Why does a cutter rig point higher & sail faster?

the Slot Effect, etc

First let me correct a previous staement I wrote,

I meant to say unirig rather than cat rig

There are still some number of people who continue to refer to the 'slot effect' as a myth, ie;

I made a reference to Paul Bogataj's excellent paper earlier "How Sails Work". Well here is an excerpt from that paper that speaks to this interaction of headsail and mainsail, and then relates it to an actual sailing phenomenon that we experience on the water as well (rather than just theory):
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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."
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Tom Speer has made a number of references to AMO Smith's paper. It has been awhile since I read this paper, and I must admit it is rather 'technical', but if I remember correctly, it does imply that multiple, multiple slots (multi-element airfoils) do work in a similar manner to the single slot in that each preceding foil gains in its ability to 'point'. The question here is how many foils (multi-element headsails) are really practical on our sailing craft.

I'm more concerned with their most efficient interaction than how many their are.

For fun I included a figure from Mr Smith's paper. I need to reread this paper again and get back to Tom for proper interpetations and questions. As suggested by

I think I will revert back to the old Sail Aerodynamics for these discussions.

 

Taking a break from laughing at Frank... Seems to me that this discussion of "slot effect" is diverging because people assume different definitions of what the slot effect is. For example, I would have agreed with Skinny Boy that the slot effect theory is bogus, yet what I read in Brian Eiland's post does not describe what I thought the slot effect theory was.

I'll go first - My understanding is that the "slot effect" theory holds that air accelerates in the slot between two sails and by that means increases the lift of the more aft sail.

 

Slot Effect

.....some excerpts from previous discussions....
Subject: How Sails Work, the slot effect

I noted that this forum (another one) has had quite a bit of discussion recently of "lee side air speed". Well, I couldn’t resist bringing up the closely related, and also much maligned subject of ‘air flow thru the slot’ created by the mainsail and the headsail.

....Now look at the rig’s aerodynamic configuration. We’ve had many years of controversy over the flow of air in the slot between the jib and the mainsail, and many incorrect explanations. We now know irrefutably that the flow between these two sails is slowed rather than speeded up, and that results in a higher pressure on lee side of the main and on the windward side of the jib; ie, the jib’s drive is improved, and the main’s drive is degraded!! Put another way, the mainsail provides an upwash for the jib that makes the jib both more efficient and able to point higher (its operating in the safe leeward position). The jib meanwhile creates a downwash on the mainsail that decreases its efficiency.

There are any number of reference sites for the correct explaination now, but I'll just add one more that appeared in the archives of my website. Granted, it is not as easy to understand as it speaks to the 'circulation explanation', but it also has some questions by persons with pretty weird misconceptions about the old explanations of how sails work.

Colin wrote:
So what about the 'slot effect' everybody seems to assume in our club - i.e. that the jib accelerates air over the lee of the main, reducing pressure and increasing left. Is that happening as well further away from the sails?

Brian responded:
No, the air is not speeded up in the slot (on the lee side of the main). The air is speeded up outboard of the sail-plan (the lee side of the headsail). This speeded up air on the lee side of the headsail must eventually slow down to the free stream conditions as it leaves the sail-plan as a whole...but trhats another topic that Tom Speer can tell you much about.

I'll repeat an excerpt from my previous posting,"....that like a restricted water hose, the air is speeded up in the slot. This is another of those axioms that the textbooks have got wrong and have taught us wrong for so many years. But as Tom Speer noted, “there’s no way to finally put a stake through the heart of that old explanation—it just keeps coming back to life”.

 

My point wasn't that I don't understand "slot effect" theory, or that I think it's right or wrong. My point was that different people here seem to have different opinions of what the theory is.

Maybe we can agree that air does not accelerate through the slot; there is interaction between sails. Leave the term "slot effect" out and everyone's back on the same page.

That's all I want to say. Carry on.

 

Hmm. I would note that AIUI Cogito closes their slots and converts the rig to una upwind, where they require low drag and close windedness, and opens the slots downwind where they require high power and aren't worries about angle of attack or a drag penalty.

While anyone who attempts to compare empirically the performance of different styles of boats is being foolish, it is to say the least not obvious that in the highest performance regions sloop rigs are point higher. Another example would be the Formula 40 cats of a few years ago, who rapidly evolved from the planned sloop rig to a quasi una rig which had never had a jib larger than the minimum size storm jib required by the rule set.

Those are probably among the most close winded boats of all time. But there are loads of other factors that come into play, and for very many types of craft the slightly higher lift slightly higher drag two sail configuration is probably a better solution upwind.

While such a phenomenum undoubtedly exists I would question how often rigs would actually be close enough together for this to have a significant effect. Consider that the jib of the leading craft would need to be as close to the jib of the trailing craft as a jib normally is to a mainsail for it to affect the pointing angle. On the other hand the downwash, and more importantly the trailing votices from the rig persist for a considerably greater distance. The well known photo of Volvo 60s in light mist at Capetown in the last race demonstrates what an immense distance the trailing vortices can persist for in some conditions.

 

I don't recall this exact rig. Do you have a photo? I think this is a solid wing-sail arrangement, right?
I suspect this is because the slot between these two elements is so small as to not benefit the overall situation. Her leading element is probably too small & narrow an area to benefit from the extra drive pressure it might gain from having extra flow go to the outside of the leading element

I am familiar with that photo you speak of with the V60's, and it certainly displays the bad air downwind behind the leading vessel. But ask any top racer or the America's Cup guys about the 'safe leeward position', it is a very real phenomenum. In general the trailing vessel must have its bow up to the aft quarter or better of the leading vessel. Very importantly, remember the air begins to change direction well in advance of the forestay as a result of the rig's configuration. It does not just change direction as it hits the forestay.

 

Oh sure any and every half decent racer knows about that. But its not that the leading boat starts performing better. The point is that when you're in the trailing position you're sailing where the wind flow is being bent by the rig of the leading boat and so the wind getting to you is a degree or so different to the free flow condition and you're sailing in a header. If you look at all the wind tunnel outputs you'll see that the wind bend on the windward hip of the rig persists over a greater distance than the bend in front of the rig - its got to if you think about it, with those great trailing vortices disturbing all the wind flow aft of the rig.

 

Safe Leeward Position

No, if you (leeward boat) were then "sailing in a header" you would be at a disadvantage. Rather you are sailing in a slight lift as a result of the flow being bent around the outside of the leading boat's sails (in particular the headsail). A "header" would cause you to have to fall off slightly. Rather you can point up slightly.

 

gggGuest: ... when you're in the trailing position you're sailing where the wind flow is being bent by the rig of the leading boat and so ... you're sailing in a header.

brian eiland: No, ... you (leeward boat) ... are sailing in a slight lift as a result of the flow being bent around the outside of the leading boat's sails (in particular the headsail).

This is why in my aerodynamics post yesterday, I used the combination "leading/leeward" to describe the relative position of the jib, and "trailing/windward" for the main. Both of you are right, because you're talking about two different things. If you're in a trailing position, gggGuest is right. You're in the lead boat's backwash, which is air that has been turned into your bow. If you're slightly ahead of the other boat on the leeward side, then Brian is right. You benefit from the circulation effect #2: The air has not turned completely around the other boat's jib yet, so it's still moving somewhat more across your heading.

But to be fair to Brian, I think he does have one more point: If the trailing boat attempts to overtake the leading boat on the weather side, then the leading/leeward boat will benefit from the circulation effect, being propelled forward by the trailing/windward boat's upwash. While the trailing/windward boat will be sailing into the leading/leeward boat's backwash. So in that case, the leading/leeward boat has two effects helping it, not just one. That's the "safe leeward" position: It's leeward and leading.

 

Something different...a gentleman wrote to me recently;

I have been interested in the idea of a forward raked mast with an
unusual sail layout for quite some time now, originally seeing the
idea at a student design show. The boat looked like the sailboat
equivalent to a future speculating auto show "Concept Car", with wild
ideas that gave little concession to practicality.
Kyle


Have a look at this futuristic design

 

Arven Gentry is a GOD!!!

That was the classical explanation of slots on wings and accelerated flow was suggested by an important aerodynamicist (Prandtl? from memory) almost a century ago.

The sad thing was that no-one actually did the science. They just took the aerodynamicist at his word because he was considered important.

Experiment shows the opposite.

The first time I was made aware of it were the articles by Arvel Gentry in the late '70s printed in "Sail" magazine.

With a reprint in a book "The Best of Sail Trim". Others may have been before him, but he mixed experimentation with reportage and explanation.

His articles are available at
http://www.arvelgentry.com/magaz.htm

Read them and you will know EXACTLY why Cutters and Sloops are slower than cat rigged boats upwind for equal sail areas.
Why sloops and cutters may be slightly faster on a reach or run in certain conditions than cat rigged boats for equal sail areas.

Read the articles, take the effort to understand them - and then you will be able to work out the answer to this thread!!!

Best Regards
Michael Storer
my boat pages

 

Upwind Performance by Hall Spars

Pretty broad statement there. Why don't you try explaining that to Eric Hall of Hall Spars who I referenced an excerpt from under the subject thread Sail Aerodynamics;

"More recently I ran across a news article in the Sept issue of Seahorse magazine which discusses the very interesting full scale prototyping work being carried out on a J-90 class boat by Eric Hall of Hall Spars. Eric is now on his third-generation, free standing ,carbon wing rotating mast, with a una-rig mainsail. His “ thought process (and maybe not entirely logical) was: If biplanes became monoplanes and monoplane wings shed wires, why not an unstayed una-rig upwind” Boy, you would surely think this was the ideal upwind rig. In responding to an inquiry on upwind performance, Eric responds, “ first, of course, the boat would be improved upwind with a No.1 jib. Generally, we could not point as high as the others here (Block Island) and therefore had difficulty holding lanes.” He goes on to say, “this is a very interesting project that we especially want to succeed. I have been accused of a missionary zeal, which frankly keeps moving it along. It’s a real problem sometimes keeping focused on what we are trying to do in view of all that is ingrained in our minds about what makes sailboats work.

 

The split rig should be faster when maximum lift per unit area is needed. The single element rig should be faster when maximum L/D is needed.

Sailplanes need max L/D ... no canards that I know of.

Ice boats sail faster than the wind on all points of sail and use single element rigs.

A single element sailplan may well be a poor choice for a typical mono that sails in displacement mode most of the time. The AR of the sailplan is limited by the boat's RM (or lack of RM). For a given RM a multi element system can develop more power for the same area than a single element at the same AR.

 

Hi Firends: I have to Sailboats one Columbia 39 and a Newport 27. It is worthy make the 27 cutter that can give me more performance and speed. Or a 27 is to small for that. The boat sail very fast but I read here that point more and go faster. What was a correct answer.

 

"The split rig should be faster when maximum lift per unit area is needed.
The single element rig should be faster when maximum L/D is needed."

You have been given the key to the answer.

For a really fast racer , in competiton the max LD will let you squeak out the highest pointing angle .

For a heavier cruiser that is slogging up a hill of big water , the angle will never be as high as much FORCE is need to drag the boat uphill.

That takes maximum lift , to create the force needed.

So if your in a Soling playing racer for an afternoon the sloop would be best choice.

If your attempting to drag a fat heavy Westsail to windward ,length of luff wire makes the power needed to climb hills.

The old byplanes were fantastic at low speed high angle of attack , only suffered when the speeds got too high for fat wings and them damn rig wires.

Not a problem for a 8K sailboat.

FAST FRED