sail aerodynamics

[Mikko Brummer]

In this attachment, the vertical distribution of lift & drag for the boat in the static case. From the graphs, you can distinguish at least kinks in the drag at the spreaders, the hollow in the lift at the boat deck level, the vortex under the boom, the kink at the jib head height and separation in the top of the main above forestay (or is it the tip vortex there?). Anyhow, most of the lift & drag appears to lie in the "guts" of the sails, not as concentrated into the tip vortices as you would perhaps expect.

I also want to add, that not all of this time spent on simulation was waisted, as the boats with these sails scored last weekend a double victory at the X-35 Worlds in Copenhagen ;-).

 

[SukiSolo]

Good work Mikko, nice when things work out.

I suspect that regarding pitching, the use of T foil rudders and other devices, helps smooth out the motion of the hull and allow more efficiency ie less drag, in the sails. That is for non foiling craft. Your graph (post 416) also shows nicely how sail trim change can also help through waves, if I'm interpreting it correctly.

 

[Jamie Kennedy]

In the attachment, graphs of Cl and CD for the X-35 pitching in waves more or less or not at all. The average CL and CD for both pitching cases (which also include heave/yaw/roll) is in nearly the same as for the "no motion" case represented by the straight line - L/D for no motion is 5.12, smaller pitch 5.07 and larger pich 5.03.

The sail trim for the case is for smooth water, and the motions borrowed from the Star, with hardly an optimal steering for this case. There's no rig dynamics in the simulation, the sails and rig are "stiff", which would be very much true for this type of boat in this small waves. Allowing for the rig dynamics for boats like the Star & Finn, I'm convinced that the "pitched" numbers for the sails of these boats can be better than the static ones, actually extracting energy from the waves as philSweet suggests.

This would be in contrast with all the VPPs that I know of (including my own personal VPP), which assume the sail aerodynamics deteriorate quickly in a seaway (up to 20% or so).

Also, it makes you think something similar is happening under the water, with the keel tip pitching fore & aft and rollin sideways, even more complicated by the circular motion of the water inside the waves. The "apparent water angle" and speed is ever changing.

A lot to think about there. Great stuff. Sailing never stops getting interesting.

 

[brian eiland]

Double Headsails vs Fractional & Masthead Sloops

You fellows have really presented some very interesting discussions on the mathematics and computer analysis of the airflow over our sailing rigs. Regrettable I got lost in some of these very technical discussions.

Now I would ask if some of you would care to speculate on a premise I put forward on several occasions ?,...the superiority of the flow dynamics of a double-headsailed rig where the two leading edges of the 2 sails are virtually parallel, ....verses the situation with the two leading edges of a jib and a mainsail,... in either a fractional rigged, or masthead rigged sloop configuration?

For reference I refer back to this posting I made titled,
Overlap Dynamics
"We know that the restriction presented by the ‘slot’ tends to divert more air around the two sides of the slot, i.e. the windward side of the main and the leeward side of the genoa. This higher flow rate on the lee side of the headsail increases its effectiveness. Now if we also overlap the mainsail with the trailing edge of the headsail, we further increase the effectiveness of the headsail, as it is able to carry this increased flow rate much further aft along its span than if it was to have to dump its flow at free stream velocities up at the leading edge of the mainsail. This overlap is important."

On another subject thread I posted,...

...I thought it might be interesting to revisit this portion of Paul Bogataj's paper

SAILS IN COMBINATION
Each sail by itself is much simpler than the combination of a foresail and mainsail as in the sloop rig. The sails are operating so close to each other that they both have significant interaction with the other. The most interesting feature of this is that the two sails together produce more force to pull the boat than the sum of their forces if they were each alone.

Earlier, upwash was identified as the increase in flow angle immediately upstream of a wing. There is also a corresponding change in angle, called downwash, just behind a wing, where the flow leaving the wing has been turned to an angle lower than the original flow. This is the cause of the well known “bad-air” that a boat just to windward and behind another boat experiences.

The mainsail of a sloop rig operates in the downwash of the forward sail, causing the flow angle approaching the mainsail to be significantly reduced from what it would be otherwise. This decreases the amount of force that the mainsail produces. The observed affect commonly referred to as “backwinding” is partially a result of downwash from the foresail, but is also due to the higher pressure on the windward side of the genoa being very close to the forward, leeward side of the mainsail, causing the flexible material of the mainsail to move away from that higher pressure.

The foresail of a sloop rig operates in the upwash of the mainsail. The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of force that the forward sail produces. So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail. Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity (lower pressure) and more force. The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses.

There is a converse affect to a windward boat receiving “bad air” (downwash) from a boat ahead and to leeward. A leeward boat gains additional upwash (“good-air”?) from a boat just to windward and slightly behind that acts like a lifting windshift until it moves ahead of the windward boat. This is the same phenomenon from which a foresail of a sloop rig benefits.

Another consequence of the difference in flow angles that the two sails experience in each others’ presence is that the mainsail must be trimmed to a much closer angle with the boat’s centerline than the foresail, which is able to be trimmed to a lead position well outboard. This angle represents the difference in upwash on the foresail and downwash on the mainsail due to each other.

MASTHEAD RIG.
On a masthead rig, where the forestay is attached to the top of the mast and both sails taper to basically zero chord length at their heads in a similar fashion, the interference effects of the sails on each other are similar along the entire height of the mast. The mainsail ends up being rather tightly trimmed all the way up because of the genoa’s downwash, and the genoa gains from favorable upwash all the way up.

FRACTIONAL RIG.
A fractional rig has the more complicated characteristic that the top of foresail is not as high as the top of the mainsail. This means that the top of the foresail is very close to the front of the mainsail at a height where there is still an ample amount of chord length in the mainsail. As the foresail luff approaches the mainsail luff, the upwash on the foresail due to the mainsail increases, because the low pressure behind the mainsail has more affect the closer the flow gets to it. This causes the top of the foresail to experience even more upwash and contributes to a fractional rig’s foresail being trimmed more twisted than a masthead rig’s foresail.

The top of the main on a fractional rig extends well above the foresail, leaving the upper portion of the mainsail free to experience the apparent wind without the downwash interference of the foresail. Apparent wind toward the top of the mast comes from a much higher angle, so the mainsail above the foresail experiences much higher wind angles than the lower portion of the mainsail where the genoa is causing substantial downwash. This change in flow angle with height on a mainsail is quite dramatic with a fractional rig and leads to trimming a fractional rig’s mainsail with more twist than a masthead rig’s mainsail

... a couple of flow sketches (please excuse their older rough nature). Please note I have presented the profile view of the rig design, along with the overhead view of the sail's shape at each level


DOUBLE-HEADSAIL RIG
As noted on its sketch below there is a more consistent slot between the 2 sails throughout the height of the rig, and the slot is less restrictive to processing a greater volume of airflow that occurs with increasing height off the surface of the water/deck,...unlike the more restrictive slot of the fractional or masthead sloop.

At the upper region above the hounds, there is even the possibility that the mast tube itself will act in a similar manner as the inner/trailing sail to create a 'slot effect' for that region.

A great majority of the 'most productive portion' of the sail plan area has a nice overlap feature for better dumping velocities for the genoa's leech,...a even more productive genoa

 

[markdrela]

You fellows have really presented some very interesting discussions on the mathematics and computer analysis of the airflow over our sailing rigs. Regrettable I got lost in some of these very technical discussions.

Now I would ask if some of you would care to speculate on a premise I put forward on several occasions ?,...the superiority of the flow dynamics of a double-headsailed rig where the two leading edges of the 2 sails are virtually parallel, ....verses the situation with the two leading edges of a jib and a mainsail,... in either a fractional rigged, or masthead rigged sloop configuration?

For reference I refer back to this posting I made titled,
[...]
"We know that the restriction presented by the ‘slot’ tends to divert more air around the two sides of the slot, i.e. the windward side of the main and the leeward side of the genoa. This higher flow rate on the lee side of the headsail increases its effectiveness. Now if we also overlap the mainsail with the trailing edge of the headsail, we further increase the effectiveness of the headsail, as it is able to carry this increased flow rate much further aft along its span than if it was to have to dump its flow at free stream velocities up at the leading edge of the mainsail. This overlap is important."

I don't see what the term "effectiveness" signifies here. The reality is that the overall lift of a rig is almost entirely determined by its planform and by the local incidence of the rearmost portions of the rearmost sail. In contrast, the lift is almost unaffected by:
* the incidence of the front parts of the sail (i.e. the jib or jibs)
* the size, number, or orientation of the slots, if any
* any overlap between the sheets
These can be verified by playing with Javafoil in inviscid mode. Even "reversing" a slot by putting the jib trailing edge to windward of the mainsail mast will not significantly affect the overall rig lift. An example is shown here:
www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-18.html#post192017

The primary function of slots is to better manage boundary layers, and thus allow larger rear-sheet incidence angles and hence larger lift without separation. The slot also reduces drag on the mainsail mast as a side effect. I'll repeat what I said here: www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-19.html#post192023
"
Multielement airfoils on aircraft have nearly zero overlap --- maybe 1% chord at most. It doesn't help to increase lift.
...
The point I'm trying to make is that designing a slot geometry based on inviscid lift arguments is utterly pointless. The slot should be designed by viscous flow considerations.
"

 

[brian eiland]

I don't see what the term "effectiveness" signifies here. The reality is that the overall lift of a rig is almost entirely determined by its planform and by the local incidence of the rearmost portions of the rearmost sail. In contrast, the lift is almost unaffected by:
* the incidence of the front parts of the sail (i.e. the jib or jibs)
* the size, number, or orientation of the slots, if any
* any overlap between the sheets
These can be verified by playing with Javafoil in inviscid mode. Even "reversing" a slot by putting the jib trailing edge to windward of the mainsail mast will not significantly affect the overall rig lift. An example is shown here:
www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-18.html#post192017

Are you telling me that the jib/main configurations on page 1 of that PDF and page 2 are going to be equally effective at propelling a boat forward?

Have you ever been sailing?

 

[markdrela]

Are you telling me that the jib/main configurations on page 1 of that PDF and page 2 are going to be equally effective at propelling a boat forward?
Have you ever been sailing?

Yes, I've been sailing. But I'm basing my arguments on airfoil theory. Adding the third smaller jib in your sketch 2 will not add significant lift to the existing jib/mainsail configuration, unless the mainsail is sheeted in. You can see this in the 2D panel calculations in the attached PDF. The small jib, which adds 32% to the total sailcloth area, increases the CL from 1.793 to 1.828, which is only a 2% increase. The added loading on the new jib is mostly offset by decreased loading on the existing mainsail, and also slightly on the existing larger jib. The only way to increase overall loading is to sheet in the mainsail, which was the point of my previous post.

One could argue that that the small added jib will likely allow the mainsail to be sheeted in before flow separation occurs, and thus increase the available drive. But the added jib doesn't appear to alleviate the suction peaks on the existing jib and mainsail, so I can't see how it will help in this regard. The only benefit appears to be a reduced suction peak on the mast.

Basically, adding sailcloth in the form of another jib doesn't appear to help much except to reduce mast drag, and it certainly will increase sail skin friction drag. It would be much better to add the sailcloth to the existing jib or mainsail, since here the increase in maximum lift and hence in available drive would be certain.

 

[brian eiland]

... Adding the third smaller jib in your sketch 2 will not add significant lift to the existing jib/mainsail configuration, unless the mainsail is sheeted in. You can see this in the 2D panel calculations in the attached PDF. The small jib, which adds 32% to the total sailcloth area, increases the CL from 1.793 to 1.828, which is only a 2% increase. The added loading on the new jib is mostly offset by decreased loading on the existing mainsail, and also slightly on the existing larger jib. The only way to increase overall loading is to sheet in the mainsail, which was the point of my previous post.

Perhaps we need to get on the 'same page' in our discussion here. That sketch 2 of mine perhaps confused things. There is NO mainsail in that sketch of mine,...only two headsails in front of a bare mast. I cropped this attached dwg so as to not confuse things with my added mizzen sail.

One could argue that that the small added jib will likely allow the mainsail to be sheeted in before flow separation occurs, and thus increase the available drive. But the added jib doesn't appear to alleviate the suction peaks on the existing jib and mainsail, so I can't see how it will help in this regard. The only benefit appears to be a reduced suction peak on the mast.

Basically, adding sailcloth in the form of another jib doesn't appear to help much except to reduce mast drag, and it certainly will increase sail skin friction drag. It would be much better to add the sailcloth to the existing jib or mainsail, since here the increase in maximum lift and hence in available drive would be certain.

I would tend to agree with you here, but these observations are similar to one I recently posted to a 'cutter rig' discussion.
http://www.boatdesign.net/forums/sailboats/why-does-cutter-rig-point-higher-sail-faster-5596-10.html#post740759

 

[Mikko Brummer]

I don't see what the term "effectiveness" signifies here. The reality is that the overall lift of a rig is almost entirely determined by its planform and by the local incidence of the rearmost portions of the rearmost sail.
"

Great to read this from such an authority as prof. Drela. The angle of the leech area to the centerline of the boat is often called "leech return" in sailmaker jargon, and it is what designers and coaches look at when asessing sail shape & trim. I have come to the conclusion that leech return and twist are by far the most important sail performance factors - to the extent, that I have long been putting forward "the front part of the mainsail is only there to support & keep the leech in the right position in space, with reference to the luff. The shape of the front part has little importance per se".

Brian: Sailboats have two sails in order to be able to adjust the power from light winds to a stiff breeze. The mainsail is attached to the mast in order to be able to flatten & depower it through the bending of the mast. The problem with a forestays-only rig would be that forestays tend to sag with the increasing wind, making the sails fuller when you would want them to be flatter. Also, a fractional sloop rig is far better for power control than the masthead rig, even if it could be aerodynamically inferior.

 

[Jamie Kennedy]

Key point by Mikko, the ability to power up and power down. Also for long distance offshore boats close reaching and beam reaching performance becomes as important as upwind and downwind, whereas for course racing the rig and sails can be over-optimized for upwind and downwind performance. Close reaching and beam reaching can be quite the different animal than upwind and downwind, and can really challenge how you set different sails and use combinations most effectively.

 

[Guillermo]

.... I have come to the conclusion that leech return and twist are by far the most important sail performance factors - to the extent, that I have long been putting forward "the front part of the mainsail is only there to support & keep the leech in the right position in space, with reference to the luff. The shape of the front part has little importance per se".

Absolutely. It's a well known fact that we may sail effectively to winward even with the luff fluttering, if the rear part of the sail is working properly.

 

[brian eiland]

Great to read this from such an authority as prof. Drela. The angle of the leech area to the centerline of the boat is often called "leech return" in sailmaker jargon, and it is what designers and coaches look at when asessing sail shape & trim. I have come to the conclusion that leech return and twist are by far the most important sail performance factors - to the extent, that I have long been putting forward "the front part of the mainsail is only there to support & keep the leech in the right position in space, with reference to the luff. The shape of the front part has little importance per se".

How do you get forward drive out of this 'leech return' area? Can you show this to me by a vector dwg or analysis?

Brian: Sailboats have two sails in order to be able to adjust the power from light winds to a stiff breeze. The mainsail is attached to the mast in order to be able to flatten & depower it through the bending of the mast. The problem with a forestays-only rig would be that forestays tend to sag with the increasing wind, making the sails fuller when you would want them to be flatter. Also, a fractional sloop rig is far better for power control than the masthead rig, even if it could be aerodynamically inferior.

I suspect you meant to say superior rather than inferior?

I FULLY understand what you are saying about the forestay sag making our sails fuller with increasing winds. I also understand the bendy mast technology, and have utilized it on numerous occasions.

But lets say we could hypothetically maintain very tight forestays. Could that make this twin parallel forestays (foresails) arrangement aerodynamically superior to the inverted 'V-slot' arrangement presented by either the fractional or masthead rig??

 

[brian eiland]

Absolutely. It's a well known fact that we may sail effectively to windward even with the luff fluttering, if the rear part of the sail is working properly.

I never thought a fluttering leech was 'effective' :?:

 

[CT249]

How do you get forward drive out of this 'leech return' area? Can you show this to me by a vector dwg or analysis?

How do you want anyone to show you anything by vectors or analysis, when you have just dismissed the theory explained to you by an MIT professor who is renowned for his expertise in record-breaking aerodynamic and hydrodynamic designs?

Decades of practical experience have shown that Mikko's points are correct, years of theoretical development have shown that Mark's points are correct. What more can one want?

PS - Surely Guillermo was perfectly correct when he used the word "inferior", and he didn't mention a fluttering leach. Finally, what is the relevance of a "very tight forestay" when we are already talking of "very tight" forestays, and a perfectly tight forestay is not in the realms of reality?

 

[daiquiri]

How do you want anyone to show you anything by vectors or analysis, when you have just dismissed the theory explained to you by an MIT professor who is renowned for his expertise in record-breaking aerodynamic and hydrodynamic designs?

Decades of practical experience have shown that Mikko's points are correct, years of theoretical development have shown that Mark's points are correct. What more can one want?

Don't be too harsh, there is nothing wrong in wanting to understand better things which are in effect not so intuitive.
For example, let's consider it via the momentum theory - see the attached drawing:



In the reference frame of the moving boat, the air mass enters with the velocity vector V1 and exits with the velocity vector V2. The force imparted to the air by the sails (and by the air to the sails) is
Eq.1 :

F = air_mass_flow * (V2 - V1)​

where V1, V2 and F are vectors.

But if the sail creates a wake, and the wake is of a significant size, The resulting vector V2 is the mean value of the two exit velocities V2,w (winward) and V2,l (leeward) of the relative streamlines at the boundary of the wake:
Eq.2 :

V2 = (V2,w+V2,l) / 2​

So, intuitively:

- F depends on the vector V2, as by the Eq.1
- the vector V2 depends on the wake shape and thickness at the trailing edge, as from the Eq.2
- the wake depends not only on the leech return but also on what happens upwind along the sail.​

Hence, one would rightfully expect to see that the sail performance depends on the forward part of the sail too, and not only on the so-called "leech return". So, IMO, Mr. Eiland's question is legitimate.

I would really like to hear from Mr. Drela and Mikko a rational explanation about why the above reasoning does not work in practice.

Cheers