Airplanes Fly and Sailboats Go Up Wind
Most of know how airplanes fly. Aircraft wings have a curved surface on top and a flatter surface on the bottom. Air moving from front to back over the top of wing has a greater distance to move than the air at the bottom. To have lift, the air has to recombine at the wing's trailing edge. This difference in the air speed across the wing produces a differential in pressure that we call lift. We have less pressure on the top of the wing than we have on the bottom. This lift theory is the same as the venturi effect used in other applications. Carburetors on cars use that same method to distribute gasoline to the motor's cylinders.
As a new sailor, I was told somewhere along the way that sailboat keels provide lift. Since the keels on sailboats have the same shape on both sides, I didn't understand how they could generate lift. I guessed that sailboat lift was not the same thing as aircraft lift. It took awhile to figure out what they were talking about. Sails on sailboats work the same as wings on airplanes. Their shape produces lift in both cases. Most sails can produce lift or power when they are trimmed to about forty-five degrees to the wind. Any closer and a sail will luff. When sailing close to the wind, the sails will force the boat forward. The sails will also produce some power that's at an angle to the boats forward direction. That sail power would cause the boat to slide sideways if it didn't have a keel. This sideways movement generates more water pressure on the leeward side of the keel and less on the windward side. This is what we call keel lift on sailboats.
We can say that the sails produce the force that produces keel lift. If we didn't move upwind, we couldn't get a differential in water pressure on the keel. What does the shape of the keel have to do with lift? The answer is nothing. The keel needs a shape that produces less drag, hold the ballast and has enough area to produce a pressure differential to produce lift while sailing upwind. Rudders work the same way. Large boat rudders need the strength of thickness and they are shaped on the outside to produce less drag. Rudders should be judged on required strength with minimum drag factors for their application. I really like the flat metal rudders on the Com-Pac 16. It has great strength and minimum drag. Some people like the more expensive shaped rudders that are available. Customers say they feel better. We need to have a race some day using both types of rudders. Guess who going to win?
The picture below shows me moving in no wind. I'm using the genoa to catch the wind and direct it towards the main. You can see the wind up in front of the boat. I need to sail through the "No Wind" condition to get there. Water pressure on the leeward side of the keel is low because wind and boat speed are low. The pressure is even lower on the windward side of the keel because I'm sailing close to the wind. How close did I come to the boat that took the picture? I call it very close. We were racing at the time. Larger boats were stationary objects to my rear. When sailing in "No Wind" or very light wind, sit on the leeward side towards the stern and point the keel. You will get an extra five degrees.
We just used a "No Wind" example of keel lift. We happen to have a good picture of light wind sailing. When it's blowing twenty-five plus, not many people have the time to take pictures, so we don't have one for this example. We had a new Precision 18 in stock and the Marines were going to have a race at Camp Lejeune. It sounded like fun and the wife and I trailered over and launched at the marina. The wind at the marina wasn't that bad so I bent on a genoa. That may have been my first mistake. Putting a new boat together and racing on the same day isn't too smart. I think that was my second mistake. Most of the boat parts were there and away we went to the starting line. When we got there, I noticed that we were the smallest boat present. Everyone else was racing thirty plus foot boats. The modest wind at the marina had increased to more than twenty and the first leg of the race would be on the wind. I was going to have my hands full with that genoa.
We were racing boat for boat and not PHRF. The first mark was directly into the wind. I got a pretty good start and because I had never sailed a Precision 18 before, I didn't know what was going to happen. The Precision 18 is a shoal draft sailboat with a centerboard. Its displacement is one hundred pounds more than the Com-Pac 16.
What happen was that the little boat sailed almost directly into the wind. The rest of the fleet was sailing forty-five degrees off the wind and I was a lot closer. My course was a bunch of scallops with some fast close-hauled sailing in-between. I was rounding up and falling off a little and sailing close to the wind. The roundups were caused by too much sail power on a light little boat. It worked because I was the first boat to round the first mark. I was looking good. The beam reach leg wasn't as good as the first leg. I had a big boat getting close and I wasn't rounding up anymore. We both turned the last mark together I knew I was in trouble. I was still in the lead, but only by a few yards. He pulled up behind me and killed my air, then passed me and headed for the finish. I came in second in that race.
Did I raise my centerboard on the last leg? I can't remember, but I should have. Maybe I should have raised it on the second leg? I do it now as a matter of course when I race centerboard or dagerboard boats. Less drag can make a big difference. What happed on that first leg? I had lots of keel lift from too much sail power. The boat moved to within ten or twenty degrees of being on the wind. The roundups were caused by sails that were too big and not balanced. Some of my straight-line speed was sails trimmed on the edge of a luff and a keel that liked all that lift.
We have two examples of keel lift. Little wind and lots of wind. Little boats are similar to little airplanes that can do acrobatics. Little boats can twist and turn and change their attitude using crew weight. Little boats with ballast can normally sail circles around big boats in light wind.
The two examples above prove that keel lift exists. Both examples are at wind speed extremes. Knowing you’re boat's windward performance is a safety feature that all sailors should know. That knowledge proved useful several times when I sailed home without an engine.
The amount of keel lift varies with wind speed. If that statement is true, it means that your boat will sail at different angles to the wind at different wind speeds. Most sailboat owners can point their boats to within 45 degrees of the wind. The big question is at what wind speed did that occur? They might measure their boat's pointing angle and be disappointed in their performance. They think it's their ability or the sails or the boat or whatever. In reality, they are checking their boat's performance in a wind speed that's too low. In high winds, the keel will have more lift and the boat will point higher. If you have a cruising boat, you need to know how well your boat points. A good reference would be between 10 to 15 knots of wind. White caps occur at 12 knots of wind and that would be even more accurate. If you have a reference point for your ability, the boat's performance and a wind speed, then you can evaluate the effects of waves and other conditions. Lots of sailors try to evaluate too many variables at one time. You can get the wrong answer if you have too many variables. How well does your boat point in low and high winds?