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pastmember
Master Marine Consultant

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Response Posted - 09/14/2014 : 16:47:04

My first keel boat was named Barber Hauler.

Edited by - on

Arlyn Stewart
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Response Posted - 09/14/2014 : 18:48:59

quote:
Originally posted by islander

quote:
The bugaroo is that they will only work if there is a lead block ahead of the winch,

Wouldn't another car work slid back on the track placed in front of the winch say where you would normally place it for strong winds. Then when the wind picks up you just ease your twang and the second car takes over.

In the case of the early 250s, the tracks were too short and such a fairlead position would defeat the twing/twang effort to produce a more aft sheeting angle.

Edited by - on

Arlyn Stewart
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Response Posted - 09/14/2014 : 19:14:57

quote:
Originally posted by Stinkpotter

quote:
Originally posted by Arlyn Stewart

Dave, you are correct that the athwardship angle changes slightly but not much for a hardened condition where the car would normally be far forward, so the twing block is pulled down fairly close to the normal track car position...
i get that part, but the car forward (or "twang" pulled down) would be for light air. I was wondering about heavier air where you want to pull the clew back, so would ease the twang and harden the sheet, if I understand the rig. That can also be when you want to point as high as possible...

Dave... best that I can recall, with roller furling, twisting the jib off to depower wasn't much of the game plan.

Edited by - on

Arlyn Stewart
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Response Posted - 09/14/2014 : 19:20:40

quote:
Originally posted by pastmember

My first keel boat was named Barber Hauler.

Frank... thanks for refreshing my memory of a barber hauler. They were used for tightening the slot IIRC?

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pastmember
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Response Posted - 09/15/2014 : 06:37:10

I have been a registered Barber since 1977 even though I stopped cutting in '86. In '82 I was excited to find a term in the reading I was doing about how to make a keel boat go that used the term Barber and took it as the name for my Spirit 23. John Kolius was a a young sailmaker in Seabrook Tx then and he built a main for me and came to Wichita to bend it on and tune my rig for me. I remember the Barber Hauler to be a slot closer but I have read they were used a slot openers too. I always saw closing the slot to sheet in more and sail higher as the goal on a boat that lacked inboard tracks for small headsails. As I recall the Barber brothers got credit for what has probably been rigged on boats since a week after the Marconi rig was developed.

Edited by - on

Dave5041
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Response Posted - 09/15/2014 : 19:45:46

Love where this thread has gone. New thoughts

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Arlyn Stewart
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Response Posted - 09/16/2014 : 04:19:40

Dave... I recall a lot of discussions about slot effect many years ago with many saying a magic slot supercharged the main (constricted the air flowing aft off the windward side of the headsail and forcing it speed up across the leeward side of the main) and others saying nay, the benefit wasn't from supercharging the main but because of a condition where the two sails appeared as one and thus reduced drag.

I'm not smart enough to know which, but certainly have experienced whatever it is, a real kick because when the slot is working, the boat is boogieing.

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Stinkpotter
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Djibouti
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Response Posted - 09/16/2014 : 06:05:55

Arlyn, I've always agreed with the former--based on the Bernoulli effect. It explains (to me) why a larger, overlapping headsail, with virtually all of its increased area in the "wind shadow" of the main, generates so much more power going to windward. As I see it, the big sail holds the fast-moving air against more of the leeward side of the main, creating more of a pressure differential. (But I've been known sometimes make up my own physics...

)

New many designs are favoring bigger, higher-aspect mains with fractional, self-tending jibs on Hoyt booms, obviously with no overlap. I wonder.....

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Arlyn Stewart
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Response Posted - 09/16/2014 : 07:39:50

quote:
Originally posted by Stinkpotter

Arlyn, I've always agreed with the former--based on the Bernoulli effect. It explains (to me) why a larger, overlapping headsail, with virtually all of its increased area in the "wind shadow" of the main, generates so much more power going to windward. As I see it, the big sail holds the fast-moving air against more of the leeward side of the main, creating more of a pressure differential. (But I've been known sometimes make up my own physics... )

New many designs are favoring bigger, higher-aspect mains with fractional, self-tending jibs on Hoyt booms, obviously with no overlap. I wonder.....

An interesting memory. I of course was very familiar with when the slot was working on my Hobie Cat but on my 250, I can only recall one occasion when I was certain that the slot was working. It was the first year I sailed the Great Lakes with it. She was doing a mile and a half better than hull speed. I never saw it again in another six years of sailing it there.

My conclusion was, the boat was lighter that first year. I'd not fitted her out with cruising amenities yet. Thus, if the slot was ever working after that, the extra power couldn't overcome the extra drag. As you pointed out, perhaps the twings had something to do with losing a little inboard sheeting angle. However, best that I can recall, the twings were used perhaps the last two cruises so there should have been plenty of times in four cruises to experience it again... but never did.

When racing cats... it went without saying on a light to moderate wind day, the lighter the combined crew, they had the advantage.

Edited by - on

Steve Milby
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Response Posted - 09/16/2014 : 07:50:17

The theory of the slot effect has been largely discredited since [edit: around 1969-70], when Arvel Gentry, an aerodynamics expert working for Douglas Aircraft, published his conclusions on the subject. His writings are generally very technical, and not easy to understand, and, when I tried reading his articles, I finally gave up. Nevertheless, the following, taken from wikipedia, is a fair summary of his thinking.

"A correct explanation of the interaction between jib and mainsail was published by aerodynamicist and yachtsman Arvel Gentry in 1981,[4] and "is much more complicated than the old theories imply". This states that the widely believed explanation of the slot effect is "completely wrong" and shows that this is not due to the venturi effect (or "valve effect" to use Curry's term) accelerating the air in the slot. Instead it is shown that the air in the slot is slowed down and its pressure increased reducing the tendency of the mainsail to stall, that the mainsail reduces the air pressure on the lee side of the jib accelerating that airflow, and that the mainsail increases the angle at which the air meets the luff of the jib, allowing the boat to point higher. Gentry points out that proper understanding of sail interaction allows better sail trimming."

In short, as I understand it, his thinking is that the slot effect has little to do with increasing the power generated by the sails, but it has much to do with the pointing ability of the boat. That would seem to make sense, because the mainsail is generally capable of generating more power than the boat is capable of using efficiently, and the sail trimmer's task is to de-power it when that power peak has been exceeded. De-powering the mainsail keeps the boat on it's feet and permits it to point higher.

Edited by - Steve Milby on 09/16/2014 16:37:59

OJ
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Response Posted - 09/16/2014 : 09:09:03

quote:
Originally posted by Steve Milby

. . . when Arvel Gentry, an aerodynamics expert working for Douglas Aircraft, published his conclusions on the subject. His writings are generally very technical, and not easy to understand, and, when I tried reading his articles, I finally gave up . . .

Reminds me of "A Manual of Sail Trim" by Stuart Walker. I felt like a moron by the middle of the first chapter.

Edited by - on

Stinkpotter
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Djibouti
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Response Posted - 09/16/2014 : 10:41:28

quote:
Originally posted by Steve Milby

...Instead it is shown that the air in the slot is slowed down and its pressure increased reducing the tendency of the mainsail to stall, that the mainsail reduces the air pressure on the lee side (?) of the jib accelerating that airflow, and that the mainsail increases the angle at which the air meets the luff of the jib (?), allowing the boat to point higher.

Wow... There are some hugely counter-intuitive (to me) assertions there. I think I'd better give up and go for a ride on Sarge!

Edited by - Stinkpotter on 09/16/2014 10:43:54

Tradewind
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Response Posted - 09/16/2014 : 15:06:57

quote:
Originally posted by Stinkpotter

quote:
Originally posted by Steve Milby

...Instead it is shown that the air in the slot is slowed down and its pressure increased reducing the tendency of the mainsail to stall, that the mainsail reduces the air pressure on the lee side (?) of the jib accelerating that airflow, and that the mainsail increases the angle at which the air meets the luff of the jib (?), allowing the boat to point higher.
Wow... There are some hugely counter-intuitive (to me) assertions there. I think I'd better give up and go for a ride on Sarge!

Gotta agree with Dave on this one, makes no sense to me. I've been sailing and flying for over 40 years and even taught aerodynamics here and there. Arvel would have to draw some pictures for me on that one, he may be right but it's beyond me.

Edited by - on

pastmember
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Response Posted - 09/16/2014 : 15:24:31

quote:
Originally posted by OJ

quote:
Originally posted by Steve Milby

. . . when Arvel Gentry, an aerodynamics expert working for Douglas Aircraft, published his conclusions on the subject. His writings are generally very technical, and not easy to understand, and, when I tried reading his articles, I finally gave up . . .

Reminds me of "A Manual of Sail Trim" by Stuart Walker. I felt like a moron by the middle of the first chapter.

Wasn't he the Snipe champion?

Edited by - on

Steve Milby
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Response Posted - 09/16/2014 : 16:34:24

quote:
Originally posted by Tradewind

quote:
Originally posted by Stinkpotter

quote:
Originally posted by Steve Milby

...Instead it is shown that the air in the slot is slowed down and its pressure increased reducing the tendency of the mainsail to stall, that the mainsail reduces the air pressure on the lee side (?) of the jib accelerating that airflow, and that the mainsail increases the angle at which the air meets the luff of the jib (?), allowing the boat to point higher.
Wow... There are some hugely counter-intuitive (to me) assertions there. I think I'd better give up and go for a ride on Sarge!

Gotta agree with Dave on this one, makes no sense to me. I've been sailing and flying for over 40 years and even taught aerodynamics here and there. Arvel would have to draw some pictures for me on that one, he may be right but it's beyond me.

You guys aren't alone. I was skeptical at first, too, but it has been many years since he first published his original article, and the folks who are smart enough to understand what he said seem to have universally accepted it as gospel.

The following is a link to an article that was published in Sail Magazine, apparently back in 1973. It isn't nearly as technical as his original articles, but it's still tedious reading. As you can see, he drew pictures and used scientific methods. This isn't just his unproven theory.

http://www.bethandevans.com/pdf/another_look_at_slot_effect.pdf

Edited by - on

GaryB
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Response Posted - 09/16/2014 : 18:12:17

If I read what he said correctly it sounds like the airflow between the lee side of the main and the windward side of the jib/genoa slows as it goes through the slot which effectively semi-plugs the slot. This higher pressure area forces some of the air that would have gone through the slot to go around either the windward side of the main or the leeward side of the the jib/genoa. By doing that it kind of makes the main and jib become one large sail and because they are separated by some distance fore and aft it increases the effective area thus forcing the airflow on the lee side of the jib/genoa to really accelerate to keep up with the air passing on the windward side of the main/jib combination. When it accelerates it decreases the pressure on the lee side of the main/jib/genoa combo which pulls the boat ahead faster.

On most airplanes the top side of the wing has more curvature in it than the bottom. If two air molecules are one above the other and are then separated by an aircraft wing or sail they try to rejoin each other on the back side of the wing/sail at the same time. Since the top side of the wing/sail has more curvature it forces the molecules to travel farther and thus the air particles traveling over the top side of the wing have to accelerate. Based on the Bernoulli Principle this causes the pressure to drop on the topside of the wing or the lee side of the sail.

Remember the wing, slats, and flaps on an old Boeing 727 or 747? When the slats and flaps are extended on the wing of these planes it lengthens the chord and increases the area of the wing and also effectively puts more curvature into the shape of the wing which forces the airflow over the top of the wing to accelerate creating more lift. You will also notice when the flaps are extended past a certain point on these planes they open up a slot between the leading edge of the flap and the wing trailing edge. This allows "some" of the high pressure air underneath the wing to accelerate over the flap itself and create additional lift.

May not make sense to you when you read the above but I'm pretty sure I understand what was being explained. Then again, I probably sound like a nut.

Edited by - GaryB on 09/16/2014 18:28:18

Steve Milby
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Response Posted - 09/16/2014 : 19:55:02

quote:
Originally posted by GaryB

May not make sense to you when you read the above but I'm pretty sure I understand what was being explained. Then again, I probably sound like a nut.

As Kermit the frog said, "It Ain't Easy Being Green!"

Imagine how difficult it must have been in 1973 for Arvil Gentry when he sprang this theory on folks who knew it just couldn't be true. He must have known that the penalty for heresy was to have your eyes put out with a hot poker!

Edited by - Steve Milby on 09/16/2014 20:08:36

Arlyn Stewart
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Response Posted - 09/16/2014 : 20:01:37

If I recall correctly, more credit is now given to a Newton's third law.

"Newton's third law says that for every action there is an equal and opposite re-action. When an airfoil deflects air downwards, the air exerts an upward force on the airfoil." Wiki http://en.wikipedia.org/wiki/Lift_%28force%29

Flaps increase drag and slow a plane but also increase lift by vectoring more air downward.

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Stinkpotter
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Djibouti
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Response Posted - 09/17/2014 : 07:47:39

Arlyn, that's gotta be part of it, but if that was the whole story, I'm thinking the most efficient sail would be perfectly flat.

As many have acknowledged, there's some advantage to a sail with an airfoil shape. But an airplane wing doesn't have the same curvature on both sides--the top has more, creating the pressure differential discussed here. A sail on a boat is curved the same on both sides, so by itself would seem to lack the characteristic that creates lift on a wing. But I'm beginning to grasp the proposition that two sails, particularly overlapping, create that difference between the overall shape of the windward and leeward sides. And given that air doesn't precisely follow the surfaces it moves past (thereby creating pressure differentials), the effect might even apply to a non-overlapping jib that directs the flow somewhat away from the leeward side of the main, more so than the flow past a the leeward side of a main with no headsail.

But I'm still afraid to read that stuff--everything I'm thinking I'm beginning to understand might get blown up! (Steve: Remember the physicist who asserted that a curve-ball in baseball had to be an optical illusion?)

How to blow up a cocktail party: Ask a sailor how a sailboat can sail into the wind...

Edited by - Stinkpotter on 09/17/2014 07:52:00

Steve Milby
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Response Posted - 09/17/2014 : 08:40:21

Gentry's articles were published in Sail magazine 40 years ago, and since then, they have been re-published in "The Best of Sail Trim." Nevertheless, most sailors still focus on the slot effect, and all the sailing courses that I took through the years taught the slot effect, and never discussed Gentry. Maybe it's because it's so difficult to understand Gentry's theories, and to explain them clearly and in lay terms. Maybe it's because we all learned the Bernouille effect in school, and it's easier for us to understand and explain, even if the slot effect theory is based on science that was disproved 40 years ago.

Based on my reading, Gentry wasn't just a recreational sailor with a cockamamie theory. He was a professional aerodynamics expert who held one of the premier aerodynamics jobs in the world, working for Douglas Aircraft, and he had access to the best aerodynamic test equipment of the day, and he used that equipment to test and prove the validity of his theories. To the best of my knowledge, nobody has disproved his theories.

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Arlyn Stewart
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Response Posted - 09/17/2014 : 12:27:17

A flat plate is not as efficient for redirecting air as a foil for several reasons. The most obvious is the impossibility of it being strong enough and resistant to flex and warping under loads but that isn't the big one, which is that an airfoil redirects air better than does a flat plate. A flat plate produces a lot of drag due to a high reynolds number (a measurement of drag due to poor laminar flow.

I suspect that there are few modern aerobatic planes that don't have symmetrical foils because of the need to be able to redirect air equally well in both directions. Back in the early days, when undercambered and clark Y foils were used, the aircraft was designed primarily for lift in one direction... hence to some extent why the outside loop was so difficult, there was not near as much lift. Don't jump on me for oversimplifying that... there are several other factors involved that made the outside loop difficult.

When a foil is given a positive angle of attack, it directs air to the negative side and does so with both air coming from the negative and positive side of the foil.

Bernoulli theory that lift is created by air pressure differences between the two sides of the foil has pretty much been abandoned.

Again, I'm not smart enough to argue for either theory about slot effect. All sailors know that apparent wind is their friend, the faster the boat goes, its speed is added to the wind velocity. What I think I do know, is that I've experienced power gains that increased speed beyond hull speed and it was rare and something had to have caused it. It could have simply been the apparent wind multiplier combined with a boat light enough and drag free enough to set it free to boogie.

In the cat days, when we'd claim we had the slot working, it was a statement that everything was hardened to the extend our muscles would haul the sheets. Things were so flat that if going submarine with the bows and killing speed, they only way to recover speed was to ease the sheets to a lower gear and go back through the gears again by slowly hardening until they ware again tighter than Dicks hat band.

Apparent wind then was likely the big player and our use of "working the slot" was likely more about that then we admitted or understood.

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Arlyn Stewart
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Response Posted - 09/18/2014 : 04:39:26

Above I comment about going submarine and that deserves some explanation. There comes a point that the sailplan is exerting so much power (forward lift) and that power opposing the drag of the hull/s leverages the bow/s down. On a cat it can cause the bows to submerse and actually dive and in some cases even cartwheel the boat.

It illustrates the kind of super power that is generated by the increasing speed of the laminar flow of air past the foil, a product of apparent wind.

Ice boats are an even better example because they don't suffer as much drag as does a cat. Where cats may average top speeds of perhaps 30 mph before submarining the bows, ice boats double that and triple and more.

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Stinkpotter
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Djibouti
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Response Posted - 09/18/2014 : 07:24:36

This hijack is outdoing our winter douldrum discussions... but what the heck!

Having flown hulls on a Hobie 16 and 14T, I came to believe "hull speed" didn't apply to that hull design (like a knife). It seemed the leeward hull just sliced the water instead of being forced upward on plane by it, and indeed, the drag or other factors could eventually submarine it, but at speeds way above "theoretical hull speed". It could be like sailing into a fire hose!

Back to foils... Arlyn, you know more about them than I do--I was oversimplifying to make the point that there appear to be more factors than just deflection involved--to the point where (almost) nobody knows exactly what they all are. I've long been amused by these kinds of discussions, which tell me that the modern Marconi Rig and sail shapes are probably a product more of decades of incremental trial-and-error experimentation and theorizing than of precise aeronautical calculation.

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glivs
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Response Posted - 09/18/2014 : 16:36:39

quote:
...more factors than just deflection involved--to the point where (almost) nobody knows exactly what they all are.
I've long been amused by these kinds of discussions, which tell me that the modern Marconi Rig and sail shapes are probably a product more of decades of incremental trial-and-error experimentation and theorizing than of precise aeronautical calculation.

Dave as a career scientist I'm always amazed at just how incredibly long it takes knowledge to work its way both through the scientific community and the public. But to your point, don't underestimate just how far aeronautics has come in recent years even if we are out of the loop. NASA Ames and NASA Langley as well as a variety of Universities have tremendous aeronautical modeling capabilities. I have no doubt that aspects of that modeling has filtered down into modern sail design software.

Edited by - glivs on 09/19/2014 04:02:54

Arlyn Stewart
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Response Posted - 09/19/2014 : 04:46:07

Yeah Dave, the values of experience, creative thought, engineering and design disciplines all contribute.

I once exchanged a series of emails with a Brit who did International One Meter RC racing. They now use hull shapes (think of the hull as a foil) that when heeling, lifts to weather. Traditionally, most sailboat hull forms probably lifted leeward.

Which means that an older design isn't even in the game any more.

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