Don... Can you help me understand "twist" some more?

[weinie]

Ooops... I posted the wrong link... sorry..

http://en.wikipedia.org/wiki/Wind_gradient

scroll down to the part that is titled "Saililng" of course!

 

[Bill Roosa]

I've not read Don G's book but the concept is certianly to encouraged. I think that it all breaks down (for me) to two very simple to understand concepts. Look at the tell-tails, look at the boat speed. The rules: (forward tail is the one you have to look through the jib sail to see)
forward streaming, aft streaming, all is good with the world
forward lufting, aft streaming, head up (turn upwind) [forward luft turn up]
forward streaming, aft lufting, bear off (turn downwind)
for the main just keep the leach tails streaming straight out.

then it just becomes understanding how to shape the sails to get that to happen. And I really don't care why. so instead of trying to understand what that line does to the sails airodynamicaly I try to understand what that line does to the tell-tails

 

[Joe]

If the head of the sail is moving in the same direction as the foot relative to the "ground" (which of course, it must) then assuming the true wind's direction is constant from top to bottom, both the wind's apparent and true direction must be the same from top to bottom.
Of course this doesn't take into account other effects like the vortexes and turbulence described above.

Hey weinie (I just love saying that) you're thinking too much. Forget about vortexes and turbulences for a sec. This is a simple vector relationship.

True wind speed affects apparent wind DIRECTION. Increases in TRUE wind SPEED will move APPARENT wind DIRECTION AFT....... the true wind direction will stay the same, but the increase in true wind velocity, or gradient, higher up changes the apparent direction. (as long as the wind is not coming from directly ahead, or behind)

You can think of true wind as WOW ... wind over water. and the apparent wind as WOB wind over boat.

 

[red_dolphin]

... It is the apparent wind that acts on the sail, not the true wind. Take a minute to refresh your understanding of the difference between TRUE wind and APPARENT wind. i.e. a change in TRUE wind speed will move the APPARENT wind direction AFT. ....

Joe,
I'm pretty sure you meant to say, "...an increase in TRUE wind speed..." as opposed to just a change. With that minor edit, I couldn't agree more.

This is the basis for the 'up in the puffs, down in the lulls' tactic for sailing up wind.

Moreover, the difference between TRUE and APPARENT wind really is important.
On that subject I say to the OP, weinie, that taking the time to refresh your understanding of the difference between TRUE and APPARENT will pay dividends.

Bob

 

[red_dolphin]

If the head of the sail is moving in the same direction as the foot relative to the "ground" (which of course, it must) then assuming the true wind's direction is constant from top to bottom, both the wind's apparent and true direction must be the same from top to bottom.
Of course this doesn't take into account other effects like the vortexes and turbulence described above.

And this goes to remind all students of symbolic logic that you can prove anything from false premises.

The false premise, in this case, is that the true wind is the same from the peak of the sail to the foot.
It is not.
If you really want to know why not, you open a whole sack-of-snakes:
-- physics of the air/sea boundary
-- laminar flow
-- wind/sea-state interaction
The list goes on.
I hope you paid close attention in those upper-division math courses on differential equations.

Bill Roosa is right.
This stuff goes far beyond the knowledge needed to trim your sail.
Don G's guidelines are what my HS German teacher called, "Simple rules for simple people."
They may be short on physics theory, but they work.
I certainly follow them when I sail.

But you may still want to get more into the science of all this.
If so, I suggest learning the vector geometry that defines True and Apparent wind.
That will help a lot in your understanding of what the wind that you see is doing.
Moreover, learning and practicing this vector geometry could lead you toward your learning about what the USN called 'maneuvering board solutions'.
Maneuvering board problems routinely deal with collision avoidance, a handy skill to have if you are sailing near a shipping lane.
[OK, on the submarine I was on, we were trying to get closer to the target, not avoid it.
But that was an application of the same geometry with a different objective.]

A final note:
I use the Weems & Plath 'VectorMaster' [enhanced] circular slide rule plotting tool for vector geometry at sea.
Is it necessary? Of course not.
Is it fun to play with? You bet, and sometimes it really helps.

Bob

 

[weinie]

Hey weinie (I just love saying that) you're thinking too much. Forget about vortexes and turbulences for a sec. This is a simple vector relationship.

True wind speed affects apparent wind DIRECTION. Increases in TRUE wind SPEED will move APPARENT wind DIRECTION AFT....... the true wind direction will stay the same, but the increase in true wind velocity, or gradient, higher up changes the apparent direction. (as long as the wind is not coming from directly ahead, or behind)

You can think of true wind as WOW ... wind over water. and the apparent wind as WOB wind over boat.

You are right!!! I must of had a brain fart... It it me when I was driving in the car with the window open today.

 

[RichH]

Hey weinie (I just love saying that) you're thinking too much. Forget about vortexes and turbulences for a sec. This is a simple vector relationship.

True wind speed affects apparent wind DIRECTION. Increases in TRUE wind SPEED will move APPARENT wind DIRECTION AFT....... the true wind direction will stay the same, but the increase in true wind velocity, or gradient, higher up changes the apparent direction. (as long as the wind is not coming from directly ahead, or behind)

You can think of true wind as WOW ... wind over water. and the apparent wind as WOB wind over boat.

Ill state again ... true wind is never ever experienced by a boat or sails as the wind must 'get out of the way' (upwash) for a sail to correctly transfer its energy to the sail. A sail never ever ever 'sees' true wind, even a boat or sail that is NOT moving. True wind only exists 'far away' (well up stream) of any sail/wing/foil. It is only a 'reference' to compare the variability of the apparent wind and the velocity of the boat/sail.

Simple speak: adjust the trim/shape solely by what the tell tales are telling you. ;-)

 

[Joe]

Ill state again ... true wind is never ever experienced by a boat or sails as the wind must 'get out of the way' (upwash) for a sail to correctly transfer its energy to the sail. A sail never ever ever 'sees' true wind, even a boat or sail that is NOT moving. True wind only exists 'far away' (well up stream) of any sail/wing/foil. It is only a 'reference' to compare the variability of the apparent wind and the velocity of the boat/sail.

Simple speak: adjust the trim/shape solely by what the tell tales are telling you. ;-)

Okay....... whatever.... I don't think anyone, god forbid, was criticizing you in any way. Then again I haven't read your book so I had no idea that the definition of True Wind had changed. Just curious, but, how far away is far away?

 

[Joe]

Joe,
I'm pretty sure you meant to say, "...an increase in TRUE wind speed..." as opposed to just a change. With that minor edit, I couldn't agree more.

This is the basis for the 'up in the puffs, down in the lulls' tactic for sailing up wind.

Moreover, the difference between TRUE and APPARENT wind really is important.
On that subject I say to the OP, weinie, that taking the time to refresh your understanding of the difference between TRUE and APPARENT will pay dividends.

Bob

Thanks for that correction, RD.... you're right, that's what I meant.... glad you caught it, I'll go back and change it so I don't get flamed by the engineers, heh, heh.

Btw, I appreciated your comment on the importance understanding vector theory and collision avoidance..

 

[weinie]

And this goes to remind all students of symbolic logic that you can prove anything from false premises.

The false premise, in this case, is that the true wind is the same from the peak of the sail to the foot.
It is not.
If you really want to know why not, you open a whole sack-of-snakes:
-- physics of the air/sea boundary
-- laminar flow
-- wind/sea-state interaction
The list goes on.
I hope you paid close attention in those upper-division math courses on differential equations.

Bill Roosa is right.
This stuff goes far beyond the knowledge needed to trim your sail.
Don G's guidelines are what my HS German teacher called, "Simple rules for simple people."
They may be short on physics theory, but they work.
I certainly follow them when I sail.

But you may still want to get more into the science of all this.
If so, I suggest learning the vector geometry that defines True and Apparent wind.
That will help a lot in your understanding of what the wind that you see is doing.
Moreover, learning and practicing this vector geometry could lead you toward your learning about what the USN called 'maneuvering board solutions'.
Maneuvering board problems routinely deal with collision avoidance, a handy skill to have if you are sailing near a shipping lane.
[OK, on the submarine I was on, we were trying to get closer to the target, not avoid it.
But that was an application of the same geometry with a different objective.]

A final note:
I use the Weems & Plath 'VectorMaster' [enhanced] circular slide rule plotting tool for vector geometry at sea.
Is it necessary? Of course not.
Is it fun to play with? You bet, and sometimes it really helps.

Bob

In the Gene Hackman/Crimson Tide submarine vernacular:

I concur!!!

But here is where I am going with all this and please point out where I am going wrong again:

Isn't it true that if the rate at which the wind increased as you went up mast was constant, then a sail maker could build in a particular twist into a sail and one would never have to adjust it? Then the only time one would adjust the twist of one's sail was if this rate increased or decreased; i.e. the wind at the top went from being 20% greater than the wind speed at the boom to 30% greater. If the rate were relatively constant, then all we would need to do is adjust our traveler to move the angle of attack of the entire sail. Therefore, when we are adjusting our twist, aren't we are simply accounting for this change in wind speed differences at the mast head to the boom?

 

[FourPoints]

In the Gene Hackman/Crimson Tide submarine vernacular:

I concur!!!

But here is where I am going with all this and please point out where I am going wrong again:

Isn't it true that if the rate at which the wind increased as you went up mast was constant, then a sail maker could build in a particular twist into a sail and one would never have to adjust it? Then the only time one would adjust the twist of one's sail was if this rate increased or decreased; i.e. the wind at the top went from being 20% greater than the wind speed at the boom to 30% greater. If the rate were relatively constant, then all we would need to do is adjust our traveler to move the angle of attack of the entire sail. Therefore, when we are adjusting our twist, aren't we are simply accounting for this change in wind speed differences at the mast head to the boom?

The fault with this is the wind sheer is not consistent with altitude alone. As true wind speed increases, so does sheer, at 5kts of true wind the sheer will be minimal as the surface boundry does not exert much drag, as the true wind speed increases so does the degree of wind sheer. At 25kts it is quite significant.

Try hoisting a line up the mast with tell tails all along its length in different wind conditions and you will be able to visually see the sheering at work. Of course the taller your mast the easier and more pronounced it will be to see.

If a sail maker was to try to build in twist they would make a sail that is only useful for one specific wind condition, and it would be useless any other time. Most boats have the necessary controls to manage twist correctly and thus can effectively use the same sail in numerous wind conditions, always shaping the sail to the optimal shape for the given situation.

 

[Bill Roosa]

Ah! As my boss is fond of saying here is your fatal assumption. The sail maker does not build in twist. That is just a consequence of using triangular sails, no more no less. Sail makers can build in camber, take it out with a flattening reef, reduce it further for a fully reefed configuration and that is about it. Other than the actual design of the sail (lengths of edges, reef levels, cunningham or no Cunningham etc) stuff I'm not aware of anything other than camber that the sail maker can build in. I'm not a professional sail maker however so others will no doubt chime in on my ignorance.

My understanding is that sail makers build the edges so they are curved (convex from the center). When you put them on a straight mast or pull them straight with the sheet that "drives" the material into the sail and "makes it baggy". More curve on the edges yields more bagginess. He can also cut the panels of the sail proper on a "gore" (orange peel shape) that can add bagginess in or take it out of specific places. The amount of leach curve built in by the sail maker can't effect the twist. It can however affect the camber (bagginess) of the sail along the twist perhaps this is what you are describing. He could set up the sail so that the entry angle is “ideal” for some set of sheet tension and wind gradient I suppose by adjusting the location and depth of max camber. I’m sure that, while it may not be deliberate on the sail makers part, there is some ideal sheet tension and wind gradient that every sail is best suited for. This takes us down the “a sail for every wind” route however and not the “one sail to do it all and in the sunlight drive them” route which most of us ascribe to.

 

[Bill Roosa]

Course if you just set the tell-tails correctly this is accomplished automagically.

 

[weinie]

I have an automagic pilot on my need to get list!

 

[Joe]

The fault with this is the wind sheer is not consistent with altitude alone. As true wind speed increases, so does sheer, at 5kts of true wind the sheer will be minimal as the surface boundry does not exert much drag, as the true wind speed increases so does the degree of wind sheer. At 25kts it is quite significant.

For what it's worth, I found these comments regarding wind sheer or gradient in the not so authoritative Wikipedia.... albeit there are references. If you don't want to read the whole thing, I've highlighted the critical statements in the last paragraph.
Conclusion...... at higher wind speeds less twist is needed. Rely on your tell tales.

In sailing, wind gradient affects sailboats by presenting a different wind speed to the sail at different heights along the mast. The direction also varies with height, but sailors refer to this as "wind shear."[35]

The mast head instruments indication of apparent wind speed and direction is different from what the sailor sees and feels near the surface.[36][37] Sailmakers may introduce sail twist in the design of the sail, where the head of the sail is set at a different angle of attack from the foot of the sail in order to change the lift distribution with height. The effect of wind gradient can be factored into the selection of twist in the sail design, but this can be difficult to predict since the wind gradient may vary widely in different weather conditions.[37] Sailors may also adjust the trim of the sail to account for wind gradient, for example using a boom vang.[37]

According to one source,[38] the wind gradient is not significant for sailboats when the wind is over 6 knots (because a wind speed of 10 knots at the surface corresponds to 15 knots at 300 meters, so the change in speed is negligible over the height of a sailboat's mast). According to the same source, the wind increases steadily with height up to about 10 meters in 5 knot winds but less if there is less wind. That source states that in winds with average speeds of six knots or more, the change of speed with height is confined almost entirely to the one or two meters closest to the surface.[39] This is consistent with another source, which shows that the change in wind speed is very small for heights over 2 meters[40] and with a statement by the Australian Government Bureau of Meterology[41] according to which differences can be as little as 5% in unstable air.[42]

 

[RichH]

And to add more confusion -

.... the *Coriolis Effect* will cause 'wind shear' to be unequal when on differing tacks with respect to ... "the rotation of the earth".
Simpler speak: the 'right side' of the wind flow will have more 'shear' (in the northern hemisphere).

... and the wind driven wave patterns/directions will also be 'different' on each tack, requiring a different sail shape from tack to tack to optimize the set/shape, especially at the lower wind speeds. The faster the boat is traveling the less the effect of coriolis effect.

 

[red_dolphin]

.....

Isn't it true that if the rate at which the wind increased as you went up mast was constant, then a sail maker could build in a particular twist into a sail and one would never have to adjust it?

My first thought here was, "See above regarding proving things from false premises."
The rate at which the wind increases as you go up the mast is not constant.
Wind speed and sea state are major variables here.
Wind direction, wave direction and wave period are secondary factors, but still significant.
We are absolutely talking about differential equations, and not just of the first order.

My second thought was, "Weinie, do you sail much?"
If so, why not test your theories [and those of others] on the water.

But please remember,
As many posters in this forum have suggested ...
The simple solution is probably the best -- Accam's razor does apply.
Rather than get involved with wind modeling [or false assumptions] ...
why not do what RichH and others in this forum have suggested?

That is: Read your telltales and trim your sails accordingly.
Accordingly?
Well, Don Guillette's Sail Trimming Guide is a pretty good start toward 'accordingly'.

On the water, you may find variations or exceptions to Don's guide that apply to specific conditions or specific vessels.
I'm sure that all [including Don] would be interested in your posts about the variations and exceptions you encountered on the water.
I know I would.

Red Dolphin