What is Keel Lift?? 

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!

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.

Touché! My usages were the other definition of ton “a large number or amount”. 😆

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.

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.

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!!

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.

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.

“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.