sail aerodynamics

Slot Effect Myth

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Brian responded:
I might suggest you get yourself a copy of Tom Whidden's book, "The Art and Science of Sails". There is an interesting discussion of the slot effect herein by the knowledgable author. Mr Whidden was, and may still be the CEO of North Sails.

 

DynaRig slot effect

Here is another affirmation of the slot effect....look at this new DynaRig proposed for the 285' Maltese Falcon:
<http://www.doylesails.com/newsletter-03-pg8.htm>

Note the sheeting angles for the three sails, and particularly that of the very forward sail. Now why is this?....look at the explantions of the slot effect.

And which sail do you think is providing the greatest drive forward, and which sail is providing the greatest leeway??

Interestingly, they are not trying to account for the wind gradient (twist) with this new design even though it is an extremely tall rig.

 

Tom Whidden's book The Art and Science of Sails is pretty good about where the air actually flows. It illustrates the theory as well as the practice. I think it's out of print, though.

I think it's pretty well understood that, on a given hull, a plan with a big jib is faster upwind, maybe all around. The downsides are that it costs more in sails, equipment and crew. Perhaps you can get a faster boat with a longer hull and a cheaper sail plan.

 

Brian wrote:
Just found this rather interesting discussion by Steve Dashew commenting on the vortex at the bottom of our sails. Check out the very sizable increase in weatherliness he quotes!! I suspect this is far in excess of what a CFA would predict.

Hi Steve and Linda, Thanks for all of the excellent books and tapes on you adventures. They have been a great help. I have noticed the winglets on airplane wings over the last few years. Has any one tried making a "plate" at the top of the mast, maybe using carbon fiber as a frame covered with sail cloth, to form a device which would reduce the vortexes created by a headsail & main combination? If if would work with a plate on each side of the mast, to tending would be needed during tacking or gybing. Asked my sailmaker about it but he deals with racers more than cruisers, so he is not too interested in the idea. Since you seem to be interested in making cruisers go faster with less effort, thought this idea might be for you. Thanks for thinking about it. Crawford


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Hi Crawford: Interesting concept, and as a glider pilot, with some very long and exotically shaped "winglets" I can relate to what you are suggesting. However, in a sailboat situation there are a whole series of variable which make this idea impractical.

On the other hand, there is another approach which we've used over the years which does work in some cases. This is to "endplate" or seal off the bottom of the boomed sails. If you can achieve this for even half of the foot length, the increase in efficiency is dramatic.

On our 67' ketch, Sundeer, we were able to pick up five degrees in weatherliness--without losing boat speed, when we sealed the main and mizzen. We've just had seals made for Beowulf which we'll be testing in the near future, and will write up for SetSail.

The area added is down low, where it is in turbulent air flow and where the breeze is much lighter. However, the seal effect is very powerful, and if you can make it work with your rig and deck structure, will generate a huge improvement. Note--the less efficient your keel, the more this will help as it reduces induced drag--which hit cruising keels harder than those found on racing boats. Regards--Steve

 

Crab/Claw sails

Woodenboat did a few articles on Crab Claw sails, I forget the author, which implied a better efficiency. I think that if we follow this thread much further, some of us are going to have to take a look at the boom vortex,, anyone sitting in a cockpit of a sailboat knows well the airflow around the boom. At the risk of being redundant, vortexs indicate what I said in an earlier post' that the lift is caused by redirecting the airflow, a temporary condition, if you extend flow lines past the sail far enough you see the turbulence moves in the same direction as the "mean" air flow. The "Bermudan rigs" current favour could in all possability be to two factors, reduced chaff and the rule making bodies that be. I for one would like to spend some time with a "Sprit Rig".

 

Great Proa site

Here is a really nice Proa site with lots of other info including the rigs on these vessels:
http://www.schachtdesign.com/proafile/volume_3/rig_comparison_table.html

 

FYI, I've updated my article on wingmast aerodynamics.

 

Masts

I wonder if a new mast is more important than a new mainsail in class racing? Somewhere I have run across the pearl, that the first 20 % of an airfoil is of the most importance in determining reynolds number. Perhaps a flawlessly finished mast is of more than esthetic importance?? Thank you TSPEER and Brian

 

The problem with dropping the boom to create an end-plate is that AFAIK there are some examples that go the other way, with great success. Even something as old as the Snipe has for decades only used the top of their range for the boom height. They'd rather have the rig high up (accelerated windspeed AFAIK) than try to seal it. Of course, maybe the boom even in the bottom position is too high to get a real end-plate effect.

But also look at 18' skiffs. They are technically sophisticated, yet they have very high booms for boat-handling reasons (ie you have to be a ble to run under the boom to tack ans gybe fast). Any improvement in having a lower boom must be less than the loss in tacks and gybes. In older boats the booms were lower, yet the sailors didn't hesistate to lift them up when hulls and techniques changed.

Another example is the International Canoes. They have very low tac ks on the mains, yet the booms poke up AND many skippers tack and gybe BEHIND the main so there is no reaosn that the boom has to be high for handling. Nor is the IC unsophisicated- current world champ is Steve Clark, owner of the wing-masted LAC holder Cogito.

In boards, there's a lot of talk about the end plate effect but how can this be separate from other effects that a low tack and rake create? Sure, there may be a theoretical advantqage, but there's a theoretical advantage in tilting the rig to windward but it's total B.S. in practise as proven by boards all the time - they go fastest with the rig almost vertical (sideways, not fore and aft).

So is the end plate that important??? It seems obvious from the many performacne boats that ignore it, that it can't be a huge improvement.

 

On the subject of end plate effect...

The air passing over the boat just above the hull has to be filled with som pretty mean vortices. So the airflow is severely disturbed in that area, and that can probably be the reason why the endplate effect does not make as much difference as it could in theory.
I think that boatspeed will be higher because of an efficient crew work area with a higher boom rather than trying to get a little end plate effect. As the slot between decklevel/cockpit sides and the boom is quite a distance aerodynamically.

So does the same happen at the foredeck...
When the boat heels 15-20 degrees the airflow at deck level is pretty banged up. So what gives the best effect? Discarding the eventual effect the endplate effect can have and hoist the foresail a little higher for more undisturbed airflow, or trust the endplate effect to do it's thing and make sure that the lower part of the sail is as close to the deck as possible?

I need to try to position som tellteles around the sail just above the deck to see if a lower or higher hoist makes any difference in the lower part of the sail. Also we have the thing with the airflow always being slower at the surface...

 

I agree. That's why I think there's a good arguement for wishbone booms and a raised clew. The area in the vicinity of a conventional clew can be better used elsewhere.

The problem as I see it is trying to combine wishbone boom, rotating wing mast and reefing. Two out of three is straightforward. All three is tough. I wish Team Phillips had lasted long enough to wring out her rig.

 

increased slot pressure, overpowered jib

Possible extreme illustration of this effect?

When sailing my light (725lbs) sportboat to windward in heavier air, we have taken to reducing heel by easing the jib before easing the main. it's a very effective technique on a boat that a 25kt wind will capsize with the main fully eased and the jib trimmed.

the jib's heeling moment (not necessarily a sign of efficiency) appears to increase with the increased pressure in the slot, and if I understand Frank Bethwaite's arguments in Performance Sailing, you will sail higher, flatter and faster in heavy air situations under main alone. (I haven't had enough experience to verify Bethwait's assertion for myself, but seems to make sense).

in a short-ish puff, often, with the crew easing the jib until it bubbles at the luff, I leave the main fully trimmed and feather the boat to windward for the duration of the puff. not positive whether there is a net gain or not, but we don't seem to lose speed and obviously we're sailing momentarily higher. on the other hand, feathering for an extended time definitely does not seem to pay.

reactions / explanations / suggestions for improvement?

 

Slot Effect

This question of the 'slot effect' has come up again on a subject thread entitled "Why does a cutter rig point higher & sail faster?" , with several persons referring to it as a myth and unexplainable. I thought we had disposed of that notion.

I thought I would bring up this thread discussion again so the contributors to both threads would be aware of each others previous discussions on this subject.

 

Tom mentioned five forms of sail or foil interaction from an article by A.M.O. Smith:
Smith, A. M. O., "High Lift Aerodynamics", AIAA Journal of Aircraft, Vol. 12, No. 6, June, 1975, pp. 501-530.

I hope nobody minds if I call the two elements the "jib" and the "main".
jib === the leading/leeward element
main === the trailing/windward element
"freestream" means the "external" apparent wind velocity that you would have without the sails, at the same boat velocity.

Could Tom and/or anybody else please tell me where any mistakes are here.

1. Slat effect: In the vicinity of the mast, the velocities due to circulation around the jib run counter to the velocities around the main, and so reduce pressure peaks on the main.
Since the jib is sheeted out more than the main, the air circulation around the jib resists the leeward airflow around the mast. The mast then slices the air more evenly on both sides, reducing both the lift and the drag of the main. I'm not sure "peak" pressures are important in and of themselves. The main benefit seems to be reduced flow around the high-drag mast.
The cause of this effect is the differing orientations of the sails.

2. Circulation effect: In turn, the main causes the trailing edge of the jib to be in a region of high velocity that is inclined to the mean line at the rear of the jib. Such flow inclination induces considerably greater circulation on the jib.
More air is passing leeward of the jib, so less air is going through the slot. To some degree, the two sails are acting together as though they were one big fat foil.

3. Dumping effect: Because the trailing edge of the jib is in a region of velocity appreciably higher than freestream, the boundary layer "dumps" at a high velocity. The higher discharge velocity relieves the pressure rise impressed on the boundary layer, thus alleviating separation problems or permitting increased lift.
The leading portion of the lee side of the main has a negative pressure gradient. This reduces the "pile-up" of air (adverse pressure gradient) on the weather side of the jib, and therefore keeps the air flowing more smoothly along it in light winds.
The benefit of this effect occurs in the slot, but is caused by an area farther downstream.

4. Off-the-surface pressure recovery: The boundary layer from the jib is dumped at velocities appreciably higher than freestream. The final deceleration to freestream velocity is done in an efficient manner. The deceleration of the wake occurs out of contact with a wall. Such a method is more effective than the best possible deceleration in contact with a wall.
This occurs downstream of the slot.

5. Fresh-boundary-layer effect: Each new element starts out with a fresh boundary layer at its leading edge. Thin boundary layers can withstand stronger adverse gradients than thick ones.
Multiple elements can be smaller than an equivalent single element, and therefore have a shorter chord, which is more efficient (lower Reynolds numbers).
This effect applies to each element individually. It's not a product of their aerodynamic interaction, via the slot or otherwise. It's allowed by the mechanical fact that the two elements are attached to the same vessel, both helping to propel it.


The "circulation" effect #2 seems to be most closely associated with the slot itself. As Tom pointed out, it's the opposite of the venturi-effect theory. Furthermore, its main effect seems to be to transfer lift from the main to the jib. I would like to know whether it really is advantageous in applications such as two foils, where the trailing/windward foil is just as efficient as the leading/leeward foil.

The "fresh-boundary-layer" effect #5, I would say has nothing to do with the slot at all, and would apply just as well, for example, to a parallel twin-rig catamaran, no matter how far apart the hulls and sails are.

 

1. The peak pressures are important because ultimately the flow outside the boundary layer has to slow down to near (actually a little below) the freestream speed at the trailing edge. The higher the peak velocity, the steeper the deceleration has to be, and if the deceleration is too great you get flow separation.

Bob Liebeck did a very interesting study of the tradeoffs involved with peak velocity and maximum lift. The Stratford pressure distribution has a constant margin from separation all along its length so it is the shortest, steepest way to decelerate the flow from a given peak velocity. Therefore, the suction surface velocity profile that produces the maximum lift for a given peak velocity consists of a constant "rooftop" velocity distribution from the leading edge to the beginning of the recovery region and then a Stratford distribution from there to the trailing edge. He then looked at the tradeoff between a short, high rooftop and a long, low rooftop to find the maximum area under the curve for the greatest contribution of the suction side to maximum lift.

With regard to the mast, I don't think there's much reduction in the drag of the mast due to a change in dynamic pressure. But by reducing the peak velocity as the flow accelerates around the mast leading edge, the tendency toward separation on the back side of the peak is reduced.

2. Air passing to leeward of the jib is not what he meant. The trailing edge acts like a volume control knob for the lift on the rest of the surface. The circulation (lift = velocity * circulation) adjusts itself to match the conditions at the trailing edge. What's most important is the component of the freestream that is perpendicular to the trailing edge, because this is the component that is essentially cancelled out by the circulation. When you change the angle of attack of a surface, you are changing the perpendicular component at the trailing edge. When you deflect a flap or camber the surface, you are changing the flow component perpendicular to the trailing edge. And when you place another airfoil behind the trailing edge of the first, you change the flow component perpendicular to the trailing edge. In every case, the circulation adjusts itself to cancel out these changes at the trailing edge. As Smith shows in his article, even if you place a bluff body like a circular cylinder behind and below the trailing edge, you get an increase in lift on the surface due to the deflection of the flow at the trailing edge.

3. You've basically got it. The adverse pressure gradient is like driving on ice - if you slow down too fast, you'll break loose. From a given starting speed, you don't have to break as hard if you're only slowing down for a new speed limit, instead of coming to a complete stop. The dumping velocity is like slowing down to a less restrictive speed limit. For example, here's a Liebeck designed slotted section with its pressure distributions for a few angles of attack:



Notice now the forward section's trailing edge is at a pressure coefficient around -1.3 while that of the flap trailing edge is at a pressure coefficient of 0.6 or so. The flap boundary layer has a much lower dumping velocity than that of the main wing.

4. Yes, it not only occurs downstream of the slot, it can occur downstream of the whole airfoil.

5. Lower Reynolds numbers are not more efficient. Skin friction drag coefficients decrease with Reynolds number, so breaking the area up into smaller pieces is less efficient from this standpoint.

Breaking the chord up into searate pieces is an advantage because each piece can have more of a rooftop pressure distribution and a shorter pressure recovery region. The absolute maximum lift you could get from a given peak velocity level would be to have a rooftop pressure distribution that extended all the way to the trailing edge. Anything that cuts away at this rectangle is a loss of lift. With a multielement section, you chop off a number of corners to form the pressure recovery region for each element. But these are not as much as you have to cut away to form the pressure recovery for a single element section with the same peak velocity.

The main really does transfer lift to the jib. The combination of the two is more effective than the same area allocated to either one separately. This is true even if both of the elements are good performers in their own right.

For example, the NACA tested a 23012 section with a 23012 used as a flap. This figure shows the pressure distribution of the plain and flapped sections at the same lift coefficient, and at the same angle of attack. As the flap is deflected, you can see the circulation effect increasing the lift on the whole wing surface. The suction surface velocities are increased and the pressure surface velocities are decreased, showing the circulation effect. You can also see the increase in dumping velocity with increased flap deflection.