A sailboat can't sail upwind without a keel. A sailboat can't sail on a beam reach without a keel. A sailboat would have a hard time sailing on a broad reach without a keel. You may be able to sail downwind without a keel. What do you know about your keel? At what speed does the keel generate lift? How does the keel and sails work together? What happens when the keel loses lift? We are going to try and answer these questions and maybe a few more in the following paragraphs.
We need to talk about a wind reference when we talk about keels and our reference will be what the water surface looks like while we are sailing. Slick is the water surface we will see up to 3 knots. A combo condition is a combination of slick water and ripples. This could be from 4 to 6 knots of wind. Ripples everywhere is between 7 and 10 knots of wind. One whitecap is 11 knots of wind and whitecaps everywhere is 12 knots of wind. Water and wind is the same everywhere and our reference system will work everywhere. I have used this system before on this Web site and those numbers may be a little different. Sorry about that. The principle remains the same. You need to pick your own numbers and keep them in your head as a reference. At what wind speed does your keel provide lift?
I have a short story that tells a good keel story. After racing a Yamaha 21, I was returning to the slip to park the boat. This is a 2000-pound boat with a 5-foot fin keel. It was designed for ocean racing around the island of Japan and I was racing it on a river and I didn't do very well. I needed more sail area and more water line length to win in our light air conditions. Anyway, as I was coming into the slip, I slowed the boat until the keel lost its directional capabilities and the wind on the hull took control and we docked sideways in a 2 boat slips. The wind on the hull above the water line was more powerful than the water going pass the keel. These forces on the keel and the hull happen all the time. Sometimes the keel and the sails work together and sometimes they don't.
If we have a keel on our boat and we are motoring, the boat will have directional capabilities. It will go where we point the boat. Deep "V" powerboats have directional capabilities and flat bottom powerboats do not. A keelboat under power can't generate lift. If this statement is true, we could say that keel directional capabilities and keel lift are two different things. They are related, but important differences do exist. We need to know the "what, when and how" if we are going to be good sailors.
Directional control is created by the same amount of water that (at the same speed) moves by both sides of the keel at the same time. That's a mouth full of words. Long keel boats track well and short front to back centerboard boats do not. When we talk about tracking, this is what we mean. Lift is when the same amount of water doesn't move by both sides of the keel at the same time. How is this possible? When we are sailing on the wind, the power being generated by the sails makes the hull and the keel move sideways. The water pressure difference on the keel caused by the sails generates keel lift. We generate more lift when we are on the wind, less lift on a beam reach and almost no lift on a broad reach. How much lift do we generate going downwind? No lift, but the keel still has lots of drag. The big question is: At what speed does my keel generate directional control and when does the keel generate lift?
All sailboats and all keels are different. If you own a sailboat and if you have sailed in some strong winds, you should have experienced keel lift. Long keels need more wind and short front to back keels need less wind to generate lift. You can see that sailboats are designed for a given wind environment and your intended use. We have done some research on the boat that we sail the most. Keel lift happens at 8 knots of wind with our Com-Pac 23. It has directional capabilities all the way down to nothing with its long keel. I do not want to race against a light centerboard boat when the wind is less than 8 knots.
If you own a light centerboard boat, it going to be difficult to see the keel lift point. It will happen quickly when you are on the wind. The lift point on a long shoal keel is pretty easy to identify. It's when the boat points the best. It may be 5 or 10 degrees better than pointing in light wind and it will be when the boat is moving at close to hull speed. Pointing when you can't point doesn't work. If you know when your boat can point, you can plan better and get good at sailing.