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Sail Trim and Aeronautic Theories.

RussC

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Sep 11, 2015
1,519
Merit 22- Oregon lakes
When I first got SV Hadley, I was in constant motion. Trimming this, adjusting that, correcting the person at the helm for drifting while I adjusted the sails. (Boy that bugged me. I set the course then trimmed the sails then the dude shifted the course 5 degrees which screwed up the trim) Again this was because the feel was not right. Finally a good friend, who was manning the helm, as I was reaching again to adjust the course asked "Do you ever sit back and just enjoy the ride?"

I thought... No.
Yup
"sitting back and enjoying the ride" is not sailing. that would be....... being a passenger, which is a whole other experience. I also prefer sailing. :beer:
 
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Oct 22, 2014
15,695
CAL 35 Cruiser moored EVERETT WA
Russ. I have to say... I am an evolving sailor. I cruise better now. And I think this new enlightenment may get a bit more out of the few times I race on friends boats.
 
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Sep 22, 2018
1,869
Hunter 216 Kingston
John

I’ve read the PDF once and sort of understand but will need to re-read a few times before I have any confidence in saying I get it ;)

My first impression is that he explains how “lift” happens. Of course I was a believer in the prevalent theories that this disproves so I’m not sure it will have any impact in how I go about sailing. The basic premise is you have the outside of the curve of the sail downwind right? ;)

I absorbed what I read through a lens of my windsurfing background and how that equipment has evolved.

I started with an original windsurfer, teak boom, baggy sail outfit that would try to beat you to death if the wind came up.Too much apparent and the sail would back wind you right into the water surface.
Sails evolved to more battens, then camber induced battens to today’s race sails that have the ability to dynamically twist and reshape letting the rider go through a gust or lull without altering anything. Now people are riding foils so they are above the chop so again sails are evolving to take that into account.

My point is that the average windsurf sailor’s ability hasn’t changed, the equipment has evolved to the point where a relative novice can go out and have fun in conditions that would have beaten up a Robbie Naish in his prime.

It is highly likely that the windsurfer equipment designers have absorbed this knowledge quite some time ago and used it to build equipment that helps the sailor be more efficient. I just get to enjoy the benefits.
 
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Oct 22, 2014
15,695
CAL 35 Cruiser moored EVERETT WA
I just get to enjoy the benefits.
It is all about enjoying the benefits, be it wind surfing or sailing.

The math corrects the previous beliefs by identifying what is actually happening on the sail.

Think of it as the understanding of how to make ok beer verses really excellent beer.

While if all you want is a cold beer and the buzz/relief that happens from drinking it well any beer will do. But perhaps one day you want more. you want to taste the flavor of the hops the cleanness of the water used. The malt flavorings in the brew. Any old bear won't do.

Same with sailing. Some folk are happy with just being on the water. A baggy sail and a boat is fine. Others want to see their boat go as smooth and fast as it is capable of going. They will add twist and move the center of effort to where it optimizes the angle of attack. They will monitor the leech and when they see it happening they will take action to reduce the induced drag of the Mainsail.

Gentry identified these natures of a sail and quantified them. Bethwaite went the next step and connected the Gentry science with the actions and behaviors of the sailors to control the sails.

Like a stunt pilot learns to manage the cockpit of an airplane - adjusting power, ailerons, rudder, flaps in a way to get the maximum performance from his plane. The top sailors do the same by learning the skills to enable them to trim the sails faster and smoother enabling their boat to go it's fastest.

Why discuss it here? So those who may want to explore the the edges of their boats envelope might be encouraged. :biggrin:
 
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Likes: jon hansen
May 25, 2012
3,785
john alden caravelle 42 sturgeon bay, wis
i like the airplane analogy john. i'm in the final steps setting up a new rc plane tonight.

foils
lift
balance
performance

on my sailing vessels, they 'ride' nicer when sailed well, pushing the hulls makes for a nicer ride, "it's all about the ride", jon
 
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Likes: jssailem
Oct 24, 2010
2,380
Hunter 30 Everett, WA
Thanks, John for sharing this (A review of modern sail theory). I used to know a brilliant aerodynamicist (33 years at Boeing). He was a patient man who attempted to explain some theory to me, but the greek letters and calculus got in my way. This was written is a way that I can understand. Now I need to stop by the boat this afternoon and test what I learned so it might stick.

This also helps in my understanding of modern flap and slat design. but that's really no longer needed for me.

Ken
 
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Sep 22, 2018
1,869
Hunter 216 Kingston
While I was re-reading the article (having a beer as John suggested ;)) I am a little stuck on some wording. I will paste a picture of the section and then explain my issue below it.
6984FD21-E2F3-4449-BCDC-EA5C684E623E.jpeg


So my question isn’t about how to trim a sail it’s helping me get my head around what appears to be conflicting statements

At the top of the section I posted he talks about the separation bubble becoming larger the farther OFF THE WIND you go.

Near the bottom he talks about bearing off slightly to get speed back up.

So to me you want to avoid a separation bubble as that could lead to a stall. BAD

Heading too high will cause a sail to stall, bearing off or FALLING OFF THE WIND he says (and I agree) increases power. GOOD

How can going OFF THE WIND more be BAD???
 
Jan 1, 2006
5,950
Slickcraft 26 Greenport, NY
I think we are assuming that the sail angle isn't changed. So when you head off the wind without adjusting the sail, the bubble grows? Stall increases and drag increases.
I'm still reading it and struggling to understand some of it. I really hope I don't have to go to the bathtub. The circular flow evades me. Does that mean if I put tangs on the windward side of my mainsail, they would point towards the bow. Has anyone done this?
 
Oct 22, 2014
15,695
CAL 35 Cruiser moored EVERETT WA
I'll give it a try to clarify...
First we are talking about the sails. They are at a fixed trim. Not changed for this experience.

When the rudder is moved to let the boat fall off the angle of attack is changed on the sail.

Gentry talks about this event in the section above referencing his experience with a plane wing Figure 19.
8D5A1F29-98F4-4010-B997-5AE063532F87_4_5005_c.jpeg


As you fall further off the wind the angle of attack increases, your bubble increases in size just like it did on the plane wing. This happens as the "speed" increases.

Eventually if you keep falling off, the wind flowing across the sail (laminar flow) is completely disrupted and the sail stalls.

This can happen in a couple of ways to a sailboat. Your have the sails set, and the your charging along on a set tack. The wind gusts (increasing the wind speed across the sail) and slightly changes direction ( this changes the angle of attack). The increased wind speed starts the bubble at the leading edge of the sail. The slight shift can increases the bubble and as it moves across the sail you feel the boat slowing. You pull back on the tiller to fall off. This may correct the problem or make it worse depending on the wind change.

What can a sailor do? Well the falling off a little should help correct for any wind shift, and you can ease the sail to adjust for the increase in angle of attack.

Does that help?
 
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Likes: Will Gilmore
Sep 22, 2018
1,869
Hunter 216 Kingston
So when you head off the wind without adjusting the sail, the bubble grows?
That’s the part that has me puzzled. I get that a bubble forms. I’ve seen the leading edge of a sail start to curve inward (is this the bubble?) but the remedy is to bear off. I think he is saying this would make the problem bigger not smaller??????
 
Oct 1, 2007
1,723
Boston Whaler Super Sport Pt. Judith
The single best way to learn about sail trim and the effect on boat speed and pointing is to sail one designs. Start with dinghy type and move up. I began sailing Sunfish, moved up to Snipes, then Lightenings, then Ensigns. By the time I bought my first cruising boat, a Pearson 30, sailing was like walking. Everything was instinctive and automatic.
 
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Sep 22, 2018
1,869
Hunter 216 Kingston
@jssailem

Thanks for having a go but the wording conflict remains. Likely caused by how I framed my question ;)

To be clear I’m not disputing the theories presented, angle of attack causing stall etc.

I’m getting hung up simply on how he is wording this.

He doesn’t talk about sail trim adjustments, wind shift or strength changes so my assumption is that they are static.

He describes sailing farther off the wind will cause the bubble to increase. Take the airplane wing, rotate it so it looks like a sail and put it on the boat with no ability to adjust it, increase the angle of attack by turning the boat off the wind and at some point it stalls. The flow over the lee side becomes detached and you lose lift. Makes sense to me.

Then he proceeds to talk about using tells to help see stalling.

At one point in this section he speaks to “regaining lost speed” by bearing off so the first several tufts swirl.

I could infer that he is saying induce a separation bubble or the start of a stall by increasing the angle of attack will provide MORE power.

Again just a phrasing thing for me.

I think what he is trying to tell the reader is the tufts provide visual clues so the sailor can tread the fine line between steering too high (luff) and too low (stall) so that windward progress is optimized without adjusting sail trim.

This entire ramble of mine is on you as you suggested drinking “good beer” ;) :beer:
 
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Sep 22, 2018
1,869
Hunter 216 Kingston
I had to go see if the information is true. Today I managed to sail closer hauled. Nice day. Thanks.
Well there you go - science prevailing again ! :)

Curious about what you changed in your approach and what part of the article altered your thinking??
 
Oct 19, 2017
6,850
O'Day 19 Littleton, NH
I’ve seen the leading edge of a sail start to curve inward (is this the bubble?)
No. He's not saying that. Luffing is quite the opposite from the leading edge bubble. I think he's trying to say that, among other points.

The bubble is a cavitation caused by the angle of attack trying to change the direction of airflow too quickly, eventually leading to detachment and a stall. Luffing is sort of the reverse of that effect.

The bubble is a low pressure area that keeps the driving force forward of perpendicular. There's a balance. To much forward and you lose detachment and consequently the forward directive force vectors. To little bubble forward and the vectors don't add to a forward force.

-Will (Dragonfly)
 
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Likes: Hunter216
Oct 22, 2014
15,695
CAL 35 Cruiser moored EVERETT WA
I think what he is trying to tell the reader is the tufts provide visual clues so the sailor can tread the fine line between steering too high (luff) and too low (stall) so that windward progress is optimized without adjusting sail trim.
And steering for optimum power. As you fall off you will see the boat speed up. Fall off to much and the sails will stall. The tuffs help to indicate what that fine point of sail is.

Referring back to the first post where I suggest reading the NorthSail article, Whidden identifies the stall on the mainsail by watching the leech telltales near the upper third of the sail. I believe off the first batten. Gentry and Bethwaite talk about the sail stalling like a plane wing at the tip. A ruling vortices that is created. When you look at the new planes you see all of them with "Winglets". These combat the tip vortices. Stop them from slowing the plane's wing.

@jon hansen introduced Bethwaite to the mix. And Bethwaite talks about not using the steering to optimize the power of the boat. Steering is the "old way" thought of as the "Natural Way. It is slow. Use sail trim to accomplish the sail's change of angle of attack.

As an example you select a course bearing. (One that is not directly into the wind. duh...).. then you set the sails. Bethwaite talks about the great sailors using the sail trim and the helm in combination to sail the boat FAST. These sailors are recognizing the theories of Gentry and then applying them to their course and trim methods. As Bethwaite says "using both hands".
 
Sep 22, 2018
1,869
Hunter 216 Kingston
No. He's not saying that. Luffing is quite the opposite from the leading edge bubble. I think he's trying to say that, among other points.

The bubble is a cavitation caused by the angle of attack trying to change the direction of airflow too quickly, eventually leading to detachment and a stall. Luffing is sort of the reverse of that effect.

The bubble is a low pressure area that keeps the driving force forward of perpendicular. There's a balance. To much forward and you lose detachment and consequently the forward directive force vectors. To little bubble forward and the vectors don't add to a forward force.

-Will (Dragonfly)
Will

Yes the question I posed about the bubble was wrong headed on my part. Fingers going faster than brain!

Root cause - good beer coupled with a hilarious image that formed when @shemandr spoke about going to the bathtub, mental picture of people raiding the pantry for pepper to pour in the bathtub while family looks at them with disbelief ;) :beer:

The bubble that I had in mind at the time was from my memory of being on my original windsurfer. All powered up, hiked out, stance wide, fully committed, teeth clenched, fighting for control, board zooming along. Look at the area of the sail close to the boom/mast connection and notice the sail material starting to bubble inward towards you. No time to correct, no real options anyway and the sail just power slams you into the water.

I don’t think this ‘event” has much to do with leading edge, separation bubble stall. More like baggy sail, COE moving around.

This is a picture of what I was thinking. This guy is right on the edge of getting slammed.


FB37234B-9167-44D5-8AA1-DF41284B7212.jpeg
 
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