Super Secret Settled Science of Sailing?
- Thread starter Will Gilmore
- Start date
I think sometimes whether or not a scientific explanation is 'settled' or not is less important than whether or not it's simple to understand, accurate 99% of the time, and therefore USEFUL to more people. Try this: Lift is created 2 ways: (1) by air or water 'pushing' on something like a sail, keel or rudder, and (2) by centrifugal force. Method (1) won't confuse many people, unless perhaps we complicate it by citing Newton's laws. Method (2) is very poorly understood by even the most brilliant among us. But if we think about forcing air (or water) to bend around a sail, keel or rudder (foil) and the centrifugal force that generates, it becomes much more simple and useful than trying to explain one of the Bernoulli's equations. The air or water on the 'outside' of the foil tries to go straight, but can't, and slings itself outward creating lower pressure. The air or water on the 'inside' of the foil also tries to go straight, but can't, and bunches up creating high pressure. If the foil (sail, keel, rudder, etc.) is at too big an angle (usually about 20 degrees for air; 'angle of attack') to the oncoming air or water, the air or water can't 'stick' to the foil any longer, and forms turbulence and eddies. This condition is a 'stall'. If you let it happen to your sail, you go slow. You can tell it's happening if the jib's leeward telltales are dancing around, or the main's leach tells aren't streaming aft. If it happens to your keel, you slide sideways through the water and you can't point for crap. If you let it happen to your rudder, you loose control of your boat, usually by violently rounding up or broaching if you've got the spinnaker out.
I've seen WAY too many sailors sailing (racing) very poorly because they just don't understand lift and stalls.
'When in doubt, let it out' (not stalled), but never let it luff. Learn to tell if you're sliding sideways more than going forward when upwind (keel stall). Keep the boat reasonably upright so your 'engines' (sails) aren't pulling the boat from 20' out over the water away from the drag (hull), forcing the rudder to work overtime, creating massive amounts of drag, or worse, stalling.
(If you're questioning how this works on a symmetric foil like the keel and rudder, envision how the water hits them at an angle (rather than straight on), creating more curvature on the leeward side than the windward side - this explanation works whether you buy into the centrifugal force explanation or not)
(Mechanical Engineer, Professional pilot, former flight instructor, longtime sailor, love to race!)
I've seen WAY too many sailors sailing (racing) very poorly because they just don't understand lift and stalls.
'When in doubt, let it out' (not stalled), but never let it luff. Learn to tell if you're sliding sideways more than going forward when upwind (keel stall). Keep the boat reasonably upright so your 'engines' (sails) aren't pulling the boat from 20' out over the water away from the drag (hull), forcing the rudder to work overtime, creating massive amounts of drag, or worse, stalling.
(If you're questioning how this works on a symmetric foil like the keel and rudder, envision how the water hits them at an angle (rather than straight on), creating more curvature on the leeward side than the windward side - this explanation works whether you buy into the centrifugal force explanation or not)
(Mechanical Engineer, Professional pilot, former flight instructor, longtime sailor, love to race!)
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That's not centrifugal force. And I confess that in all my years I have never hear centrifugal force suggested as a component of lift; nor do I find that simple to understand!
I think what you describe is 'deflection.'
Think about coming off the interstate at too a high speed. As you try to negotiate the curve, your body and the car are thrown to the outside of the curve, that is centrifugal force.
If I'm understanding this correctly, as the air molecules hit the luff those on the leeward are deflected outwards. This outward deflection starts the molecules sailing in an arc where they experience centrifugal force in part causing a pressure reduction due to the centrifugal force not just the speed of the air molecules. Conversely, the air molecules on the windward side are also deflected outwards with the centrifugal force sending the molecules outward into the curve the sail transmitting the centrifugal force to the sail.
The short story is the air pressure differential is not due just to differences in velocity, rather it is the centrifugal forces acting on the sails that cause the observed pressure differentials.
Sometimes it is easier to just say "keep the telltales flying."
E.A.A. is happening right now in oshkosh. lots of lifting going on. love the dc3's
i'm in ohio, going north later this week for two months sailing
i'm in ohio, going north later this week for two months sailing
I was taught in my physics classes that there is no such thing as centrifugal force. There is momentum with Newton's first law of motion and there is centripedal force, the force that tries to pull towards a center. However, that doesn't mean the point isn't well taken. The curve of the sail exerts a deflecting force on a mass in straight line momentum.
- Will (Dragonfly)
- Will (Dragonfly)
fun fact: when driving a 1,000 ft ship into a slip it is standard to drop the stern anchor with a short fetch and then use the prop and rudder to maneuver in. if you drive too hard you will push enough water out of the slip and the vessel will settle on the bottom. then you can only stop engine and wait for the water to return, re float the ship. time loss. no reverse allowed. never back over a ship's chain.
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What a crazy thread. Its like listening to 5 year olds explain how how stuff works. Id be no better.
My bs filter filled up almost immediately and have to rinse it off frequently
Science should be easy as "better or worse".
My bs filter filled up almost immediately and have to rinse it off frequently
Science should be easy as "better or worse".
35 years of engineering practice taught me that it is best to approach anything important from 2-3 completely different directions. The problem of lift can be described in terms of momentum, Bernoulli, drag, and extrapolated from wind tunnel data. In skilled hands, they will all give you more accuracy than you can use on the water. Each highlights a different facet of reality, so which is best depends on what you are trying to understand. For example, Bernoulli seems most intuitive to windward, but a centripetal force model can be helpful for understanding a chute and its rotation to windward. Finally, remember that turbulent flow involves chaos and is probably best measured in a wind tunnel, there have been attempts at the math, but they are deep and require more information about variables then we typically have. We'll start with the fact that the airflow to the sail is somewhat turbulent, and that under the water, the boat is yawing and pitching.
Fun stuff.
Fun stuff.
The problem is people try to make science simple dichotomous, yes or no, this or that, but it just ain't the way it works.
My background is psychology and education and we spend a lot of time and energy trying figure out how kids learn and why folks do the things they do. What I've learned in the past 40 years is that if solution is simple, it's probably wrong or at best incomplete.
spinnaker, forward staysail, main, mizzen staysail, mizzen. tweeking the sheets for happy air flow is a fun game. a little off and it's very slow
In sailing we should use the MACRO model that most designers rely on.
Sail design for LIFT, seems to only include forces is 2 dimensions. I often thought if you could disrupt the exit flow off the Leach, you might get more wind power on the same Sail Area.
Jim...
As a database optimizer. Ive seen it the other way around. Complex usually means the wrong solution was deployed. Ive made a living taking the complex and reorganizing to a better usually simpler model.
Start with a baseline... try something diffetent...measure versus baseline. Whats the difference? Better or worse. Throw out worse and note the design patterns for later.
Can the improvement be used as a design pattern?
In psychology we often start with the simple and keeping adding until adding additional factors no longer make a difference.
For example, we try to predict academic achievement and begin with cognitive ability to see how much IQ contributes to achievement, then we'll another factor, say parental educational attainment, and keeping adding until we account for most of the variance. We also look at the relative contribution of each of the factors.
K.I.S.S method = MACRO Model
The simple big view.
Although your Boat is Wave topping and trapping air plus water under your hull, your boat buoyancy is for all practical purposes the same. Even though the Density of the buoyant fluid is changing from time to time.
Jim...
The leeward telltales matter far, far more than the windward telltales because they indicate a stall and a sharp loss of lift. Understanding stalls helps us understand WHY that is, but you make a good point - just keep them flying.Sometimes it is easier to just say "keep the telltales flying."
Simpler often means having lots of small concepts, that work together. The "complexity" is a misunderstanding of how the smaller simple parts work or work together. We smply lack the documentation.
https://www.cam.ac.uk/research/news/how-wings-really-work
Quick, interesting article and 1-minute video for anyone interested.
Quick, interesting article and 1-minute video for anyone interested.