bourdidn

Main sheeted to windward

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Hi,
Looking at videos of the Prada cup, I noticed multiples instances of the boats sailing with the boom sheeted to windward (see attached pic of American Magic for instance). Why? Does the twist and small sheeting angle require that the bottom of the sail be over-sheeted or is there something else I don't understand about main trimming on these boats?

Blaise

1892202521_ScreenShot2021-01-15at9_15_58PM.thumb.png.694336233b204a59e799d01cd4f4ab52.png 

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There may be other reasons, but if the boat has significant leeway, it may look like the main is over-trimmed, but in reality the foot of the sail could still be parallel with the direction of movement.

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21 minutes ago, jsn7821 said:

They were wing-on-wing in this shot

Both main and jib are on a starboard tack. That does not exactly look like wing-on-wing to me. Plus I seem to recall seing this happen on the upwind leg.

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4 minutes ago, erdb said:

There may be other reasons, but if the boat has significant leeway, it may look like the main is over-trimmed, but in reality the foot of the sail could still be parallel with the direction of movement.

Very good point. I noticed a lot of crabbing in the aerial shots.

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I saw them pull the traveler up to windward right after they lifted the foil after a tack.  almost like pumping after a tack on a board.  I wondered if maybe they could also pump a little more sometimes in the lighter puffs.

the cats also pulled the wing past center a lot, so it must be  good for some things.

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5 hours ago, bourdidn said:

Hi,
Looking at videos of the Prada cup, I noticed multiples instances of the boats sailing with the boom sheeted to windward (see attached pic of American Magic for instance). Why? Does the twist and small sheeting angle require that the bottom of the sail be over-sheeted or is there something else I don't understand about main trimming on these boats?

Blaise

1892202521_ScreenShot2021-01-15at9_15_58PM.thumb.png.694336233b204a59e799d01cd4f4ab52.png 

Upper twist

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Partly twist, partly they oversheet to stop the hull from touching down after turns, in headers and short lulls. Then as soon as they have recovered from that, drop it down the traveller for more speed. 

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Main sheet to windward, IMHO it is the consequence of ... Downwash .

More Twist to minimize lift at the top in order to bring down the CoE will lead to a a lift distribution with more downwash at the bottom, consequently, if you want your bottom part of the sail to be put at full use, you need to bring windward the boom, in order to compensate for downwash and maintain the required AoA of the sail, at the bottom.

Cheers

 

PS: a bit disappointed by American Magic, was expecting a good fight from them, hope they will "recover" soon.

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These boats work with apparent wind, so with 8 kts of real wind they get to 30 kts of speed. Having main sheet to windward is not true,  if you see the complete profile of the sail you can see the uper leech open. This loss of flow downstream favors the upper roach of the mainsail.

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Every boat I have ever sailed on brings the traveler to weather of center line in light air. This is not AC75 specific, but especially in these boats you are starved for power not optimizing the lift/drag ratio.

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The AC75 is not like a keeled monohull, once upon foils it has virtually no increase in righting moment as the boat heels over. It is more like a sailboard or high-performance skiff. The main sheet hand’s primary job is to keep the mast as close to vertical as he can. If this means over sheeting it in a lull then that’s what he is required to do. Over sheeting will slow the boat and it is up to the helmsman to bear away, increase apparent wind angle and the main hand will then ease the traveler down its track.

There can be a situation where the main has more twist than optimum and the boom is to windward in order to get the best out of the upper part of the sail. This needs to be corrected by increased leech tension by sheeting In, accompanied by easing the traveler.

The former action needs to be immediate the latter can be done in slower time. 

https://www.facebook.com/groups/2397006860595933

 

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Pretty standard stuff... When you need a lot of twist in little  wind it's pretty normal to bring the boom above centerline. Rule of thumb I was taught as a kid was looking at the bottom batten for reference  instead of the boom.

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This is not 'oversheeted' in any way. There is attached flow on both sides of the lowest part of the main, and indeed the entire rig.

To generate maximum side force and accordingly lift, the half height mid leech needs to be relatively close to the cl, with overall main twist around 10 degrees.

In conventional keelboats (e.g Farr 40 or TP52) for upwind optimum in say 8 TWS, the boom is above cl sufficiently that the 1/4 height main leech is on the cl.

Because of downwash from the jib and the vertical velocity gradient, the lower part of the main is operating in onset flow that is close to parallel to the cl.

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13 hours ago, Frogman56 said:

vertical velocity gradient

Thanks Frogman56, for your expertise.

Is the Vertical Velocity Gradient significant ? 

How is it possible to mesure it?

Cheers

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I believe that some of these AC boats are measuring wind speed at deck level and at the masthead, but on normal keelboat there is only masthead measurement.

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On 1/17/2021 at 2:27 PM, Frogman56 said:

I believe that some of these AC boats are measuring wind speed at deck level and at the masthead, but on normal keelboat there is only masthead measurement.

All of them have a second wind sensor at the tip of the bowsprit.

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On 1/17/2021 at 11:45 AM, Erwankerauzen said:

Thanks Frogman56, for your expertise.

Is the Vertical Velocity Gradient significant ? 

How is it possible to mesure it?

Cheers

It is definitely  significant, as is the fact that because of Coreolis the wind higher up is therefore blowing at a different angle. Thus you need different twist on different tacks

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The velocity gradient is way more significant than Coriolis.

Velocity change from 1m height to (say) 25 m around 20%, quite a few factors in play, e.g. fetch, land roughness, water roughness, relative temps of water and air....

Will see if I can dig out the graphs from Alan Watts book.

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Thks Enigmatically, long time ago Tornado crew training for Olympics explained me they accounted for Coriolis after each tack.

But this discussion about gradient, is a bit my fault, I just misunderstood the adjective "Vertical"  in Frogman56 post.

While it is the correct wording, I just guess wrongly  it was another gradient than the gradient along the span (vertical).

I used the famous log law for my EXCEL spreadsheets  to account for this gradient, and I "validated" it by comparison with 

the graph available in HPS by Franck Betwaithe.

So far, just between 0 and 3 feet from the surface, we can observe differences between the HPS Graph and the Log law calculations.

As a result, if you need it for foiling boats, the Log law is perfect.

 

Cheers

 

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On 1/17/2021 at 6:11 AM, Frogman56 said:

This is not 'oversheeted' in any way. There is attached flow on both sides of the lowest part of the main, and indeed the entire rig.

To generate maximum side force and accordingly lift, the half height mid leech needs to be relatively close to the cl, with overall main twist around 10 degrees.

In conventional keelboats (e.g Farr 40 or TP52) for upwind optimum in say 8 TWS, the boom is above cl sufficiently that the 1/4 height main leech is on the cl.

Because of downwash from the jib and the vertical velocity gradient, the lower part of the main is operating in onset flow that is close to parallel to the cl.

Interesting concepts..

My understanding of lift is that you are not necessarily after generating maximum side force, but generating maximum lift in a forward direction. Most side force is parasitic in the form of heeling moment and only a small vector acts in the desired forward direction. The forward direction portion of the lift vector is generated by the degree to which the Chord Line is rotated off the Centerline, which is why a broad reach is faster than close hauled as the forward vector component is far larger even though the total lift is roughly the same.

As lift acts perpendicularly to the lifting surface at any one point on a lifting surface, then the sum of the lift on a surface sheeted beyond Centerline could result in the resolution of forces for that portion of the sail "lifting you backwards"... 

I had assumed that although this was parasitic that the benefit of having a more efficient shape in the majority of the sail beyond the 1st few feet above deck outweighed the losses so was overall more efficient especially with the wind gradient plus the slowing of the wind near the deck due to drag on the deck form reduces the contribution (positive or negative) of the lowest part of the Main Sail.

Not disagreeing with you, as you seem to know your stuff, just keen to make a distinction when I think I know how something is working but find I may be wrong..

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Is the difference of opinion that you are talking about lift in different respects. These boats need significant lateral force to provide the lift for the windward foil. Especially in light air they will do whatever is needed to keep foiling.

The idea of the boom to windward is not a new one to me, but I was amused to note that on Day 2 they were sheeting more often and further to windward when going downwind than upwind. It makes sense when you think about it because the AWS is lower going downwind in those conditions so they need to accentuate the lateral force more to keep foiling. But it does seem odd vs the more conventional Archimedean thinking

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Not having the knowledge at the level you guys are discussing I have always looked at placing the leech where I needed it and the boom ends-up wherever, mostly above centerline at a certain point. I first trimmed a main this way when on a 1D35 with the big (at that time) roach from the round head. Reading your descriptions seems to validate in scientific terms what I was doing by the seat of the pants.

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2 minutes ago, ~HHN92~ said:

Not having the knowledge at the level you guys are discussing I have always looked at placing the leech where I needed it and the boom ends-up wherever, mostly above centerline at a certain point. I first trimmed a main this way when on a 1D35 with the big (at that time) roach from the round head. Reading your descriptions seems to validate in scientific terms what I was doing by the seat of the pants.

But to position the leech, you cannot fixate on one part of the leech, so on traditional boats you need to adjust separately using the main & vang and the traveller. Though in these boats who knows what other controls they have or need!

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Just now, enigmatically2 said:

But to position the leech, you cannot fixate on one part of the leech, so on traditional boats you need to adjust separately using the main & vang and the traveller. Though in these boats who knows what other controls they have or need!

Yes, absolutely. My remark is simplified, but as you state it takes all of these to get it right.

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Also bear in mind that the boat's not travelling in the direction it's pointing in (because leeway), so a force perpendicular to the fore-and-aft axis of the boat still has a forward component in the direction of travel. Draw a line through the mast in the direction of travel, and the boom's probably still to leeward of that line.

Now, you *could* toe the foils on these boats in so that the hull doesn't make any leeway at all at a certain speed, but as far as I can observe they seem to make leeway just conventional boats. I suspect that toeing the foils in would make the whole configuration very draggy when they're both in the water, and once the hull's out of the water who cares if it's going sideways?

Completely off-the-wall thought: If the hull's travelling sideways it'll be pointed into wind from the perspective of the direction of travel; could that actually make it more aerodynamic?

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4 hours ago, Dave S said:

Completely off-the-wall thought: If the hull's travelling sideways it'll be pointed into wind from the perspective of the direction of travel; could that actually make it more aerodynamic?

The hull will be pointed closer to apparent wind due to leeway, resulting less windage, nut the angle is still large eough to propably cause stalling for airflow around hull. 

But the hull is not pointed into either true or apparent wind.

And it's nowhere near as close as longitudinal structure (appearing like a hull) of sailrocket 2.

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On 1/19/2021 at 11:32 PM, enigmatically2 said:

It is definitely  significant, as is the fact that because of Coreolis the wind higher up is therefore blowing at a different angle. Thus you need different twist on different tacks

Nothing is ever caused by Coriolis. All such explanations are fake news, unfortunately even some educational books might still contain such wrong info.

Off course Coriolis effect exists, but that has nothing to do with causality, nor does it ever explain anything, it's just a name given for false observations caused by accelerating frame of reference. Coriolis force is just a mathematical concept used in engineering, or apparent/pseudo force in physics.

All phenomenon observed in atmosphere studied by meteorology (including wind gradient) can and should be explained in inertial frame of reference, and thus without any coriolis force or coriolis effect, because that allows understanding causal effects as they exist in reality, unlike using coriolis related approach.

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9 hours ago, Erwankerauzen said:

Thks Enigmatically, long time ago Tornado crew training for Olympics explained me they accounted for Coriolis after each tack.

 

Olympic tornado crew can be excellent sailors, while understanding pretty much nothing about meteorology or causal effects related to wind gradients. That is not uncommon among sailors. They can think they account for coriolis and do the correct things to sailtrim after each tack, while the reason for those chances have absolutely nothing to do with coriolis.

 

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On 1/17/2021 at 12:11 AM, Frogman56 said:

This is not 'oversheeted' in any way. There is attached flow on both sides of the lowest part of the main, and indeed the entire rig.

 

To generate maximum side force and accordingly lift, the half height mid leech needs to be relatively close to the cl, with overall main twist around 10 degrees.

In conventional keelboats (e.g Farr 40 or TP52) for upwind optimum in say 8 TWS, the boom is above cl sufficiently that the 1/4 height main leech is on the cl.

Because of downwash from the jib and the vertical velocity gradient, the lower part of the main is operating in onset flow that is close to parallel to the cl.

That depends on how each of us is defining what oversheeted means. It seems it is not the same definition for all sailors.

 

There is no reason to generate maximum sideforce from the sails during normal sailing in any boat. A catamaran about to capsize sideways might require max sideforce in the righting direction from the sails to prevent that event if the wind direction and sail setting allows for that. It might be possible for rigid wing sail cats, like F50 used in sailGP by letting one line used as traveller go and tightening the other one that was loose. Might even work from 80 degrees of heel when wind is up.

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Or we understand it enough to use short terms rather than have to use the full scientific explanation every time

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6 hours ago, waterboy42 said:

Interesting concepts..

My understanding of lift is that you are not necessarily after generating maximum side force, but generating maximum lift in a forward direction. Most side force is parasitic in the form of heeling moment and only a small vector acts in the desired forward direction.

I don't think crews are after max component of lift in forward direction while foiling, at take off perhaps. Below take off speeds they might not have enough righting moment to allow for max lift, and certainly not after take off when up to speed in most wind conditions.

Quote

The forward direction portion of the lift vector is generated by the degree to which the Chord Line is rotated off the Centerline, which is why a broad reach is faster than close hauled as the forward vector component is far larger even though the total lift is roughly the same.

The forward direction portion of the lift vector depends on magnitude of lift vector and apparent wind angle, not on the direction of chord line, which is irrelevant on its own, but does have an effect on how large the lift vector is by affecting angle of attack.

Quote

As lift acts perpendicularly to the lifting surface at any one point on a lifting surface, then the sum of the lift on a surface sheeted beyond Centerline could result in the resolution of forces for that portion of the sail "lifting you backwards"... 

Lift does not act perpendicular to lifting surface, but perpendicular by free stream by definition. A local air pressure force acts perpendicular to surface, but the resultant of all differential air pressure based forces is not dependent on direction of chordline, because magnitude of those differential pressure forces varies very significantly along chord line.

Backwards need to be defined as well. Direction against motion of the boat matters, but direction aft along the centerline might not.

 

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6 hours ago, enigmatically2 said:

Is the difference of opinion that you are talking about lift in different respects. These boats need significant lateral force to provide the lift for the windward foil. Especially in light air they will do whatever is needed to keep foiling.

The idea of the boom to windward is not a new one to me, but I was amused to note that on Day 2 they were sheeting more often and further to windward when going downwind than upwind. It makes sense when you think about it because the AWS is lower going downwind in those conditions so they need to accentuate the lateral force more to keep foiling. But it does seem odd vs the more conventional Archimedean thinking

If the windward foil is above water (as it is during most sailing in foiling mode), it does not generate significant lifting force. If it is in the water during maneuvers, it does need to provide lift upwards, not sideways, and that does not require any sideforce from sails to happen. Whenever there is sideforce and both foils are in the water, any sideways lift generated by the windward foil produces far more drag that sideways lift from the leeward foil, because low pressure side of the windward foil is opposite sides for upwards and horizontal lift, resulting a lot of induced drag compared to what the leeward foil would produce. That is just one of the significant factors slowing the boat down during maneuvers, as sideways force for the windward foil can not be prevented in that case.

 

Not sure what you mean by: 

Quote

accentuate the lateral force more to keep foiling.

Quote

 

They were sheeting more often and further to windward when going downwind than upwind. It makes sense when you think about it because the AWS is lower going downwind in those conditions, so they need greater angle of attack for the sail plan going down wind to produce as much lift and drive as the righting moment of the boat allows for.


 

That sentence would make more sense edited that way. Lateral force from sails is not useful for foiling.

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6 hours ago, Dave S said:

Also bear in mind that the boat's not travelling in the direction it's pointing in (because leeway), so a force perpendicular to the fore-and-aft axis of the boat still has a forward component in the direction of travel. Draw a line through the mast in the direction of travel, and the boom's probably still to leeward of that line.

Very good point about direction of travel due to leeway.

 

It might not, and it doesn't have to be leeward of such line.

Lets compare direction of boom to chord line of airplane wing. In the latter case upwards force is the component required, yet it's common for chordline to be directed so that a force component perpendicular to chord is pointed both upwards and aft. Similarly the boom direction or chordline of mainsail could be directed so that perpendicular vector to it has a component backwards as well for very same reasons as in case of aircraft. It's the direction of total aerodynamic force and it's components that matter, not direction of chordline or component of force perpendicular to it.

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36 minutes ago, enigmatically2 said:

Or we understand it enough to use short terms rather than have to use the full scientific explanation every time

What are you refering to by word it above?

It would have helped if you had quoted something.

 

It looks rather obvious that people posting here understand it very differently and in conflicting way, thus you should not make such statement on behalf of the unknown group of we, just for yourself.

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48 minutes ago, NotSoFast said:

Very good point about direction of travel due to leeway.

 

It might not, and it doesn't have to be leeward of such line.

Lets compare direction of boom to chord line of airplane wing. In the latter case upwards force is the component required, yet it's common for chordline to be directed so that a force component perpendicular to chord is pointed both upwards and aft. Similarly the boom direction or chordline of mainsail could be directed so that perpendicular vector to it has a component backwards as well for very same reasons as in case of aircraft. It's the direction of total aerodynamic force and it's components that matter, not direction of chordline or component of force perpendicular to it.

I always struggle with the plane wing comparison because it has an external power source for its forward propulsion, whereas a sail has to generate its forward propulsion.. so a plane can have a resultant lift vector that resolves with a slightly rearward component provided the vast majority of the lift is upward as the power source can override it. I could still sail to windward with the main right up the track and sheeted hard above CL if I have the engine on with some decent throttle, but turn the engine off and I'll stall.

A glider has to run slightly nose down to generate an angle of attack for forward motion and to prevent stall and has a limited flying time in parallel airflow over the earth surface as that angle of attack means it is slowly dropping (leeway) to create that forward motion. It has to find an updraft via a thermal or a hillside facing the wind direction to change the apparent wind angle so it can gain height.

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1 hour ago, NotSoFast said:

Lets compare direction of boom to chord line of airplane wing. In the latter case upwards force is the component required, yet it's common for chordline to be directed so that a force component perpendicular to chord is pointed both upwards and aft.

Yes. An aerofoil chord is a geometric construct, it doesn't have any physical properties and is really irrelevant for discussing forces.

1 hour ago, NotSoFast said:

Similarly the boom direction or chordline of mainsail could be directed so that perpendicular vector to it has a component backwards as well for very same reasons as in case of aircraft. It's the direction of total aerodynamic force and it's components that matter, not direction of chordline or component of force perpendicular to it.

Yes.

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OK, but if the Coriolis effect is not that serious for sailing, what about the water spinning when the bath tub get empty?

Theoricians suggest it turns in opposite direction whether you get a bath in Melbourne or in Paris!!  well honestly I couldn't check!

 

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1 minute ago, Erwankerauzen said:

OK, but if the Coriolis effect is not that serious for sailing, what about the water spinning when the bath tub get empty?

Theoricians suggest it turns in opposite direction whether you get a bath in Melbourne or in Paris!!  well honestly I couldn't check!

 

Water always goes down the drain clockwise, but clocks go the other way in the Southern Hemisphere.

 

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8 hours ago, waterboy42 said:

A glider has to run slightly nose down to generate an angle of attack for forward motion and to prevent stall and has a limited flying time in parallel airflow over the earth surface as that angle of attack means it is slowly dropping (leeway) to create that forward motion. It has to find an updraft via a thermal or a hillside facing the wind direction to change the apparent wind angle so it can gain height.

OK. The glider's an easier example to pick because it has no engines, but the physics and terminology are the same. Here's a pretty standard diagram showing the important terms:

images4r.jpg

Key points:

  • Lift is (by definition) perpendicular to what sailors call the apparent wind (relative wind in the diagram)
  • Drag is (by definition) in the same direction as the apparent wind. This will be a combination of parasitic drag (various friction-like effects) and induced drag (primarily caused by things like tip vortices, which is why long thin foils and end plates are good)
  • The chord line is not very important other than to the aircraft's designer; so is the angle of incidence (angle between chord line of wing and fuselage). In sailing terms, this means the direction the boat's pointing in and the angle of the boom relative to that are only of secondary importance

Thinking about it, I was wrong when I suggested using the direction of the boat's travel as a reference; from an aerodynamic perspective it's more correct (and easier) to reference everything to the apparent wind direction.

Unlike an aircraft we also have a set of foils in the water. It seems reasonable to consider the vertical and horizontal foils separately. Here the direction of travel is important, because that's what defines the "relative wind" for the underwater foils.

As for Coriolis - this just isn't a factor at the scales we're talking about. Water spirals down the plughole in either direction regardless of which hemisphere you're in, and the direction will be down to currents in the water at the time you pulled the plug, asymmetries in the plughole etc. It's very easy to reversee the direction of spiralling while the bath is draining, and anyone with access to a bath or kitchen sink can try this for themselves. It does affect weather systems, but they're much bigger and continue rotating for days. In perfect conditions Coriolis force could generate enough wind shear over the height of a mast to be measurable, but in practice, just like the plughole, that will be overwhelmed by local conditions. Anyone who's sailed in a narrow river with high banks will have seen this on an extreme scale.

Wind gradient can be a factor, and anyone who's landed a small aircraft or especially a glider on a windy day will be very familar with this; it's not uncommon for the wind speed to be 20 knots greater, as little as 300 feet above the ground. In general the wind direction at 300 feet is not noticeably different to ground level. What this means is that the top of a boat's mast will often be in more breeze than the bottom (but generally from the same direction). When the boat starts moving the apparent wind comes ahead, but to a lesser extent at the top of the mast because the true wind strength is greater (if you're not sure, just draw a couple of vector diagrams). This is one of the many reasons we sail with twist.

 

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10 hours ago, NotSoFast said:

If the windward foil is above water (as it is during most sailing in foiling mode), it does not generate significant lifting force. If it is in the water during maneuvers, it does need to provide lift upwards, not sideways, and that does not require any sideforce from sails to happen. Whenever there is sideforce and both foils are in the water, any sideways lift generated by the windward foil produces far more drag that sideways lift from the leeward foil, because low pressure side of the windward foil is opposite sides for upwards and horizontal lift, resulting a lot of induced drag compared to what the leeward foil would produce. That is just one of the significant factors slowing the boat down during maneuvers, as sideways force for the windward foil can not be prevented in that case.

You are missing the point, because the windward foil is not providing lift, other forces must work to ensure it doesn't drop in the water. You are absolutely correct that I used lift in my post in the English rather than scientific/engineering sense of the word in that these forces must lift the foil out of the water. 

But if you would prefer, if we consider the yacht in a rotational frame of reference looking forward. Then for the windward foil to exit the water the heeling moment (HM) must exceed the righting moment (RM)

Then RM = Mcg where M = Mass, c = distance from the centre of gravity to centre of effort of the leeward foil, g is gravity

HM = (fm(w, A, s)+fj(w, s)) cos(A) +WF.B where fm(w, A, S) is the function giving the force of the sails given the wind speed w, sail controls s and angle of the force from the  mainsail to centreline A. Fj is the corresponding function providing the force of the jib. Both Fm and Fj are monotonically increasing with w. WF is the hydrodynamic lift provided by the windward foil and B is the distance between the centre of effort of the two foils.  

So for the windward foil to exit the water, we must make make HM>RM, and as we do so WF ->0

All other things being equal the easiest way is to increase A towards pi/2 radians and thus Cos (A) to approach 1. Note that this also increases fm as well in a far more complex way by happy coincidence

Which is why you see all the boats bring the main up when they try and lift out.

Similarly if s decreases once they are flying and are trying to keep RM=HM , then fm and fj decrease so again the quickest way to bring back the equilibrium is to increase  fm by the same manner

Or as everyone else can understand it, pull the main up the traveller, the boat heels more and the windward foil lifts out of the water.

 

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11 minutes ago, turtle recall said:

we discussed this years ago with the big monos...

yep, though a quick search failed to turn up anything , just it's in the archives ...

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we need more pictures...

 

image.png.2cf2e29541661a0ddd60eb3352a94c07.pngimage.thumb.png.be9df41c2b47830dbee0efb30bdbdfa5.png

image.png.97d21471e91dca97486ccd7ed6d7d174.png

image.png

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damn you beat me...

 

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On 1/17/2021 at 9:11 AM, Frogman56 said:

This is not 'oversheeted' in any way. There is attached flow on both sides of the lowest part of the main, and indeed the entire rig.

To generate maximum side force and accordingly lift, the half height mid leech needs to be relatively close to the cl, with overall main twist around 10 degrees.

In conventional keelboats (e.g Farr 40 or TP52) for upwind optimum in say 8 TWS, the boom is above cl sufficiently that the 1/4 height main leech is on the cl.

Because of downwash from the jib and the vertical velocity gradient, the lower part of the main is operating in onset flow that is close to parallel to the cl.

In my book over-sheeting is when the sail is steered in past the point of maximum forward thrust, not anywhere near the point of flow separation.  BTW Coriolis at NZ latitude at a wind speed of 40 knots equates to a  radius of turn of approximately 12 kilometres. Wind gradient is an issue, I see NZ has two masthead anemometers space laterally and slightly aft and one on the bowsprit.

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On 1/21/2021 at 1:47 AM, waterboy42 said:

I always struggle with the plane wing comparison because it has an external power source for its forward propulsion, whereas a sail has to generate its forward propulsion.. so a plane can have a resultant lift vector that resolves with a slightly rearward component provided the vast majority of the lift is upward as the power source can override it. I could still sail to windward with the main right up the track and sheeted hard above CL if I have the engine on with some decent throttle, but turn the engine off and I'll stall.

A glider has to run slightly nose down to generate an angle of attack for forward motion and to prevent stall and has a limited flying time in parallel airflow over the earth surface as that angle of attack means it is slowly dropping (leeway) to create that forward motion. 

The point is that the direction of sum of lift + drag vectors is not directly related to chord line orientation. Pulling the main further in from chordline at centerline position can result higher drive force regardless of the fact pressureforce near trailing edge would be backwards, because near the leading edge increase of pressureforce having a forward component can more than compensate for it. It's more likely to happen when aspect ratio is large and both apparent wind angle and angle of attack are small to begin with.

Quote

It has to find an updraft via a thermal or a hillside facing the wind direction to change the apparent wind angle so it can gain height.

Not necessarily, if there is a significant difference in windspeed due to altitude (vertical wind gradient), see dynamic soaring, where updraft is not needed at all to stay airborne.

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On 1/22/2021 at 1:32 AM, alanjs said:

In my book over-sheeting is when the sail is steered in past the point of maximum forward thrust, ...

Point of maximum forward thrust for the sail in question, or all the sailplan?

Sheeting main in just so much the driving force of the mainsail reduces, typically increases driving force of the jib more so, and thus does drive of all the sailplan.

There are far too many definitions for oversheeting as there are so many sailors discussing the subject, and most of them don't even care about definitions anyway.

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On 1/21/2021 at 3:32 PM, alanjs said:

In my book over-sheeting is when the sail is steered in past the point of maximum forward thrust, not anywhere near the point of flow separation. ...

I would say over-sheeting is when the performance of the boat decreases.  That may be past the point of maximum thrust from the sail rig.

I learned about this when I was playing with a simple landyacht VPP.  I had arranged the parameters so best aerodynamic L/D would occur below stall, and I thought trimming for best L/D would give the best performance.  That turned out not to be the case.  The yacht went faster when trimmed for maximum lift, which was definitely in the zone where the drag was increasing faster than the extra lift was worth it.

It turned out the reason for the performance improvement for additional sheeting was the extra lift loaded up the wheels.  A landyacht tire acts very much like a foil.  A tire has a leeway angle when loaded sideways because of flexibility of the tire, and there is a linear range where the side force is proportional to the leeway angle.  Then the tire starts skidding and you get a plateau or loss of side force and an increase in drag, just like aerodynamic stall.  But the side force from the tires is not arbitrary.  It has to match the side force applied by the sail rig.  LIke a hull, if the yacht is sailing dead downwind, the L/D of the hull/tires is zero because there is drag but no side force is being called for from the hull/tires.  As you apply aerodynamic side force, the keel/board/tires load up and the L/D improves.

It was the increased L/D from the chassis operating with increased side force that accounted for the improved performance.  The drag angle of the chassis was decreasing faster than the aerodynamic drag angle was increasing as I changed the trim from best aerodynamic L/D to max aerodynamic lift.

Nothing on a sailboat can be optimized in isolation.  Everything is so interconnected that you have to look at the total performance to see what the best solution is.  That's why a VPP is so central to the design and to creating targets for the crew.

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A very important issue that dominates keeping an AC75 up on foils is that the lateral heeling moment of the sail must, at all times, balance the righting moment. The righting moment changes very little as a function of heel angle. Hence the mainsail trimmers job is to provide that constant heeling moment and if that means over sheeting the sail so be it. Once the sail is over sheeted it is up to the helmsman to bear off a little to increase sail pressure

 

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On 1/17/2021 at 3:59 AM, PabloFoil said:

These boats work with apparent wind, so with 8 kts of real wind they get to 30 kts of speed. Having main sheet to windward is not true,  if you see the complete profile of the sail you can see the uper leech open. This loss of flow downstream favors the upper roach of the mainsail.

If you crunch the numbers, 30kts boats speed in 10 knots of wind at say 45 degrees true, apparent wind angle is down around 11 degrees. 

Interestingly AWA is about the same upwind and down as the boats go much faster downwind so you notice the traveller go down as they bear off around a windward mark then once the boat accelerates a few seconds later it comes back up to where it was more or less.

Upwind  
   
TWS 10
Boatspeed 30
TWA 45
   
AWS 37.7
AWA 10.8

Downwind  
   
TWS 10
Boatspeed 40
TWA 145
   
AWS 32.3
AWA 10.2

 

 

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On 1/21/2021 at 1:20 AM, Dave S said:

OK. The glider's an easier example to pick because it has no engines, but the physics and terminology are the same. Here's a pretty standard diagram showing the important terms:

images4r.jpg

Key points:

  • Lift is (by definition) perpendicular to what sailors call the apparent wind (relative wind in the diagram)
  • Drag is (by definition) in the same direction as the apparent wind. This will be a combination of parasitic drag (various friction-like effects) and induced drag (primarily caused by things like tip vortices, which is why long thin foils and end plates are good)
  • The chord line is not very important other than to the aircraft's designer; so is the angle of incidence (angle between chord line of wing and fuselage). In sailing terms, this means the direction the boat's pointing in and the angle of the boom relative to that are only of secondary importance

Thinking about it, I was wrong when I suggested using the direction of the boat's travel as a reference; from an aerodynamic perspective it's more correct (and easier) to reference everything to the apparent wind direction.

Unlike an aircraft we also have a set of foils in the water. It seems reasonable to consider the vertical and horizontal foils separately. Here the direction of travel is important, because that's what defines the "relative wind" for the underwater foils.

As for Coriolis - this just isn't a factor at the scales we're talking about. Water spirals down the plughole in either direction regardless of which hemisphere you're in, and the direction will be down to currents in the water at the time you pulled the plug, asymmetries in the plughole etc. It's very easy to reversee the direction of spiralling while the bath is draining, and anyone with access to a bath or kitchen sink can try this for themselves. It does affect weather systems, but they're much bigger and continue rotating for days. In perfect conditions Coriolis force could generate enough wind shear over the height of a mast to be measurable, but in practice, just like the plughole, that will be overwhelmed by local conditions. Anyone who's sailed in a narrow river with high banks will have seen this on an extreme scale.

Wind gradient can be a factor, and anyone who's landed a small aircraft or especially a glider on a windy day will be very familar with this; it's not uncommon for the wind speed to be 20 knots greater, as little as 300 feet above the ground. In general the wind direction at 300 feet is not noticeably different to ground level. What this means is that the top of a boat's mast will often be in more breeze than the bottom (but generally from the same direction). When the boat starts moving the apparent wind comes ahead, but to a lesser extent at the top of the mast because the true wind strength is greater (if you're not sure, just draw a couple of vector diagrams). This is one of the many reasons we sail with twist.

 

Would you add a flap to that?  I remember reading something to the effect that in the system of main + jib, the main can act as a flap, which changes the flow and resultant lift around the system as a whole.  Which meant, IIRR,  a flap can be sticking “up” into the accelerated flow (or what’s left of it) and add to lift.   Although that description may be completely out of circulation :lol:  now.  Of course a blade jib might be a flap too - at what point do these things change nomenclature? Or in a Quantum Way, each sail is a flap and not a flap at the same time. :) ?  I’d like to see the mathematics describing that.....

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9 hours ago, Amati said:

Would you add a flap to that?  I remember reading something to the effect that in the system of main + jib, the main can act as a flap, which changes the flow and resultant lift around the system as a whole.  Which meant, IIRR,  a flap can be sticking “up” into the accelerated flow (or what’s left of it) and add to lift.   Although that description may be completely out of circulation :lol:  now.  Of course a blade jib might be a flap too - at what point do these things change nomenclature? Or in a Quantum Way, each sail is a flap and not a flap at the same time. :) ?  I’d like to see the mathematics describing that.....

I wouldn't get too worried about what is a flap and what isn't.  It's simple enough to say that both act as a flap, either leading or trailing edge, depending on trim and wind conditions.  And sometimes you might say that together they create a single wing, in effect.

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11 minutes ago, Grrr... said:

I wouldn't get too worried about what is a flap and what isn't.  It's simple enough to say that both act as a flap, either leading or trailing edge, depending on trim and wind conditions.  And sometimes you might say that together they create a single wing, in effect.

^ This. Think of a flap as a device that allows you to change the shape of the wing to get the optimum lift/drag characteristics for the speed you're flying (or, occasionally, add extra drag). Whether you're in a simple glider with no flaps or an airliner with all sorts of flaps, slots etc, the different elements are treated as a single wing. Translated into sailing, at least for single-masted boats, that means your entire rig is effectively a single wing; you change its shape with the sail controls, and its angle of attack with the rudder.

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On 1/16/2021 at 11:22 AM, Erwankerauzen said:

Main sheet to windward, IMHO it is the consequence of ... Downwash .

More Twist to minimize lift at the top in order to bring down the CoE will lead to a a lift distribution with more downwash at the bottom, consequently, if you want your bottom part of the sail to be put at full use, you need to bring windward the boom, in order to compensate for downwash and maintain the required AoA of the sail, at the bottom.

Cheers

 

PS: a bit disappointed by American Magic, was expecting a good fight from them, hope they will "recover" soon.

Upwash shurely ?

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This might be interesting from the perspective of the original question about pulling the main to windward? 

 

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