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      Abbreviated rules   07/28/2017

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dohertpk

Sail twist simplified

49 posts in this topic

Hi all,

After five odd years of sailing, I have yet to find a readily intelligible explanation of sail twist. I understand that air flow is 'freer' further up the mast, but that's about it. It would be great if someone could give me a really simple answer to the following questions:
 
1) How does sail twist relate to power? That is, should one twist the sail more as the wind increases, or decrease twist?
2) How do the sail controls affect sail twist? Does pulling on the cunningham and the kicker induce more twist in the sail or do these controls reduce twist?
3) I understand catamarans are different. How do the mainsheet, traveller and downhaul affect twist on cats?
 
For reference I sail an RS600 and an F18 cat. Many thanks for any insight.
 
Peter

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2 hours ago, dohertpk said:

Hi all,

After five odd years of sailing, I have yet to find a readily intelligible explanation of sail twist. I understand that air flow is 'freer' further up the mast, but that's about it. It would be great if someone could give me a really simple answer to the following questions:
 
1) How does sail twist relate to power? That is, should one twist the sail more as the wind increases, or decrease twist?
2) How do the sail controls affect sail twist? Does pulling on the cunningham and the kicker induce more twist in the sail or do these controls reduce twist?
3) I understand catamarans are different. How do the mainsheet, traveller and downhaul affect twist on cats?
 
For reference I sail an RS600 and an F18 cat. Many thanks for any insight.
 
Peter

I think if you sail alongside someone and adjust your sails to go slower or faster ( one control adjustment at a time) against them, you will see twist in action in different windspeeds, that no really simple chat can do.

High Performance & Higher Performance Sailing (Frank Bethwaite) has eazy accurate explaination.

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That's a cool link and the relevant pages describe Vertical Velocity gradient.

Essentially, the air at 40 feet is flowing faster than the air at 10 feet.   As your boat moves across this flow, the apparent wind speed increases in velocity and engages your mast at a different angle as you move up the mast.

If your boat is fast, say reaching at 15 knots in 15 knots of wind, the differential is less, and less twist is needed.   So us multis generally like less twist in moderate conditions.

Do I have that right guys?

 

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Agree that the Bethwaite books have illustrations that show the wind direction and strength gradient as the wind travels up the mast. 

Think of it this way, the wind velocity at the bottom of the mast is say 5 kts coming from 30 degrees off centerline.  At the top of a 30 foot mast the wind is perhaps as much as 9 kts coming from 50 degrees off centerline.  This disparity in wind strength and velocity is because wind is a fluid and at the surface of the earth it forms a boundary layer.  You can see this in any river, the water current is slower at the shore and gets stronger as you get further away from shore.

Your task as a sail trimmer is to try to match the twist of the sail to the boundary layer twist so that you are maximizing the flow over the sail (so each part of the sail is "looking" in the right direction).  Some twist is built into the sail by the sailmaker.  Your traveler and mainsheet are your primary tools for controlling/influencing twist. 

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11 minutes ago, paxfish said:

That's a cool link and the relevant pages describe Vertical Velocity gradient.

Essentially, the air at 40 feet is flowing faster than the air at 10 feet.   As your boat moves across this flow, the apparent wind speed increases in velocity and engages your mast at a different angle as you move up the mast.

If your boat is fast, say reaching at 15 knots in 15 knots of wind, the differential is less, and less twist is needed.   So us multis generally like less twist in moderate conditions.

Do I have that right guys?

 

Ok so a Optimist with a 12 foot mast needs no twist?

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Peter,

  This is what I know from my F18 and A-Cat sailing:

1) More twist = less power, generally speaking. It is however possible to not have enough twist on in light air and end up with a leech that is hooked to weather, which can be draggy. The posts above describe why this is.

2) Pulling on the downhaul opens up the leech, generally Cunningham on a dinghy does the same. It also tends to move draft forward. Vang on a dinghy controls the upper 1/3rd of the sail but it can also limit the boom travel. It basically acts like a mainsheet at times controlling the leech. On a cat, the mainsheet controls the leech (less = open leech, tighter = tighter leech). Tighter leech = more power, generally speaking. Traveler is a gross mainsheet control in a sense. It primarily controls the lower 1/3rd of the sail but it also basically changes the entire angle of attack of the sail.

3) Rotation. This is a critical control on a catamaran and affects how the downhaul and mainsheet control the sail.

For your F18 sailing, this is a very very good starting point: http://www.microwindracing.com/boat.html

 

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20 minutes ago, Legion of Modernrate Jack said:

Ok so a Optimist with a 12 foot mast needs no twist?

Same rules apply....

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34 minutes ago, paxfish said:

That's a cool link and the relevant pages describe Vertical Velocity gradient.

Essentially, the air at 40 feet is flowing faster than the air at 10 feet.   As your boat moves across this flow, the apparent wind speed increases in velocity and engages your mast at a different angle as you move up the mast.

If your boat is fast, say reaching at 15 knots in 15 knots of wind, the differential is less, and less twist is needed.   So us multis generally like less twist in moderate conditions.

Do I have that right guys?

 

Likely dangerous to generalize, but yea, I find the same Pax.  As a hack my general rule and observation is that the faster the boat and shorter the mast the less twist I am using to get the flow I want over the sail top to bottom.  In addition to less twist as breeze and speed come up, I am likely stating the obvious to you but as breeze continue to build and we are getting overpowered I will add twist back in to depower up high.

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8 minutes ago, Wess said:

Likely dangerous to generalize, but yea, I find the same Pax.  Faster the boat and shorter the mast the less twist I am using to get the flow I want over the sail top to bottom.  in addition to less twist as breeze and speed come up, I am likely stating the obvious to you but as breeze continue to build and we are getting overpowered I will add twist back in to depower up high.

Without getting too complicated now consider the best foil shape, as above when the breeze comes up and more twist to the main the max draft will move back = more downhaul to get that foil shape back ( less heel and being dragged sideways as a reward)

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Yea, agree,  and would just add that at least with mine we are usually at max cunningham and outhaul already (ie bladed out as much as possible) when we start adding twist back in to depower.  This is a pretty big generalization on my part however as there are exceptions to this for sure...

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Twist is most important when water is glassy since the wind is definitely laminar flow (what there is of it) and only the top of your sails is actually working.  When you start to get ripples on the water the wind has turned turbulent (otherwise there wouldn't be ripples) and so your entire sail can be used for power.  You need less twist in turbulent wind at least until you want to depower.

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Posted (edited)

 

nevermind

Edited by NYBOZO1
nevermind.....

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2 hours ago, NYBOZO1 said:

nevermind

Okay, we won't ;)

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3 hours ago, NYBOZO1 said:

 

nevermind

Helpfull leading edge twist post removed by poster! (purple font) :)

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Unfortunately, twist cannot really be simplified.

You start with the fact that to account for wind sheer, you twist the sails so that each part of the sail is presented to the wind at the same and optimum angle. That angle changes depending on a number of factors but is primarily in response top the apparent wind angle at any given height above the water. As it is apparent wind, the need for twist changes with point of sailing and with the speed of the boat relative to windspeed. Boats that sail at or faster than wind speed need a very different twist profile to ones that are slower. At the micro level, there can be a difference between tacks because of the direction the earth spins.

That's pretty straight forward because you can use leach tell tales to see how the wind is coming off the back of the sail. Get all the tell tales flying at the same angle off the leach relative to the final run of the back of the sail and your basic settings are right. You can adjust this through 3 controls, mainsheet, vang and downhaul/cunningham (samc99us - surely they are the same thing just called differently depending on your background). Each has secondary effects, so you need to know which to pull and this is where it gets confusing. Simply, until you are fully powered up, use the mainsheet. Once fully powered, it is a combination depending on how you want to bend the mast and move the draught around the sail. On most dinghies, I think the order of control of twist is mainsheet, vang and then downhaul. On a cat, there is no vang so it can only be mainsheet and downhaul, (plus maybe some mast rotation?)

To add complication, twist is also a major factor in how you can power or depower the rig. Adding twist lowers the centre of effort of the rig and "spills" power. Up to a point, it reduces drag. Reducing twist increases power, raises the CoE but can increase drag. Less twist generally allows you to point higher up until you are over powered, but closing the leach too soon in lighter winds kills all speed.

The only way of learning is doing. First learn how each control changes the sail on your boat. First look at it on land. Then do it on the water. Note that you aren't just looking at twist but the interplay between twist, mast bend, position of draft and much more. Then you go out and sail against other boats. If you can find a buddy, you can both try altering something at different times to see the effect. Otherwise, you play with it in a race. I will sometimes change my racing tactics so i can try different settings. If there is somebody I know is fast, I will go with them even if I think they are sailing the wrong way, just so I can learn. Better to lose a few places today but to pick up some lifelong skills which will improve your racing in every race in the future.

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Something simple that works in those tricky glassy conditions, is if you are slow or being passed, just let everything off & start again... most times you may even gain more than you lost, even while still dialing in the correct trim!

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A mainsail telltale 2/3 the way up the sail and halfway between leach and luff, both sides, will indicate flow. Keep the leeward yarn flowing aft. I was surprised at how much twist is sometimes needed. I watch that indicator almost as much as the jib yarns

Dave Ellis  Now on big Lake Guntersville, Alabama!

 

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A few figures  for an A-Cat:

1-The Floater case (13/14 knts TWS)

Windward, sailing 13knts @45° of TWA, the apparent wind total "twist" will be in the 7° to 8° range

Downwind sailing 17knts @115° of TWA the apparent wind total twist is around 17°

 

2-The Foiler case (13/14 knts TWS)

Windward foiling 20knts @ 48° of TWA the apparent wind total twist will be around  5°

Downwind foiling 26knts @ 115° of TWA the apparent wind total twist will be around 10°

So if you foil you have to pull the mainsheet harder, everything else equal.

Incidentaly, if you have a small teardrop mast it will manage the big twist for the floater case without any problem, while a large chord wing mast would not.

But if you are a foiler, the apparent wind twist is smaller and a large wingmast would not be penalized, while the big mast chord would improve  the windward separation behind the mast and decrease the section drag accordingly. With these apparent wind velocities, low aero drag is king.

Good Week-end 

 

 

 

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3 hours ago, Erwankerauzen said:

A few figures  for an A-Cat:

1-The Floater case (13/14 knts TWS)

Windward, sailing 13knts @45° of TWA, the apparent wind total "twist" will be in the 7° to 8° range

Downwind sailing 17knts @115° of TWA the apparent wind total twist is around 17°

 

2-The Foiler case (13/14 knts TWS)

Windward foiling 20knts @ 48° of TWA the apparent wind total twist will be around  5°

Downwind foiling 26knts @ 115° of TWA the apparent wind total twist will be around 10°

So if you foil you have to pull the mainsheet harder, everything else equal.

Incidentaly, if you have a small teardrop mast it will manage the big twist for the floater case without any problem, while a large chord wing mast would not.

But if you are a foiler, the apparent wind twist is smaller and a large wingmast would not be penalized, while the big mast chord would improve  the windward separation behind the mast and decrease the section drag accordingly. With these apparent wind velocities, low aero drag is king.

Good Week-end 

 

 

 

What assumptions did you make about the velocity gradient in the Earth's boundary layer. In other words, what was the difference in true wind speed & over what distance?

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I extrapolate the wind gradient as plublished by Franck Bethwaithe in his book "High Perfprmance Sailing".

But I have used also the Log formula which is a classic proxy used by engineers.

I guess you can find it easily with Google

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I am not sure for the Floater Downwind the true wind angle was actually 115°, probably 125° or 135° but I can t find the xls spreadsheet.

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14 minutes ago, Erwankerauzen said:

I extrapolate the wind gradient as plublished by Franck Bethwaithe in his book "High Perfprmance Sailing".

But I have used also the Log formula which is a classic proxy used by engineers.

I guess you can find it easily with Google

But, FrankB also says you can ignore gradient with windspeeds above 6 kts true wind because the wind is turbulent at that speed and the gradient can be ignored for most mast heights.  Here's an excerpt from Wikipedia which purports to synopsize his observations " According to one source,[38] (this is Bethwaite High Performance Sailing 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 Meteorology[41] according to which differences can be as little as 5% in unstable air.[42]"

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MultiThom, agree, that is what I understand from Bethwaithe Book: no appreciable velocity gradient in winds above 6 kt (and by the way, no change in wind direction with height either)

On the other hand, Paul Bogataj in the article I linked above, says that sails see a different aparent wind angle because: (a) velocity gradient; (b) different upwash due to the sails planform (and upwind sails). That would seem to contradict Bethwaithe...

So, according to Bethwaithe, in winds above 6 kt, twist is only a result of (b) above? I need to go read it again 

 

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What we need is an AC45 mainsail trimmer to see what he adjusts wrt twist...unless a wing sail has no upwash; confess I don't know nuthin' 'bout no wings'ls. 

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I used a proxy function to mimick and extrapolate the Frank Bethwaite graph, it was a kind of polynomial regression which fitted perfectly the gradient above 3 feet height and overestimated the TWS speed below.

When you must extract as much a possible HP from a maximum sail area, like it is the case for C-Cat and A-Cat, it is a bit different than for "Yatching" & "Champagne sailing" on big monohulls with Marconi rigs. 

Gradient , both windward and downwind, must be considered, just to check is there is something to be optimized, I have been always very disappointed by generalities heard here or there on catamarans parking, so I always make my "fact checking" and on this point, I believe that gradient worth to be considered even above 6 knts in turbulent BL.

 

 

 

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From Bethwaithe graph

With 10 knts TWS @ 18 feet height

TWS is 6 knts @ 2 feet/  7.5knts @ 4 feet 

 

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1 hour ago, Erwankerauzen said:

From Bethwaithe graph

With 10 knts TWS @ 18 feet height

TWS is 6 knts @ 2 feet/  7.5knts @ 4 feet 

 

OK, but that's non turbulent--ie, glassy seastate and laminar air flow.  Granted, you guys with tall powerful rigs probably do see some need for twist in turbulent winds that FrankB didn't...after all, the reason he wrote the books way back then was that nobody had sailboats that went very fast.  That's why I'd like to hear from an AC45 wing trimmer--they'd certainly know instead of just 'book larnin'.

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There also used to be the old rule of thumb: Keep top batten parallel with the boom for the right amount of twist, at least as a starting point.

There is some scope for varying amounts of twist whilst keeping the luff and telltales reasonably drawing. So overall balance is important. Best speed and twist, particularly upwind is not necessarily best VMG. I have sailed on plenty of Bermudan rigged boats where you oversheet the main, lose a bit of speed and get better overall upwind speed by sailing higher.

There is no substitute to sailing a lot, playing with the variables and looking at your telltales and digital instruments to see what works best for your particular boat and for each particular weather/sea state condition.

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You know he didn't actually measure the windspeeds to create those graphs, right?  When I saw that in his book I wondered why there were 4 different lines for each condition; then I figured he was just illustrating what it might look like.  We do know today, though, that there are methods to  approximate true behavior for engineering needs.  I found it interesting that wind gradient is considered in wind turbine design which leads one to believe that there is need for adjust twist for modern tall masts, but that wikipedia article I mentioned above also says there is a reduced wind gradient over water as compared to over land.  You might want to use the equation it provides and check your twist calcs under different stability conditions over water...just for fun.  https://en.wikipedia.org/wiki/Wind_gradient

It also introduces an additional term of air stability which I think we've all experienced...some days the wind is "all over the place" with vortices and eddies in 3D; some days you can pretty much count on a steady breeze.  I've never sailed as fast as you guys...at most I reached 18 kts downwind and occasionally 15 to weather.  Never considered twist and when I built mainsails or jibs for my boats, I normally did not create twist as part of the design-mostly because I didn't race much in laminar winds.  Funny though, the sails all did have twist simply because the fabric tension transmitted through the mainsheet went through the leach (My sails were not square tops) and the differences in drag through the height.

 

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Something to ponder about much of Frank`s work in the 70s was the fixed by rule sail area on the NS14 class.

Getting the most out of that 100sq foot sail area led to the over rotating wing mast & by the late 1970s fat batten assisted sail heads that dictate much of what the square top mainsail sailors are now just discovering.

http://ns14.org/classinformation#history

Unable to find historic dacron main pics online, old round profile not far away, shown on google images, the diagonal squared off obviously the more recent shapes.

 

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Something to ponder about much of Frank`s work in the 70s was the fixed by rule sail area on the NS14 class.

Getting the most out of that 100sq foot sail area led to the over rotating wing mast & by the late 1970s fat batten assisted sail heads that dictate much of what the square top mainsail sailors are now just discovering.

http://ns14.org/classinformation#history

Unable to find historic dacron main pics online, old round profile not far away, shown on google images, the diagonal squared off obviously the more recent shapes.

 

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I went ahead and calculated the % wind seen at different heights along a 40 ft (12 meter) mast using the normal air value of Hellmann's constant.  At the base of the mainsail you'll see 80% of the wind seen at 10meters and 102% at the top of the mast.  So in 10 m/s true wind (about 20 kts) at 10 meters above the boat, you see 8 m/s (16 kts) at the boom and 20+ kts at the very tip top.  Those are the TWS...you'll actually see the vector sum of boatspeed plus TWS on the angle of attack.  In stable air the differences will be more pronounced (Hellmann's constant would be 0.27 instead of 0.1).  Very unstable air would be even less at 0.06.  Which is pretty much what FrankB said, the more turbulent the breeze, the less twist you need.

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Posted (edited)

On 8/11/2017 at 3:19 AM, Erwankerauzen said:

A few figures  for an A-Cat:

1-The Floater case (13/14 knts TWS)

Windward, sailing 13knts @45° of TWA, the apparent wind total "twist" will be in the 7° to 8° range

Downwind sailing 17knts @115° of TWA the apparent wind total twist is around 17°

 

I am curious as to how the twist is derived.  Is it just vector sums? So in the windward case, the boatspeed is 13kt, the windspeed is 13kt coming from 45 TWA (FAST BOAT!).  SO the AWA is 22.5 and AWS is 24.  Assume that's top of mast.  SO bottom of mast is seeing same boatspeed, but TWS of 10.2 (80% of top of mast-assuming you believe the folks who design wind turbines) and AWA is 19.6...so twist needed is 22.5-19.6 or  2.9 degrees...did I do that right?

Edited by MultiThom
math errors

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MultiThom,

You are perfectly right, I mentionned figures from memory, and my Alzheimer mislead me:

Windward with 13knts TWS and 45° TWA with boat sailing @ 13knts

at 18 feet height AWS is 24 knts and AWA is 22.5°

at 6 feet height AWS is 21.6 knts and AWA is 19.86°

at 2 feet height AWS is 19.3 knts and AWA is 16.58°

Downwind with 13 knts TWS and 115° TWA with boat sailing @ 17 knts

at 18 feet height AWS is 16.4 knts and AWA is 45.6°

at 6 feet height AWS is 15.74 knts and AWA is 36.8°

at 2 feet height AWS is 15.42 knts and AWA is 27.3°

In the same conditions for a foiler sailing around 20 knts windward and 26 knts downwind twists are reduced by  circa: 40% .

Happy Sunday 

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I know that all this has been said better above, but I feel like saying it.

The wind higher up has less friction from the surface, so it is moving faster, so the apparent wind is bent less by the speed of the boat and is not as far forward. Higher up on the sail also has more leverage, so power up there results in more heeling moment. So reducing the angle of attack up there gets a better angle of attack on that wind and reduces drag/heeling moment more readily than down low.

Twist is induced by raising the boom. That is controlled by the vang and/or the sheet. Angle of attack is controlled by the traveler, and in the case that the vang is used, the sheet.

The Cunningham controls the depth or camber of the shape of the sail, and the place on the sail that has the deepest point, not usually the twist, by putting tension on the luff, thereby lengthening it, and so the part of the sail next to the luff is under more tension and is flatter. Reducing the depth/camber of the sail and moving the deepest point of the sail aft reduces power.

On most rigs, the mast can be bent more or less to flatten (more curved), by pulling the luff forward, or deepen (more straight) the sail. But that really only works when the point of sail is high - close hauled/tacking because that is the axis of the mast, and most masts don't rotate, so if the sail is out for reaching, putting curve in the mast does not have the same effect.

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12 hours ago, nroose said:

I know that all this has been said better above, but I feel like saying it.

The wind higher up has less friction from the surface, so it is moving faster, so the apparent wind is bent less by the speed of the boat and is not as far forward. Higher up on the sail also has more leverage, so power up there results in more heeling moment. So reducing the angle of attack up there gets a better angle of attack on that wind and reduces drag/heeling moment more readily than down low.

Twist is induced by raising the boom. That is controlled by the vang and/or the sheet. Angle of attack is controlled by the traveler, and in the case that the vang is used, the sheet.

The Cunningham controls the depth or camber of the shape of the sail, and the place on the sail that has the deepest point, not usually the twist, by putting tension on the luff, thereby lengthening it, and so the part of the sail next to the luff is under more tension and is flatter. Reducing the depth/camber of the sail and moving the deepest point of the sail aft reduces power.

On most rigs, the mast can be bent more or less to flatten (more curved), by pulling the luff forward, or deepen (more straight) the sail. But that really only works when the point of sail is high - close hauled/tacking because that is the axis of the mast, and most masts don't rotate, so if the sail is out for reaching, putting curve in the mast does not have the same effect.

Excellent summary.

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nroose , Great basic over view of control/ reaction. The basics however ignore that the wind, in general, comes from above so interaction with the surface is moot. Until we get low enough that the wind is being disturbed by water surface and prior to that the interaction of end plate, or lack there of. as we get towards the boom.

 

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As this discussion is all/mostly based on Bethwaite's HPS-2, I'll add that I recently got my hands on Fast Handling Technique, and it is very nice.

To a certain extent, it is a rehash of HPS-2, chapters 25 onwards, updated a bit to cover AC-45 catamarans.

I don't mind the redundancy; I'll be lending FHT to friends wanting to crew with me ;-) -- it is a lot more practical and approachable than HPS-2. 

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A new breeze filling in over cold water with a stable atmosphere can result in >45 degrees twist.  Saw it in Seattle NOODs a few years ago.  Cold Puget Sound water, southerly filling from the convergence zone effect, stable air with the fleet fighting to get south to the new breeze.  Stood on the cockpit seats with the boom on my shoulder and got >45 degrees twist.  Sailed right through the fleet, so of course they cancelled the race...

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1 hour ago, Kenny Dumas said:

A new breeze filling in over cold water with a stable atmosphere can result in >45 degrees twist.  Saw it in Seattle NOODs a few years ago.  Cold Puget Sound water, southerly filling from the convergence zone effect, stable air with the fleet fighting to get south to the new breeze.  Stood on the cockpit seats with the boom on my shoulder and got >45 degrees twist.  Sailed right through the fleet, so of course they cancelled the race...

Don't know I ever saw anything that extreme but I get your point.  Knowing how to control twist is good.  All the book and theoretical references (you need 17 degrees at this height) I don't as a practical matter. Heck they can't predict tomorrow's weather.

Check your tell tails.  Adjust sails accordingly.  B)

Maybe a topping lift instead of the shoulder, LOL!

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11 hours ago, Legion of Modernrate Jack said:

nroose , Great basic over view of control/ reaction. The basics however ignore that the wind, in general, comes from above so interaction with the surface is moot. Until we get low enough that the wind is being disturbed by water surface and prior to that the interaction of end plate, or lack there of. as we get towards the boom.

 

That seems true to me in conditions that don't involve chop. On SF Bay, you can have a 5 foot chop, and that causes a lot of wind shear, especially in smaller boats.

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28 minutes ago, nroose said:

That seems true to me in conditions that don't involve chop. On SF Bay, you can have a 5 foot chop, and that causes a lot of wind shear, especially in smaller boats.

How fun to set up a model that includes the rotational effect of the boat, caused by the chop, that changes according to hull length & mast height, flailing around for interaction of laminar flow V air rowing. :)

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nroose

I need to respectfully disagree with you.  The OP asked the question on both the dinghy forum and this one, and it is important to realise that there is quite a difference between how you control twist on the 2 boats he mentions. As this is a multi forum, we have answered the beach cat question and as such most of your comments don't actually apply. For instance, we do have rotating masts, don't have a vang and don't use the traveller to control angle of attack in the same way as you do with a monohull. 

On beach cats downhaul is a primary controller of twist. I am not sure about how things work on a dinghy rig, but I think having a fully battened sail also makes a significant difference. For the cats, downhaul controls the fullness of the sail and up high, as you pull on downhaul, the sail gets flatter and the leech opens. I sail upwind with the traveller on the centreline. The problem with controlling twist with the mainsheet is that without a vang, if you ease the sail to get twist, the sail gets fuller. So the downhaul becomes the power control and does so through controlling fullness and twist. Rotation is also used to control the shape of the sail and therefore the power. Depending on the mast section, it also controls the effective camber and position of max draft in the sail.

My usual depowering chain of events is to start with a little downhaul to flatten the sail, then start dialing out rotation as I pull the downhaul on more and more. I have the rotation fully in before I have max downhaul. My ideal is to keep the mainsheet hard in all the time. It begins to get interesting when you are trying to foil upwind, but that isn't what the OP is asking as he sails an F18.

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^ Nice to hear that perspective. I'd like to try cat sailing. Hopefully someday.

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Funny story about sail twist on cats. Back in the day when the Tornado was in the Olympics, the then Team GBR coach couldn't do a particular event and the legendary British coach, Jim Saltonstall, stood in. He had never watched cat racing properly and at the first evening debrief, he commented that he thought everybody needed to be more careful about letting off vang at the top mark.:D

Probably very unfair to tell that story, because he really has been more responsible for the success of Team GBR than any other individual. He ran the youth programme when all the now big names were going through and he set them up for success. Then lottery money kicked in to enable people to transition from Sunny Jim to the main squad. Many ferrets owe a lot to one of the all time greats. Best coach I have ever worked with.

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