channel=ClintonLakeSailingAssociation and much of it is clear, but it has left me with a couple of questions.

There are terms and techniques for sailing at various angles to the wind. Conceptually the simplest is a run straight downwind: you orient the sails sideways, and the wind gives you a straight push.

The instructor (around 20:00) recommends newbie sailors avoid doing this, going so far as to recommend that if the wind happens to be blowing in exactly the direction you need to go, you should zigzag somewhat to avoid having it directly behind you. The given reason is that a shift in the wind could send the boom swinging across the boat.

But it seems to me that for this to happen, the wind would have to shift fully ninety degrees, which seems unlikely; if we should be prepared for arbitrarily large shifts of wind direction, then by the same token the recommended broad reach is also not safe.

Is it the case that, counterintuitively, the wind is much more likely to shift 90 than 135 degrees? Or is there something else I am missing?

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    Are you considering only the case when you're the only boat in the middle of an open area of water, when you think of those wind changes, or have you thought about other boats, cliffs, buildings etc. My sailing was all in dinghies, much of it in docks with tall buildings around. There the wind could easily shift 90° when passing a gap between the buildings
    – Chris H
    Jul 19 at 13:42
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    "The wind will have to swing by 90 degrees". If the wind drops and then swings, the sail won't stay pulled out to one side, but can drift back in, to be caught by the changed wind. The sheet doesn't stop the boom coming inwards, only outward. Jul 19 at 14:12
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    Sailing straight downwind is dangerous. By jibing, the point of danger (the boom hurtling across the boat at the same moment you swap sides, powered by the wind and hard enough to crack your skull) is controlled, which makes it a known, not an unexpected, danger. Jul 19 at 14:45
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    @WeatherVane I think the question concentrates on the likelihood of jibing and we've both mentioned factors the OP may not have considered that make it more likely. I'd struggle to answer well enough and don't have time to try now, but anyway I should probably defer to your username
    – Chris H
    Jul 19 at 14:51
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    @ChrisH knowing the liklihood would come with experience, and the advice in the video is well given. Jul 19 at 14:59
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You are under the assumption that it requires a 90 degree shift. Having had my fair set of accidential or uncontrolled jibes before I switched to cat sailing (where we do not run straight downwind for speed reasons), I can tell you this is not the case.

First, there is waves. It might be some ocean swell or a motor boat passing by. When passing through a wave coming from the side, this can heel your boat over to the wrong side and the boom follows due to gravity. As soon as it passes through the wind, the boom will accelerate and sweep across the deck.
Another risk with waves is that a waving coming from behind at an angle might push you in a certain direction. This changes the relative angle of the wind and might push you through the wind.

Second, there is a huge risk of sailing too deep. Take another look at this video and watch the jib. Also take a look at the main sail. Those guys are already sailing way deeper than just down wind. All it takes then is a slight change of the wind direction, a wave or a steering error to lose control

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    Another aspect on some boats is that you are at extreme risk of pitch-poling, where the bow goes under the water and your whole boat flips over. Jul 20 at 18:24
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As you're sailing downwind, your apparent wind is very slow (it's real wind speed minus boat speed).

This means the tension to keep the sail - and the boom - taught is also very low.

In turn, this means that the boom will be moving around easily, not staying fully sideways as you'd imagine, and it doesn't take a lot to get the boom going in the wrong direction, catch the wind backwards, and sweep the deck.

A slight low wind followed by an angled gust, a wave rocking the boat and pushing the boom by gravity, as Manziel explained...

And the risk is serious too, as you'd easily end up in the water, injured and possibly unconscious, with the boat immediately downwind from you, in the worst place to come rescue you.

At the same time, on smaller boats, you loose the stability of having the sail pushing the boat sideways, so the boat is more susceptible to rocking, making matters worse, and navigation less comfortable.

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