Monday, April 23, 2007

Gradients, Tacking Angles and Coriolis

Four of my distinguished and erudite readers have already commented on the question raised by my Dutch correspondent about the Coriolis effect and its part in sailing tactics. They all were sceptical about the suggestions in the email I quoted.

They may be right. But just ponder these questions before you dismiss the subject...

1. Can wind direction at the top of a mast be different from the wind direction at water level?

2. If so, and you measure wind direction with a mast-head instrument and boat direction with a deck-level compass, then won't the angles between the close-hauled course and measured wind direction (tacking angle) be different on port and starboard tacks?

3. What causes the difference in wind direction between the top of the mast and water level? Isn't it the shifting of the high altitude wind by the Coriolis effect? Isn't it always to the right in the northern hemisphere?

Just asking. I haven't a clue about this issue as it really doesn't come into play on a Laser. But if your mast is 292 ft high... what then?


Ant said...

The answer is very definately yes.. the wind at the mast-head can often be different to that at water/boom/jib level.

I've found this particularly so in the Enterprise around the Midlands circuit where you sail on some damn small and shifty ponds... when the wind is coming over trees, flats, mountains or whatever you can definately have quite a different breeze from the burgee on the masthead to the main body of the sail. I've routinely experienced differences in wind-angle of upto 30 degrees

JB said...

No No No No No

Wind shear is common and caused by the non zero viscosity of the air.

the atmospheric Coriliois effect is a fake force introdced so that when treating the earth as stationary (rather than rotating) amoshpeic models arent total nonsense.

I cant think of any sensible way it affects whats happeneing to a sail

Carol Anne said...

Sure, there can be different winds at the top of the mast and at the surface of the water -- I experience this condition just about every time I sail. The wind indicator at the top of the mast will often show very different winds from the telltales on the shrouds.

But this wind gradient is NOT caused by the Coriolis effect. It is caused partly by terrain (there's a lot of it where I sail) and partly by changes in the wind that happen first higher up and then work their way downward to the surface as the higher winds gradually bring the surface winds up to speed.

Fred said...

Yes there is a difference also out on the sea. Might be less friction up higher. You might have noticed that you can sail into an odd looking glassy spot but still have some pressure. Even with a short rigged Laser.

This Coriolis effect will be something we can have endless discussion like the one with lee bowing a tide or current. Does it lift you or not. There is definetely a better feel, lee bowing a current, but is there an advantage?

Fred said...

uuuhps, just found this on the Valencia Blog:

Put the blame on gradient wind
During the last few days we have been constantly hearing about "gradient winds" and how they influence the development of the seabreeze. What are gradient winds? Why do they kill the developing seabreeze. Here is the answer from Marta Weores, former Hungarian Olympic sailor and meteorology strategist for Shosholoza.

The gradient wind is the horizontal wind in the upper atmosphere that moves parallel to curved isobars. It results from a balance between pressure gradient force (i.e. difference between pressures), influenced by the Coriolis force and the centripetal force.

The gradient wind can either help or hinder seabreeze development depending on its strength and direction. We are having light gradient winds these days which are OK for the seabreeze. Depending on the orientation of the shoreline near any race course area, the direction of the gradient wind will influence the seabreeze development.

In the Valencia race area, the NE gradient wind is the worst for seabreeze development, and unfortunately that is what we have. This is one of the reasons why seabreeze is struggling to develop.

Tim said...

It is quite simple. There will be a different wind strength high up because the resistance caused by friction between the moving air and the water surface.
This means that as the boat moves across the wind there will be a different apparent wind direction and strength at different hights. The wind will be more on the beam the higher up the sail you go.

The relatively short hight of most masts probaly means that anyh of theother effects cause are likely to be so small as to be unimportant.

The Enterprise dinghy (the boat I sail) has a jib which affects the wind flow around the lower 2 3rds of the main sail so the effects of wind differences are more pronounced than they might be on a laser rig.

The technique with an Enterprise rig is too set the kicker tension on the main to give suffiecent twist to keep the leach tell tails flying. When sailing close hauled and you need to point high you pull on enough kicker to tighten the leach except when punching your way through waves where you loosen the kicker a little to increase the fullness and increase the power.

Cunningham tension also effects the leach, the more tention opens up the leach as the draught in the sail moves foward towards the mast.
The Enterprise rig doen't tend to need much cunningham, normally just enough to remover the worst creases from the clue caused by kicker tension bending the mast.

Litoralis said...

1. Yes.

2. Yes.

3. Boundary layer effect (frictional effect on velocity and direction varying by height. Wind shear/gradient (environmental and terrain effect of direction and velocity).

milo said...

The biggest thing is the wind gradient. The molecule of air touching the water surface molecule has zero velocity due to friction. As you go away from the surface the velocity increases as the friction is lost. Dramatically just above the surface and less so up the mast.

Consider a sailboat in 15 knots true wind speed at the masthead, sailing at 8 knots 40 degrees off the true wind. At the mast head, the apparent wind is 21.75 knots at 26.3 degrees. If the wind at boom level is only 10 knots, the apparent wind at the bottom of the main is 16.93 knots at 22.3 degrees. The apparent wind is more beamy at the bottom.
If the main is trimmed so that the end of the top batten is parallel to the boom, the bottom of the main curving back to the boom is actually at a closer angle of attack than the top. This is good because the bottom needs 26.3-22.3=4 degrees more angle of attack.

Tim said...

The thing that strikes me most about Milo's sums is that the effect of the sail at the top of the mast is sigificant due to the higher wind speed with potential to develop a lot of power.
Thus it is important to watch that top leach telltail; get it wrong and you are throwing away boat speed!

Carol Anne said...

Tim and Milo have it right ... What's up at the top of the mast is important.

Yair said...

the known fact is that the wind increases with height which leads to different apprent wind speed and direction at different heights of the sail.
to adjust the sail angle of attack and camber we use thetwist to flaten the sail and "open" it at the top.
my question is: with the change in "true" wind speed what is the change in "true" wind direction and how can it be calculated or does anyone have any measurements of it over sea?

clayton shaffer said...

My coach was talking to me about this today. The direction is usually slightly clocked at the masthead however, lasers have vangs and triangular sails so there is no way to have two different sail shapes like we try to do on f18s.

Post a Comment