What is Keel Lift??
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- čas přidán 29. 08. 2024
- A short description of keel lift on a sailboat and how a symmetrical foil (the keel or centerboard) needs an angle of attack in order to create lift. This angle of attack is created by the lee way (sideways drift) of the sailboat as it moves through the water. The lift developed by the keel is a force called "lateral resistance", that then counteracts and reduces leeway. Clear as mud, right?
Thanks! I've been looking for an explanation of this for years. Very satisfying to finally hear an answer and understand it.
It was gnawing at me for a few years as well! My inquisitive mind would not let it go, so I finally just had to do some research. Glad you enjoyed it…many thanks!
Well done 👍 buddy .. I’ve cheeed on the subject n the diagram n presentation are very helpful tech info n you lay it out very logically n clearly. Great job buddy. Looking forward to more videos.
Cheers.. Nick.
Nice discussion, Dave!
Wooo ! That’s cool .. but need to take that one in again. I’ve flown n studied lift … buttttt . Need to hear that again .. ôi trời ơi (Vn for oh my goodness)! We’ll explain but my 77 yr old brain 🧠 needs to chew on that one again. Thank goodness for replay buddy. Much appreciated ! Cheers 🥂 Nick or Papa J
So my question is what keeps the boat from heeling over even more? Say you are looking at the boat from the aft, and the wind is coming from the left, blowing towards the right side of the boat (you are on a port tack). The center of pressure on the sail (CP) is creating at that point a clockwise moment about the Center of Mass (CoM). That will cause the boat to start drifting to the right, meaning the keel is now generating its OWN lift to the left to reduce the leeway. However, the CP of the keel is now generating ANOTHER clockwise moment about the CoM, meaning the boat SHOULD be heeling over MUCH more with the keel down than without it. Obviously from my own personal experience, this isn't the case. In fact, its the opposite.
My guess for why is that when the boat would heel over, the keel goes with the apparent current, reducing the lift generated by the keel pretty dramatically at the bottom point, allowing the boat to keep from heeling too much further. I guess also the keel serves as a massive heavy ballast that naturally resists heel over. And just the increased drag from the keel trying to rotate in the water would serve to resist that motion as well. I don't know though. Any insight would be appreciated.
@@cadenorris4009 Sailboats and the dynamics of sailing are all about balance. For every force, there is an opposing force that tries to restore the balance. So, yes, when the keel develops lift (lateral resistance) it does contribute to heeling. But, there are several other things that are also at work. First, as the keel foil is heeled at an angle, its efficiency at producing lift (lateral resistance) is reduced. This reduced lateral resistance increases the leeway the more the boat heels, but also reduces the heeling effects from the keel. Thus, one of the reasons why it is said: “flatter is faster.” Because keeping the boat flat reduces leeway and gets you to your destination on a more direct and faster route. Second, the heeling force on the sails is reduced as the boat heels. The sails become less efficient and produce less heeling force as the boat heels because the angle to the wind changes. Less sail area is presented to the wind in the horizontal plane, so the sails have less drag as well,“spilling” air out the top. Third, the components that counteract heel (buoyancy and ballast) create more righting moment as the boat heels. As the boat heels, more of the hull on the leeward side of the boat is pushed into the water. This creates more and more buoyant force (because of the additional submerged airspace), trying to push the hull back into an upright position. Similarly, the ballast also rotates below the surface more to windward, increasing the righting moment and counteracting the heeling forces (think of carrying a 20 lbs weight in your hand with your arm straight down, versus out at a 45 degree angle-much harder to do at a 45 degree angle). Finally, if you pull up a swing keel (which is ballasted) you will see a reduction in righting moment and increased heel because you are moving the ballast closer to the center of gravity and reducing the moment arm of the ballast. However, I have a lead ballasted shoal draft (fixed) keel boat with an (unballasted) centerboard. If I raise my centerboard, heeling is in fact reduced (by about 5 degrees) and only leeway increases. Hope this helps.
Thanks. Very helpful. One comment: force is newtons not tons 🙂
Touché! My usages were the other definition of ton “a large number or amount”. 😆
I think in this discussion the Term “Lift” doesn’t make sense!
It more of lateral stability. C-G stability or improved reluctance to change, lateral drag?
Anything, but in modern word of foils, and the reliance of looking at aeronautical terms! “Lift” makes little sense.
Aerobatic planes can use identical parabolic shapes, creating zero lift so controls no matter are same! Trim is used to give (actual) lift.
Foils use this aeronautical understanding to create “Lift”, by having shape like “most planes” one sized curved/ one side flatter.
Mass flows under the shorter side faster than longer curved side. Creating upward force “lift”.
Nothing wrong either article, it’s just area of Marine vocabulary that is off base! Especially since “Aero-nautical” sciences are leaned on so much
Good video! Not trying to attack this man’s knowledge.
More general statement of “??” Against, much of Naval language, in this subjects moment. Could be overlooked for last 100 years but with foiling it needs to be presented clear.
“Keel lift” has been used as a term since foil shapes were incorporated into keels (and rudders), but I agree that it’s confusing because the layperson tends to think of lift as an upward and vertical motion…not lateral. However, lift in scientific fields is defined as occurring “perpendicular to the flow” and thus lift does occur in any direction. Perhaps “foil induced lateral resistance” would be more clear.
I'm just making sure i understand, lets say the boat is on a port tack close reach, the lift is on the port side of the keel? Like the windward side of the boat? Or the opposite, on the leeward side?
Correct. The lift is to windward and creates lateral resistance below the waterline. The lift and lateral resistance counteract and reduce (but cannot fully eliminate) the leeway (sideward slippage downwind). Basically the keel’s lift helps prevent the boat from being blown downwind. If you’re thinking “wouldn’t it contribute to heeling?” It does just a bit. But luckily, as the heel increases the foil generates less lift (because the angle of the foil changes, but the direction of the water flow stays the same, making the foil less and less efficient as the boat heels more and more). That’s also one of the reasons people say “flatter is faster”-because a flat boat will have less leeway and can tack to its upwind destination more efficiently.
Great explanation, but I have a fourth question. What stops the two lifting surfaces from heeling the boat right over, because they are acting either side of a pivot point. Is that simply the buoyancy of the side in the water being counterbalanced by the weight in the air. I am working up a sailing rig for my canoe which doesn't have any width to speak of so some type of sponson may be required. All fascinating stuff.
I believe that’s right…the righting moment is a combination of the buoyant forces increasing as you heel and the effect of static ballast and/or human ballast (depending on if it’s a keel boat or dinghy). Also why it’s important to reduce sail area as the wind picks up. The amount of righting moment definitely has a limit. An outrigger will help provide additional stability with minimal drag.
I'm afraid I cannot accept the airplane wing analogy because an airfoil compresses and slows the air on the curved side creating a low pressure zone hence lift. Water cannot be compressed so this principal does not apply. Not convinced there is actually any lift going on here. It seems an eddy is created on the windward side which may actually cause some resistance.
Just a thought. Needs further investigating.
One word: hydrofoils. Right? It’s a foil attached under a board or boat that when moving really fast (by sail power or towed) creates lift and lifts the hull or board well above the water surface. We’ve all seen them in action. Exact same principle except in the vertical plane. Water cannot be compressed, but it does transfer energy and force. Put a water storage tower way up high and all the water pipes are pressurized. Not because the water is compressed, but because the force of gravity and weight of the water is transferred or conveyed directly through the incompressible water to the faucet at the end of the pipe. So a foil moving through water is creating pressure differences on each side of the foil. Just like when you move your hand through water…you can feel the force of the water pushing back resisting your movement on one side of your hand, but no pressure (low pressure) on the backside. Of course, your hand is not a foil and the angle of attack of a keel is only 1-2 degrees creating a nice, smooth low pressure flow on the windward side and a higher pressure flow on the leeward side.⛵️
I guess the use of the word pressure is the culprit. A water ski, type of foil, uses the water density, no lift involved, however quite a bit of fiction produced. The air on top of the ski does not contribute to keeping the ski on the surface, I don't think the water on the top side of hydrofoil contributes either, that foil could just as easily plane along surface. Still have doubts.
Hi William. The concept of lift arising from fluid passing around an aeroplane wing, or a sail or a keel will be easier to accept if you first accept that the air or water that passes over the longer, curved side of the foil actually expands. It doesn't compress. It expands as it rushes over a longer path, to meet up at the trailing edge with the fluid it was separated from, which went along the shorter path. Because it has expanded, the pressure the fluid exerts against the adjacent surface of the foil is lower than it is on the side where there is no expansion. The higher pressure on the side with no expansion pushes the foil away. Therefore, in the case of an aeroplane wing, it's pushed up. In the case of a sail, it's pushed out sideways, to leeward. In the case of a keel, it's pushed out sideways to windward. Why to windward for a keel? This is where the magic of leeway (or sidewards) movement of a keel-boat does its thing.
The keel is symmetrical, so that it can generate lift when the boat is on either starboard or port tack. When the boat is sailing the keel is moving mostly forwards but also slightly sideways. It's crabbing through the water. There is leeway. At the leading edge of the keel the fluid is split, to pass either side of it. For the fluid that passes on the side of the keel that the crabbing movement is going towards (the leeward side), the distance from the point of fluid separation to the point of fluid reconstitution (leading edge to trailing edge) is less than on the side of the keel that the crabbing movement is going away from (the windward side). You can picture this by imagining yourself and a friend treading water together as a sailboat approaches. It's crabbing through the water quite noticeably. As the bow is about to separate the two of you, you agree to meet at the stern. Your friend is lucky and swims along the side of the boat that was crabbing towards you, whereas you go down the other side; the side you couldn't see as the boat was coming towards you. To meet at the stern, you have to swim faster, to cover a longer distance. Now imagine you're in the water with a thousand of your friends, all bunched up as the sailboat approaches. Those who go down the leeward side of the boat stay at about the same density. However, those who go down the windward side of the boat are spread out as they swim quickly to meet everyone at the stern. They have to cover a greater distance. Their density in the water is reduced. If we switch back to thinking about a crabbing keel, this difference in density of water going either side of it pushes it to windward.
So the sails are pushed to leeward by air pressure differences and the keel is pushed to windward by water pressure differences. These opposing forces mostly cancel each other out, apart from a proportion of the forces on both foils which are pushing forwards, due to their bulbous shaped forward edges. These driving forces make the boat move forwards.
But if the sails are pushed leeward and the keel is pushed windward, why doesn't the boat just flop over onto its leeward side? The answer is that as the boat leans, the centre of buoyancy shifts sideways to leeward while the centre of gravity stays put. This creates a strong lever arm effect which wants to stand the boat upright.
And there we have it - monohull yachting 101. 🙂 Regards, Rick.
great video, i also was looking for it for years and i ended up.same conclusions, however still missing a bit the effects of body weight aft/stern and relative keel angle lift... despite many years of Laser Sailing nevrr found a full.scientific explanation
Heeling results in significant decrease in keel lift and thus more leeway. A tilt aft (or forward) by shifting weight would have very little effect as it would not change the angle of attack of the foil, the fore/aft angles are much smaller, and the airfoil shape is still pretty well lined up with the boat’s direction of travel. I would think the change in lift produced would be very slight. On my boat, however, I can raise/lower the CB which angles it aft. If I place it at a 45 degree angle, it reduces lateral resistance, reduces heel, but also shifts CLR rearward…creating lee helm.
Does keel lift counteract heel? Your drawing makes it look like the keel lift (to port), plus wind (from port) would make heel (to starboard) worse. What am I missing?
You are not missing anything. Keel lift does not counteract heel. The ballast in the keel is the main force counteracting heel. Increased buoyancy of the hull as it heels (and more of it is submerged) also counteracts heel to some extent. Finally, placement of crew (aka: “rail meat”) counteracts heel.
@@ColoradoSailing I initially thought about how keel and mast rotated around the hull's longitudinal axis. If the keel was lifting, it should add to the sail lift heeling the boat.
I needed to understand the situation from the vertical axis. The keel lifting force is keeping the hull tracking and lessening the amount of leeway. Thank you for the video and good (correct) explanation.
Nice video!!!
One question. Sailing close hauled with increasing heel would you head up or bear away? I have experienced in boats boats with a bulb that bearing away reduces heel. Which is the opposite I would do in a dinghy,(ex. Laser). Do you have an explanation?
Close hauled with increasing heel you would either want to head up in a gust (pinch) or let the mainsheet out. Depending on the boat and how it is equipped, you could also loosen the vang or move the traveler to leeward during a gust…but my boat doesn’t have a traveler and the vang isn’t easily reachable for such an adjustment in a gust. If the boat is just too overpowered sailing close-hauled, then you need to bear away and change your point of sail. I often sail on a beam reach to reduce heel or just run downwind when the wind gets too heavy.
@@ColoradoSailing 👍thank you!!