# Why do windward tell tales stream up in the sail? Spanwise airflow at the sails....

• Jurgen M
In summary, the man is saying that the airflow at the sails is caused by the sail being tapered towards the top rather than being rectangular, and that this causes the windward tell tales to stream up. He also says that if the sail is too twisty, the airflow will be reduced at the top of the sail, causing the jib to flap around.f

#### Jurgen M

This question is about sail aerodynamics, more precisely what cause spanwise airflow at the sails?

When sailing close hauled,at the jib windward middle and top tell tales stream up or at 45 degress up, all others leeward and bottom windwards tell tales stream horizontaly.

1. What cause airflow to switch direction so windward tell tales stream up?

2. How to trim the jib to all tell tales stream horizontaly?

Is it because jib is angled to leeward so airflow has component parallel with luff instead parallel with cord line?
Just like aircraft swept wing has wingspan airflow?

I know that means your angle of attack is too low, but not why the direction is up. But to make it clear for others, this photo might help. The green string on the windward side is pointing up while the red string on the leeward side is pointing back.

I know that means your angle of attack is too low, but not why the direction is up. But to make it clear for others, this photo might help. The green string on the windward side is pointing up while the red string on the leeward side is pointing back.

8View attachment 313775
Yes but at your picture it seems like windward (green tell tale) is stalled, my tell tales is not stalled ,it is straight(have tension) just stream up...

Yes but at your picture it seems like windward (green tell tale) is stalled, my tell tales is not stalled ,it is straight(have tension) just stream up...
It flops around a bit. Watch the whole video.

Hm he tells here the boat hull redirect wind up.
Do you agree with this conclusion?
Must be some
It flops around a bit. Watch the whole video.

So why windward tell tales stream up? Where this spanwise flow comes from?

Here we have two different explantion,once tell must keep tell tales parallel and other keep it apart as much you can.
So how is right?

10, 11 min

7-10min

The sail is tapered towards the top rather than being rectangular, so as with a similar shaped aeroplane wing, the airflow leaks away towards the end of the aerofoil, giving it an upward tilt.

russ_watters
The sail is tapered towards the top rather than being rectangular, so as with a similar shaped aeroplane wing, the airflow leaks away towards the end of the aerofoil, giving it an upward tilt.
Then why does the other-side telltale not follow the same argument?

The opposite telltale is not in a laminar stream as the airflow has become detached.

When sailing close hauled,at the jib windward middle and top tell tales stream up or at 45 degress up, all others leeward and bottom windwards tell tales stream horizontaly.
Sounds like a little too much twist in the sail, in other words the middle and top of the sail are too open. The top tell-tale streaming up might be OK on your boat (it isn't on mine) but you should definitely get the middle one streaming backwards: sheet in harder or move the sheeting position (jib cleat or genoa car) backwards. You will often find the best position is when the leach (rear edge) of the sail is parallel with the centre line of the boat.

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DaveE
The sail is tapered towards the top rather than being rectangular, so as with a similar shaped aeroplane wing, the airflow leaks away towards the end of the aerofoil, giving it an upward tilt.
But you can control that flow by changing the shape of the sail using the controls.

Sounds like a little too much twist in the sail, in other words the middle and top of the sail are too open.
But why tell tales stream up,where this airflow direction comes from, who change the wind in up direction?

you should definitely get the middle one streaming backwards: sheet in harder or move the sheeting position (jib cleat or genoa car) backwards.
Why this man learn people that keep tell tales parellel is "rubbish"?
Listen at 11:16min
So he learn people to set windward tell tales all the way up...

Sounds like a little too much twist in the sail, in other words the middle and top of the sail are too open...
or move the sheeting position (jib cleat or genoa car) backwards...
If I move car back, sail will twist even more.
Wouldnt be even worse?

If I need reduce twist ,then car must move forward..

But why tell tales stream up,where this airflow direction comes from, who change the wind in up direction?
Air moves from higher pressure to lower pressure. If there is too much twist there will be less pressure at the top of the sail.

Why this man learn people that keep tell tales parellel is "rubbish"?
Listen at 11:16min
So he learn people to set windward tell tales all the way up...
I haven't watched it all: I have never sailed a boat where you can get all the windward tell-tales streaming straight up and the leeward ones streaming so I can't comment on that. At around 8.00 he stresses the importance that all 3 should be the same: on most boats I have sailed that is when they are all "dancing" - streaming straight back with the occasional flick upwards to about 45%. He also mentions the parallel with the centreline "rule".

If I move car back, sail will twist even more.
Wouldnt be even worse?

If I need reduce twist ,then car must move forward..
Yes, good catch: I will edit.

Air moves from higher pressure to lower pressure. If there is too much twist there will be less pressure at the top of the sail.
At 9:45 men tell that boat hull redirect wind in upward direction, so windward tell tales stream up..This is his explanation.

Do you agree with his "theory"?

Can you set at the jib all tell tales when sail upwind(boat is angled to leeward) to stream parallel/backwaard at your boat?

At 9:45 men tell that boat hull redirect wind in upward direction, so windward tell tales stream up..This is his explanation.

Do you agree with his "theory"?
Possibly, but I'd be surprised if this were the only, or even the main, factor.

Can you set at the jib all tell tales when sail upwind(boat is angled to leeward) to stream parallel/backwaard at your boat?
Yes I can, although note that the video is talking about dinghies which are sailed flat upwind. Even on a keelboat where you are healed to leeward all telltales should stream aft (windward telltales dancing).

Of course slightly undersheeting/too much twist in the jib is much better than oversheeting/hooking the leech: it is no good having the windward telltales streaming like they are painted on the sail and the leeward ones lifting, stalled.

Possibly, but I'd be surprised if this were the only, or even the main, factor.
If boat hull is reason for spanwise flow at the windwards side of sail, then you couldn't set telltales to stream horizontal with sheet angle whatever you do.
So his theory missing something.

I think more you angled sail to leeward the more spanwise flow you have.Becuase if you lay down sail to horizontal(flip over) at beam reach,then wind stream 100% parallel with mast,no cord line flow at all.

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Possibly, but I'd be surprised if this were the only, or even the main, factor.
Listen him at 18:35, he strongly believe in this

Listen him at 18:35, he strongly believe in this
At 18:50 he says the windward telltales should be angled up at about 45° (not straight up). I aim for mainly horizontal with the occasional flick up to 45° ("dancing") for maximum VMG and I believe that is still what is generally recommended.

At 19:10 he stresses that the most important thing is that all 3 windward telltales should be similar.

The best way to go faster is to watch people who are faster than you. Look at the medal race from the 2018 470 mens Worlds for instance at 6:20 and 25:20 for instance. Telltales are horizontal with the occasional flick up.

As to the merits of all telltales straight up, or the reason for this being hull displacement (of air), well I have a great deal of respect for Michael but I think I'd like to hear this from someone else before I change the way I sail.

Oh at 20:18 he says that the "straight up" only applies to faster boats like Fireballs. Strange, a Fireball isn't that much faster upwind, and you can't get a much lower bow profile! Let's have a look at some Fireballs upwind at 0:25:

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The best way to go faster is to watch people who are faster than you. Look at the medal race from the 2018 470 mens Worlds for instance at 6:20 and 25:20 for instance. Telltales are horizontal with the occasional flick up.

But they sail upwind upright/flat.

You don't agree that angled sail to leeward cause spanwise flow?
Imagine at beam reach ,sail completely angled to leeward(flip over) then wind blow 100% spanwise...

I think sail angle to leeward cause lots of spanwise flow...

The best way to go faster is to watch people who are faster than you. Look at the medal race from the 2018 470 mens Worlds for instance at 6:20 and 25:20 for instance. Telltales are horizontal with the occasional flick up.

But they sail upwind upright/flat.

You don't agree that angled sail to leeward cause spanwise flow?
Imagine at beam reach ,sail completely angled to leeward(flip over) then wind blow 100% spanwise...

Can you describe how boat heel angle affect airlfow at jib ?

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But they sail upwind upright/flat.
Yes, all conventional monohull dinghies are faster that way.

You don't agree that angled sail to leeward cause spanwise flow?
Imagine at beam reach ,sail completely angled to leeward(flip over) then wind blow 100% spanwise...
If you are heeled on a beam reach the boom is going to be in the water: this is not good.

Can you describe how boat heel angle affect airlfow at jib ?
At the leech? Not a lot, it is still going to be horizontal. Upwards flow will only establish further back so you will have pressure spilling out of the top of the jib and backwinding the top 1/4 of the main; the foot of the jib will be contributing very little and so the centre of effort will move higher, increasing the heeling effect.

At the leech? Not a lot, it is still going to be horizontal. Upwards flow will only establish further back so you will have pressure spilling out of the top of the jib and backwinding the top 1/4 of the main; the foot of the jib will be contributing very little and so the centre of effort will move higher, increasing the heeling effect.
You don't agree that flow at the windward side of sail will become more and more spanwise when sail in angled toward horizontal?
I think this is pure geometry,isnt it?

No, I don't agree that the mast will ever be leaning to leeward if you are sailing for maximum VMG upwind.

For optimum flow over the sail, the telltales will be horizontal, and if the mast is leaning to leeward then yes the telltales will be at the angle you indicate.

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No, I don't agree that the mast will ever be leaning to windward if you are sailing for maximum VMG upwind.

For optimum flow over the sail, the telltales will be horizontal, and if the mast is leaning to windward then yes the telltales will be at the angle you indicate.
How do you mean lean to windward?? only windsurfing sail is lean to windward, every sailingboat lean sail to leeward..

Yes, I meant leeward, corrected.

Yes, I meant leeward, corrected.
If you now apply my picture to close hauled, then you again get spanwise flow when lean mast to leeward but in very smaller amount because apparent wind comes from different angle to the jib then on beam reach...

So I think some of this upward flow "comes from" sail lean to leeward. 3D geometry stuff.

What do you mean "apply to close hauled", I repeat if you are healed on a reach then the boom is in the water. And if you are heeled close hauled in a dinghy you are not going as fast as you would be if you were upright. Let's not go round and round in circles, I think we have said everything that needs to be said now.

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I just want to say that wind don't comes at same angle to the sail if boat is flat or if boat is heel to leeward.

Windex can't show correct airflow direction when mast is heel to leeward, because airflow hit windex from below at some angle..3D geometry stuff..