Sail Shape Tips
How good is my keel going to weather?
We all know that a sailboat keels prevent us from going sideways when we sail into the wind. Keels really do a lot more than that. They provide dynamic lift just like sails do when we are sailing on the wind. They help us go to weather. Deep draft fin keels and shoal draft long keels are designed to do the same job, but in different environments. Fin keels are more efficient at lower speeds and the deep draft keeps them in the water longer providing more ballast down low in a smaller package with less drag when it really blows. Fin keels are great for deep water locations. Shoal keels are a compromise designed for shallow water sailing. You really can't have a light shoal draft keel boat. The boat would be on its side (not stiff) and the keel would not be in the water and you would be going sideways.
We can say that most shoal keel boat are going to be heavy boats. We can say that the typical shoal draft boat needs to sail upright in heavy air and that design requires a modest sail plan. We are not talking about a shoal draft race boat in light air. Center board boats are somewhere between the fin keels and the shoal keels in sailing efficiency and operation.
As sailors, we want to know when our boat is going to point well. At what wind strength can we count on our keel to provide us with the lift we need to go to weather well. We all remember a time that our boat really sailed well while we were on the wind. That may have been the right wind strength for your boat's keel. Shoal keels can be overpowered and keels can stall. If you point up and the boat continues to slowly move in the same direction, you may have stalled your keel. A stalled keel provides no lift and works like a brake.
We all need to think sails, wind, water condition and keels while we are sailing. What wind and wave conditions are best suited for your keel and what kind of keel is required for your area?
What's The Best Way To Get Good At Sailing?
In 2000 the Federal Government removed some of the restrictions on the accuracy of GPS. Currently speed accuracy is so good that buying a knot meter for speed information may be a waste of money. This improved GPS performance gives sailors a way to measure the sailing performance of their boats and also a way to monitor their own sailing abilities as they improve their technique. Most sailors buy a GPS to find out where they are located. If we sail in areas with land in sight, a GPS may be unnecessary as a locating device. Most sailors today buy and have a GPS on board. To utilize the GPS as a performance measuring tools, you also need a small computer to manage and record your GPS results. PDA devices have been available for several years and they works really well as a recording device for shipboard use. A laptop computer will do the same thing, but needs to be mounted inside the boat where it stays in one place and can't be damaged.
A PDA can be mounted on your hatch board (RAM mounting system) to show speed, course, distance made good and a current plot over a map of your area. We are currently using a COMPAC iPAQ PDA, but many new devices are on the market and priced in the $200 range. Your GPS must have a NEMA-183 output capability and most GPS devices are equipped with that capability.
The computer and GPS is going to help sailors learn about those little sailing details that would have taken many years to identify and master. The computer may help us identify the point where we become really good sailors.
The Art of Sailing?
A sail needs full draft (large curvature) when the wind is light. A sail needs less draft (flat curvature) when the wind is strong. The way we change draft is by stretching the sail at the luff (leading edge) and at the foot (bottom edge) if it's a mainsail. We are looking for a steering difference of about 3 degrees on the wind in most cases. If we flatten the sail to much in very strong winds, the steering difference between a luff and a stall becomes less than 3 degrees and makes for difficult steering. A flat sail can be so flat that the grove disappears completely. If we have a lot of draft (very light wind conditions), steering is also difficult because the grove is very large. Sailing the luff with a large draft is the only way to identify the correct trim position. The boat will sail OK off the luff position, but would be sailing below the best course for upwind sailing. In light conditions with full draft, experiment with luff and stall to see how large the grove is. It can be as large as 30 degrees wide in very light conditions. Add that to the 45 degrees of a normal close haul course and you could be on a beam reach. A close haul course of 45 degrees plus the grove width of 30 degrees comes to 75 degrees. If you sail the luff in these light conditions, you will be closer to the most desirable course, 45 degrees.
Both over draft and under draft are features of poor adjustment or extreme wind conditions. Most of our sailing falls into the 3 degree grove condition and steering is a bunch of little adjustments to maintain proper trim and avoid stalling your sails.
What is the most important part of sailing? Our first requirement is to maintain boat balance while sailing. Weather helm tells us about balance. Our next decision after balance is to determine if we need to reduce or increase our sail power. We have been talking about increasing sail power through good sail shape, but many times we want to reduce sail power while maintaining boat balance. It's great to practice and talk about optimum performance, but a balanced boat is more important. However, talking about fine tuning a sailboat for optimum performance and seeing it in action now and then will increase our understanding of sailing in general, even if we don't practice the finer points of sailing all the time. Adjusting and balancing can be a lot of work if your not careful.
The Smart Sail and The Strong Sail?
A sloop has two sails, one smart and the other not so smart. The jib is the sail in free air. It's not impeded by a mast or some other sail. It's in front and that's the sail that determines how close we can come to the wind. We adjust the jib first and then the main. For the sake of argument, our boat is always going to windward, or on the wind. We know that the sail maker designed the jib to have a maximum draft at the 33% point in most cases. If we make an imaginary horizontal line from the luff to the leach (front to back), the maximum curvature would be at the 33% point. If we look at the main, the maximum curvature would be at the 50% point or half way back on the horizontal line. In our average sloop, in 8 knots of wind, medium to light tension on the jib and main luff, the maximum draft would be at the 1/3 point on the jib and 1/2 point on the main. We would have very little if any weather helm. Our boat balance would be perfect with minimum drag. This 1/3 point for the jib and 1/2 point for the main is were the sailcloth is taunt or tight. Before and after that point, the sailcloth will have some wrinkles. You can look at sails and see were the power is located. This maximum curvature will continue from the foot of the sail to the head of the sail (bottom to top).
I have three tell-tails on each side of my jib. They are position to allow me to see through the sail and observe all six at the same time. They are positioned about 6-12 inches behind the luff. Some sailors like red on one side and green on the other. If we are on the wind and we are, the windward tell-tails will be at the 45 degree point and the leeward tell-tails will be at 90 degree point or horizontal position. If I point up (steer towards the wind), the windward tell-tails will point up to the 0 degree point and my jib will luff (like a flag flutters). If I drop off (steer away from the wind), the windward tell-tails will drop down to the 90 degree point. If I continue steering away from the wind, the leeward tell-tails will fall down and I have stalled my sail. A stalled sail looks good, but has less power, more drag and causes a lot more heal. That's bad form.
When the jib is adjusted correctly, the main can be adjusted to the jib output. The main sail is getting all of the windward air off the jib through the slot between the jib and the main. That makes the main a strong sail. With single tell-tails attached to every batten on the leach of the main, observe the tell-tails trailing aft as you adjust the main's angle of attack. When the bottom ones trail aft and the top one trail aft, you have the right adjustment for the correct angle of attack and twist.
The difference between luff and stall in most conditions is 3 degrees. As you increase the tension on the luff in higher winds, the sail becomes flatter and the difference between luff and stall becomes less and makes for difficult steering. We call this 3 degrees the grove. That 3 degrees of angle of attack is the same thing as the jib's windward tell-tails moving the 90 degrees from luff to stall. If you are a lazy sailor as I am, skip the tell-tails and sail the luff. When the jib shakes, fall off some and then make a slow curve back to windward and do it again.
A Wind Speed Reference?
If we are going to adjust our sails when the wind changes speed, then we need a reference to determine wind speed. There are wind machines, flags, birds, trees and many other methods or ways to determine wind speed. The method I use is to look at the water. The water is in my line of sight and the water surface responds to wind speed changes. A reference is an event that is repeatable in the same conditions over and over again. If the water is slick, like a mirror, I reference the wind at 0-3 knots. The tension on the luff will be zero. We raise the sails to the top and do not stretch the luff. If the water has dark spots or small pools of wind, I reference the wind at 5 knots and use minimum tension, almost zero tension on the luff. If the water has small waves that are everywhere, I reference the wind at 6-9 knots. The luff should be on the low side of half maximum stretch. I reference the wind at 10 knots when I see the first white cap and 12 knots when I see white caps everywhere. This is the wind speed that I use maximum luff tension. Maximum luff tension will move the draft forward and balance your helm reducing weather helm and drag. All of these reference points may change a little with different boats. Lighter boats will adjust more often and sooner. Heavier boats will adjust less often and at a higher wind speed. The water's surface is an excellent reference and it's available to everyone, anywhere. How do we balance our boats when the wind speed is over 18 knots?
What Causes Weather Helm?
Our first question is what is weather helm? Weather helm is an imbalance in sail power that will require rudder movement to maintain a straight course. All sailboats should have some weather helm to maintain good feel at the tiller. That's a small amount, less than 3 degrees deflection at the rudder blade. If you have more than 3 degrees, the rudder blade no longer has lift and will produce a large amount of drag. You can identify rudder drag in sailboat pictures by looking for the wave or rooster tail following the rudder. If a picture of a sailboat has a rooster tail behind the boat, it has drag. Of course that's what sailing is, lift overcoming drag. If we have more lift in the sails, keel, hull and rudder than we have drag, we will move forward with speed instead of moving forward with less speed. Speed in a sailboat means control. If you have a sailboat that's not balanced, you will be slow and will have less control. What causes weather helm? As the wind increases, the draft in the sails move aft. Since power in sails is at the maximum draft point and the draft moves aft as the wind increases, we must adjust the rudder to maintain our course. To reduce weather helm when the wind increases, we increase the tension on the luff of the jib and main. The luff of sail is the leading edge of the sail. When we stretch the leading edge, the draft will move forward in the sails reducing weather helm. Sails are adjustable, stretchable airfoils. Airplanes have slats on the wings to compensate for the difference between fast cruising speeds and slow landing speeds. Most sails can be stretched a lot more than we think. If we over-stretch the sail luff, the amount of wind will not be able to completely shape the sail into a nice airfoil. There will be "S" shape at the leading edge. With a wind speed of 0 to 15 knots in most boats, the stretch in your sails is the same as the gear shift in your car. You need several gears in your car for good performance and you need a 0 stretch at 0 knots and maximum stretch at 15-18 knots in your sailboat. If you run your car in one gear, your car will be slow and you will have less control. If you run your sailboat with one sail adjustment, you will have weather helm. Make sure you adjust your luff for the current wind conditions and adjust as necessary when the wind speed changes for maximum performance.
Genoa Track Adjustment?
Adjust the Genoa luff with the jib halyard for the appropriate wind strength. Adjust the Genoa track to make all of the windward telltales move as one. When we point up, the windward telltales go up and when we fall off, they come back down. When the windward telltales move from a vertical position to a horizontal position, we have changed our heading by 3 degrees. When the windward telltales point up, we are at the luff point for the headsail and it should be shaking or losing power and when the windward telltales drop or point down, lower than horizontal, we are stalled. Going from luff to stall is 3 degrees of heading or less. If we have a very flat sail, it could be less than 3 degrees. Makes for very difficult steering.
With this boat we are going to try and keep the windward telltales at a 45 degree angle. Our boat is sailing on flat water in about 10 knots of wind. We know it's 10 knots because we see a white cap now and then. White caps everywhere would be 12 knots of wind. Our headsail twist is correct because we have all of our windward telltales at the same angle. We are doing a great job and the boat is in the grove. We test the telltales now and then by pointing up just a little bit to see the telltales move higher and then fall off just a little to regain the 45 degree position. Most sailors test the luff now and then to make sure they are not stalling the headsail. A stalled headsail looks good, but makes the boat heel more and produces less power.
The Genoa track is correctly adjusted to produce the telltale action above. We need to mark this position as our upwind or pointing position for our Genoa track. Now that we have everything the way they should be, we are going to change our heading to a beam reach. The wind will be off our beam. We need to readjust our headsail trim for this heading. The leading edge or luff of the headsail should be pointing into the wind. To do this we must let the jib sheet out to a point that the headsail almost luffs. We are back to the 3 degrees again. Let it out to much and the sail luffs. Pull it back in to much and the sail is stalled. Remember the story about the 3 bears and the 3 bowls of porridge. There is only one position that's just right. That position in sailing is "almost luffing".
When we change course to a beam reach, the twist in the headsail increases at the top of the sail to a point that a large portion of the headsail at the head is luffing. What can we do to improve this situation? We can move the Genoa track as far forward as it will go. When the Genoa track goes forward, the pull on the Genoa leach will increase and the foot will decrease and more of the headsail will be in trim from top to bottom. That means we have two adjustment points for the Genoa track, the upwind position and the beam reach position. In reality, we have more than two positions, but we will save the other adjustments for the next installment of Genoa Track Adjustment.
As a sailboat moves through the water, air at the head of the sail is moving faster than air at the foot of the sail. This is caused by drag induced by the water's surface. Air close to the water moves slower than air 10 feet above the water and air 10 feet above the water moves slower than air 30 feet above the water and so on. As we pass through the air, the trim angle of the upper mainsail is pointing at it's trim point and the mainsail bottom is pointing at it's trim point. If the sail maker did a good job, the rest of the sail in-between is also in trim.
A good sail is designed to handle this difference in wind direction from top to bottom with twist. It's cut to provide the necessary twist and with tell tales on the batten pockets, we can verify that twist is correct. The hard part is trying to maintain twist in light air with the weight of the boom pulling the sail down and removing the twist. Holding it up a little by hand normally works OK.
Our job as sailors is to maintain twist. If we pull the main sheet tight so we can point higher, we remove the twist and stall the upper part of the mainsail. A stalled upper mainsail will provide very little lift and a lot of heel. If we have a force trying to turn us over and that force is pushing at the top of the mast, it has a big advantage because of it's location at the top of the mast.
The tools of the trade for maintaining twist is the mainsheet and traveler. The mainsheet sets the twist and the traveler adjusts the angle of attack or trim angle. If you do not have a traveler, you can use a vang to prevent the boom's upward movement and then use the mainsheet for angle of attack adjustments.
The question for the month is: On a standard sloop, which luffs first when changing course, the mainsail top or the mainsail bottom and why?