Coriolis effect and sail twist

[BelleSerene]

If I understand it right, the breeze veers (comes from a bit further clockwise) higher above the water, because of the Coriolis effect and the effect of friction with the water.

So if I'm on a starboard tack, my sail twist would line up with this: further aloft, the sail sags further to port (leading edge shifted clockwise).

But on a port tack... surely the sail would be less efficient because the top sags further to starboard (leading edge shifted anticlockwise), which I'd therefore expect to stall as the wind should be shifted clockwise up there.

Clearly I'm wrong somewhere here, as there's no asymmetry between the two tacks, all other things being equal.

Can anyone enlighten me?

 

[kds]

"leading edge shifted anticlockwise),"

No ... the reason we allow a twist in our sails is not connected with the coriolis effect.
It is caused by the change of apparent wind on the sail at different heights.
I can't remember the figures - is is something like 30% stronger wind speed at a height of 30 ft. above water level ?
The wind speed reduces as the height above water level reduces, because of "friction" between moving air and the ground. As a result the component of the apparent wind which is caused by the real wind is greater at a greater height above the boat. ie. at the base of the sail, boat-speed plays a larger part in the resultant angle of the apparent wind, so the sail can be closer to the centre line.
This also explains why we need a much greater twist at low wind-speeds.
I hope this helps.
ken

 

[earlybird]

It's nothing to do with Coriolus, frictional drag from the sea surface slows the air and so changes the apparent wind angle low down, so it's the same on both tacks

 

[AntarcticPilot]

It's certainly not Coriolis effect - it would be unmeasurably tiny over the height of a mast.

 

[saltwater_gypsy]

Ah yes, the old Coriolis effect, one of the most misunderstood bits of relative motion.

Seminar Because of the rotation of the Earth, an object (eg.a stallite) moving freely from the north pole towards the south pole will appear to move in a curved path. In the northern hemisphere, when viewed from Earth it would appear to curve in a westerly direction. In contrast , a vehicle on the Earth's surface, driving from north to south and viewed from outer space would appear to move in a path curved to the East. This is always the problem with relative motion, it depends on the viewing point.

Weather All this is on a huge scale, but the most dramatic consequence is on the thermal winds which are generated near the Equater where vortices are generated which result in rotating tropical storms and hurricanes.
It also means that the winds around our low pressure systems move in an anticlockwise direction.
It is this latter aspect which causes "twist in the wind". As the wind approaches the sea surface there is viscous coupling to the surface of the sea which can slow the wind down and change its direction.

Punchline. Coriolis is at the root of the big picture, but the local effect is usually wind gradient and vertical change in wind direction.

 

[gardenshed]

The only apparatus on the boat that coriolis effects are
1. the sink
2. the toilet

As the other posters have said, there is velocity gradient between the surface and the masthead which impacts sail trimming but no measurable change due to coriolis.

 

[sarabande]

On the dimensions and masses with which the forumites are involved, even big mobos, Monsignor Coriolis is going to have damn all effect. Certainly sink drains and heads have many other forces acting upon them, far greater than any produced by the C.effect.

 

[bbg]

Sorry, wrong. Or perhaps I should say - correct, but not in any measurable way.
The Coriolis force is so small as to be virtually immeasurable over distances such as the diameter of a sink. The random motion of the water molecules that comes from filling the sink, or the direction the water enters the bowl when the toilet is flushed FAR outweigh the Coriolis effect.

It is equally true to say (in the boat environment) that the Coriolis effect also works on the can of beer that you toss across to your mate on the other side of the cockpit - you just don't see the effect because it is so (immeasurably) small.

 

[dom]

Main factor affecting twist is of course the AWA swing induced by wind slowing closer to the surface. This difference in windspeed is primariliy a function of height and to a lesser extent sea state. Other factors Coriolus and temperature have a theoretical but insignificant effect.

But that is not the whole story with regards to sail twist. Twist can be closed or induced to power or depower a sail. It can also be used to produce the appropriate sailshape for different sea states - e.g. fuller in rough seas and flatter in calm.

And sometimes twist is a function of spars that would be placed under undue strain were the vang applied to the excessive level required to reduce it!

 

[Evadne]

[ QUOTE ]
It is equally true to say (in the boat environment) that the Coriolis effect also works on the can of beer that you toss across to your mate on the other side of the cockpit - you just don't see the effect because it is so (immeasurably) small.

[/ QUOTE ]

Rubbish. I learned to play darts in South Africa (in the southern hemisphere), yet after a mere four pints of Speckled Hen I find myself incapable of hitting double top on a northern hemisphere dartboard, obviously because my finely honed athlete's senses are compensating for the wrong Coriolis effect on the missiles. In fact hitting the big dartboard at all can be a bit of an issue at that stage.

/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

 

[deep denial]

surely the sail twiists away at the top simply because the sheet which would hold it in is attached at the bottom. If the shee was attached at the top it would be the foot which sagged to leeward!

 

[tangomoon]

That's how you know the b****s have the engine working too

 

[saltwater_gypsy]

Quote: "surely the sail twists away at the top simply because the sheet which would hold it in is attached at the bottom"

With good racing sails you can adjust from "board flat" to "full as you like" using all the sail controls.

If you have mainsail telltales on the leech, the perfect sheeting is when they are all flying nicely.This indicates that the sail has the same apparent wind from foot to head.
In light conditions this is achieved with a surprising amount of "twist"

 

[Billjratt]

So we should just leave him to play his mandolin, and tune our sails as normal.
Nice thread though.