Sailing upwind in gusts

[GreenLake]

I'd like to share an unusual (to me) experience.

We had the last day of our winter series yesterday and the wind picked up with a passing cold front. (From zero to ten knots, it looked like a step function on the weather station's graph). The wind was a bit shifty and gusty, but as long as we were on the water not so strong as to be overpowering.

Now to the unusual part.

Normally, when the crew tells me that a gust is about to hit, I would get ready to luff up a bit, perhaps even anticipating the blow a bit, that seems to make it less sudden and then the boat heels and accelerates. Now, yesterday, every time I tried that, the boat would stand up and slow down.

Looks like the wind would shift direction with each gust, so it would come in more from the side than forward, so I had to fall off to stay in the wind. This was not a fluke, but a consistent pattern (the course was skewed so we sailed mostly on one tack and not the other; so I didn't have as much of an occasion to observe whether there was anything unusual there).

So, what was going on?

Normally, when a gust hits, the ratio between boat and true wind-speed changes. The effect of boat speed on the apparent wind becomes less. That makes the apparent wind move back. So the response is to luff up, because close-hauled is now closer to the true wind direction. That effect should be the same on either tack, and hold true as long as the direction of the wind and the direction of the gusts are the same.

Now, gusts are supposed to be an effect of turbulent air flow, where an eddy of faster air moves down to the water. Gusts that are caused by descending air will "splay out" on the water, and the fringes of the gust will have somewhat different direction (with a component away from the middle). However, as I'm sailing through a series of gusts, I would not expect to be on the same side of all of them. As we observed a consistent pattern, something else must be happening.

When I looked at weather station graph afterwards, I noticed that during the entire time, the wind was steadily and continuously shifting (about 90 degrees in an hour). Best I can puzzle it out is that at any time, the wind aloft was blowing further in the new direction, and each time a gust descended, it brought that direction with it.

As a result, we were headed (and de-powered) in each gust, until we clued in and anticipated that effect.

Now, for the opposite tack, this should have resulted in an extra lift on each gust. As one expects to be lifted a bit in a gust anyway, it's perhaps not as easy to spot the effect, and not as memorable, and I don't recall anything unusual.

Anyway, for me, it's the first time that I've seen this effect. It would, of course, be linked to the passing of that front, but to show up like that it would seem to require that steady change in wind direction. If it was due simply to wind shear based, for example, on the Coriolis effect (faster air affected differently than slower air) I'd think it would show up all the time.

[jeadstx]

I sail a lake with high hills all around it and the wind direction is constantly shifting. I generally just brace myself when a gust is about to hit, just keep the mainsheet in hand.

John

[GreenLake]

John, what you describe is defensive sailing, and I know places where that's the only thing you can do, because the wind comes down nearly vertically and as it flattens out along the water in a fan can hit you from any side. I know places where I wouldn't dare sail in high winds for that reason. Under more "normal" conditions, just sailing defensively isn't very fast.

What I described took place during a (casual) race in mixed fleet. Normally we count on the gusts to accelerate us more quickly than the bigger boats and thus to overcome some of their advantages (taller rigs reaching into better wind, longer hulls having higher hull speeds). So, ending up in each gust with sails flapping was a bit of a until we figured out what was happening.

[TIM WEBB]

GL, how close were you to obstacles ashore (trees, buildings, any tall objects that disturb airflow) when this was happening to you? A good rule of thumb to observe is that turbulence can be expected downwind for 10 times the height of the object. Also, the turbulence can be expected to roll in a clockwise direction when viewed from the side with the wind coming from left to right - in other words from up to down as it rolls along. This might explain some of the "symptoms" you experienced yesterday?

[GreenLake]

Tim, first off, this is the "usual" lake, so I've had years of sailing experience in all sorts conditions on it. Yet I don't recall seeing this before. It also was effective anywhere on the lake that day (I took notice of it in different places).

Second, turbulence: my understanding is that the air flow above about 6 knots is naturally turbulent. That is, even over open water, the air isn't flowing smoothly, but tumbling over itself in (what I imagine) a fractal soup of eddies. As a result, the wind speed over the water increases rapidly at first and then more slowly. In contrast to laminar flow below about 5 knots, where each "layer" of air moves just a bit faster than the one below it. In that case, there's an almost linear increase in speed from zero at the water to faster speeds as you go up the mast. (That's why you want twist in your sail when there's little wind - as you go up, the boat speed no longer dominates, but the wind speed does, in defining the apparent wind. As you go up, the AW seems to come from further behind, and therefore the angle of incidence should be wider, which you can match by letting the top of your sail twist off. With faster air, there isn't that great of a speed differential).

Third, the middle of the lake should be far enough from shore to be out of the 10x zone for any buildings, and largely even for the hills (which on the side the wind was coming from are a bit inland from shore with moderate slope, and partially parallel to the wind as opposed to across it). But, let's assume we did have a wind-roll (faster wind rolling forward and down after an obstacle). The size of the gust pattern on the water was fairly wide, as opposed to "spotty". That would mean, such rolls would have to have been wide (coordinated instead of fragmented).

Now, with wind rolling down, it has to go somewhere, and for a broad feature like this, the way out is "forward" (more localized ones can have a pronounced "fan" where the air escapes to all sides). So, for some reason the increased air flow was consistently shifted to one side.

The effect was that the gusts slowed us down, instead of speeding us up. The cause for this might be in the constantly rotating wind direction. Too bad I didn't sail for another hour (the wind rotated back - mirror image on the trace) but I'm glad I didn't because it was also building to where it wasn't fun. However, I lost the opportunity to see whether the effect reversed itself.

[TIM WEBB]

Ahhh ... if we could only "see" the effects of wind turbulence as we can see the effects of water turbulence!

Actually, we can: I once had the opportunity to observe some wind tunnel testing of SRB recovery parachutes at NASA. Very interesting! Kinda like sailing X 10K ... ;-P

[GreenLake]

I have a place near the water on a hillside that quite often is in lee of the prevailing winds. We can see the "fan" of the spreading gusts on windy days. I've observed the same thing on vacation ones, from a restaurant on the top of a bluff looking down on the water. Again, with the wind coming over the hills. The effect is most pronounced in the area close to the hill.

From the water, it's a bit trickier to see the gust fanning out. But it is possible to see the gusts coming, and having crew that can spot them and call them to my attention in time helps a lot, both in not being overwhelmed by the gusts as well as getting the most out of them.

[TIM WEBB]

Yup: that fanning out you are seeing is the downward moving air hitting the surface and spreading out. On land, it manifests itself as leaves being blown around, etc. Turbulence is annoying to a sailor, but it's pretty troublesome for any flying thing (airplane, parachute, etc.) that is trying to land ... <8~O

[jeadstx]

I think it was in 2010, I saw a video of trial runs with one of the America's Cup boats and the chase boats were using a type of LiDAR (as I recall) to map how the wind patterns affected the wing sail. The device would graphically display the wind patterns so they could make adjustments. Very interesting device and way to expensive for our use.

John

[K.C. Walker]

Yes, it's always disappointing when you can't upset the big boys!

[jeadstx]

The big boys just can afford nicer toys to tune their boats.

John

[Interim]

Greenlake--

I'm not sure I get the issue. It sounds like you think these were downdrafts that should fan out, and that it would be unusual to always be on the same side of them. But what if you were getting directional shifts with gusts? I get these all the time on our lake (gusts that have a tendency to head (or lift) us consistently.

In those cases, it seems to happen when a low is moving nearby so the direction of the wind is rotating, and gusty.

--john

[GreenLake]

John,

I think what you are describing is the pattern I was seeing. Now, from my perspective, that situation has been (for whatever reasons) so rare that I never consciously noticed it before. Our summer weather tends to be stable and dominated by local thermals or the sea breeze, which accounts perhaps for some of the rarity. (And sea breezes are said to oscillate less as well). And yes, we were sailing in a short window between fronts - perhaps not atypical for winter sailing, but the timing was perhaps just fortuitous to observe the effect.

In addition, our courses tend to not line up that well with the prevailing winds, so one tack is always the longer one. If each gust had provided a consistent header on that tack, I'm not sure I would have noticed it, because a bit of a lift is expected due to the velocity change. This time we were on the "correct" side of a passing low, then, and close enough to notice the effect.

Learned something new.

[Interim]

It is interesting to me what "normal" is in different areas. I don't know where you are, but I'm in the Great Plains and early summer (May and June) tend to be wildly unstable. Fronts come by the Rockies and go roaring through, so if the weather is bad in the morning it could well be dead calm in the afternoon.

Local knowledge is powerful, isn't it? I'd be lost in coastal waters.

--john

[GreenLake]

You mentioned the instabilities in terms of larger weather systems. But there are other effects, such as whether you have warm air over cold water or the reverse. That can affect how turbulent the air is under otherwise comparable conditions.

Or, if you have interesting topography (hills), you can get all sorts of fun effects. There's a local headland here that for some reason seems to reliably cause stronger winds in its vicinity. It's not rare to see ripples on the water around it when everywhere else is dead calm.

Or thermals. Do you get local thermals, where you are?