What happens to the wind as it slips and twirls over your sail?

[jssailem]

I saw this Facebook video, and it was one of the simplest discussions of what is happening and how you might trim your sail to improve your boat performance.

83K views · 3.2K reactions | Your textbook got lift wrong. The air on top doesn’t “wait” for the air below. The classic story: air splits at the front of a wing, both halves must meet at the back, so the curved side has to go faster. They don’t meet. Smoke released in one row shows the air on the curved side reaching the back edge first, well ahead of the other half. And on a sail that story can’t even work: it’s a single layer of cloth, both sides are the same length. The curved, downwind side really is faster. Only the reason was wrong. Here’s the part that matters on the water: that fast air is fragile. 1. Sheet in too tight and it peels off the downwind side, just behind the luff 2. The downwind telltale twirls instead of streaming 3. Fix it: ease the sail out or head up, and the flow comes back 4. Upwind telltale lifting instead? The sail is too loose, or you’re too close to the wind Both telltales streaming = a sail that’s actually working. Save this for your next sail and send it to the friend who still trims “by feel”. ⛵ Get your international skipper licence: theory online from anywhere, year-round. Sailing practice in Barcelona, Athens, Mallorca or the Canaries. DM us or tap the link in bio. #sailing #sailssearch #sailingschool #barcelonasailing #yachting | SailsSearch

Your textbook got lift wrong. The air on top doesn’t “wait” for the air below. The classic story: air splits at the front of a wing, both halves must meet at the back, so the curved side has to go...

www.facebook.com

 

[kappykaplan]

Interesting. Tks for sharing your find.

 

[shemandr]

This is a good graphical representation of how to trim a sail. I tell my inexperienced crews to move the sail towards the tell tail that isn't streaming but that doesn't always connect. Forget talking about laminar flow. I'm not a Facebook member so I don't know if I can send it along.

 

[jssailem]

I’m not sure, but it is worth a try.
The reference to Arvel Gentry and his work on the science of sails in the 70’s was a motivation to post the information. Not a big fan of Facebook either @shemandr , but occasionally there are worthwhile ideas shared.

 

[sailme88]

More difficult than trimming is finding telltales that last a season ..... The flat tape type develop a memory fold and balk at streaming while the yarn type tend to shred and become 20 or 30 individual threads...

 

[Will Gilmore]

I've never uses telltales. I look at them occasionally, out of interest, but mostly I've sailed by luff shape, testing their angles to the AW angle periodically. Since I'm not a racer, the fraction of a knot I might pickup hasn't been a big concern. Mostly, I find the way the slot affects both the jib and the main one of the more fascinating ideas.

And yes, the idea that the two streams of wind across the windward and leeward sides of the sail must meet again at the same time is a generalization based in thought more than in observation. The diversion of air at the luff helps create a low pressure across the leeward side of the sail which is the driving forced for increasing airflow. There's no "entanglement" of air molecules (can you tell I've been talking to AI about Quantum Theory lately).

-Will

 

[Joe]

I don't race much any more, but I'm never going to not use tell tales. You can spend thousands on electronic and mechanical wind devices, but they will never outperform a set of yarn tell tales. Btw, please explain why not being a "racer" has would justify not using tell tales. That makes no sense at all. Tell tales are actually way more valuable to less experienced sailors since it gives them a visual indication of the interaction between wind and sail. For that matter, you could say that watching the speedo is something a racer is more likely to monitor than the telltales.

 

[Will Gilmore]

Btw, please explain why not being a "racer" has would justify not using tell tales. That makes no sense at all.

I'm not opposed to tell tales. I didn't have them on my opti, we didn't have them on our family schooner, and I never got in the habit of using them on my hobie cat. I have a minimal set on my Mariner. I just am not in the habit, and I don't have the motivation to change the way I sail. I have never used an electrical instrument. Mostly, I look around to feel the wind direction on my face, and check where the sail trim starts to luff, and I trim it in until it stops luffing.

What I notice sometimes is that when the jib appears to be drawing well, but the main is luffing, instead of trimming the main in, I first ease the jib. That usually improves performance more than over-trimming both by sheeting the main in.

-Will

 

[jssailem]

finding telltales that last a season

The strips of nylon used on my North Sails have been in service for 2 years. Granted, they are wrapped up in the stored sails when not in use.

I have always looked at the telltale like the playing card I placed on my bicycle to flap as the spokes hit it. Never lasted more than a month or so. Meant to be replaced. I think that is why there are 52 cards in a deck.

Yes, they were a tool when I raced. Now, while I cruise, they help me find the trim setting I desire, but no longer chase as when racing.

The scientific information developed by Gentry resolves many of the theories sailors claimed by observation, regarding the air movement over and under the sails. His research influenced the current design of sails. North Sails credits Gentry with the application of computer technology in the design of their sails.

 

[shemandr]

As long as we are talking air flow over the sail I think the notion of air on the windward and leeward side of the sail arrive at the leech at different times has been debunked. I think. I've never been able to wrap my head around the notion that two streams of air separated by a millimeter of sail material travel at different rates. A sail is different than an airplane wing which is asymmetrical in shape. I think the airplane wing is more curved on the top surface in comparison to the lower surface. So the air flow around the wing does take different paths and you could say that Bernoulli's law would apply. But that's not the cast for a sail and yet they behave in a similar way. Can anyone complete the circle for me?

 

[Will Gilmore]

Without getting into the actual equation, Bernoulli states that the volume of a fluid entering a system, must equal the volume of fluid exiting the system when measured over time. In other words, it's a statement about the conservation of energy. For a pipe with various constrictions or expanses, this is so at any point you choose to make your measurements. Everywhere along the path, an equal volume over time will be measured. Hence the notion that both sides of a wing result in flow rates that end with the same air exiting as entered the system at the same time.

For a sail, the pipe isn't a closed system. While the faster air across the top of the foil, or the leeward curve of the sail will act very much in accordance with Bernoulli's laws, there is the air around the sail that can interact where the pipe example remains isolated.

If a low pressure area is created on the lee side of a sail, and consequently a high pressure region on the windward side in relationship, air beyond the separating structure of the sail will also be drawn towards the low pressure.

If the air on the windward side of the sail is a high pressure, and that high pressure is higher then the surrounding air, it makes sense that the air in the high pressure area will race towards the nearest low pressure areas. That might mean the windward air could divert up or down, as well as towards the leach, horizontal to the entrance point of the divided airflow. The tell tales usual show air flow moving smoothly in a horizontal direction, but sometimes those ribbons point up, down, or swirl around. The same can be said for the leeward side of the sail only with regards to is low pressure.

So, while the divided air has no connection to the other half of the airflow, the law of fluid dynamics suggests the same air will be reunited at the leach, if there is good laminar flow.

-Will