Balancing the boat between weather and lee helm creates the most efficient and effective way to sail any boat. Weather helm is the tendency of a boat to turn itself into the wind (weather). Lee helm is the tendency for a boat to turn itself away from the wind or downwind. Too much weather helm and she's not only hard to steer but slower than if balanced. Too much lee helm and she wants to sail down wind. To understand how a sailboat balances we'll take a look at a simple weather vane.
Weather vanes are designed to always point into the wind. They do this because the back vane is larger than the forward vane. Another important factor is where the axis of rotation is located. Here, it's located in the middle. Since the vane in the back is larger the point at which the two vanes balance each other is behind the rotation point. Move the axis of rotation further and further back and sooner or later you'll reach the balance point of the two vanes and the weather vane will no longer point into the wind. Instead, it will swing back and forth never really coming to a direction. Move the rotation point even further back, behind this balance point, and the weather vane will point down wind.
Here, the weather vane points downwind because its point of rotation has been moved to the very back. With the weather vane there are two variables at work: 1) The relative size of the two vanes and 2) the position of the axis of rotation. Sailboats work exactly the same way, let's see how.
The sailboat here shows the center of effort for the jib and the main and the combined total CE for the sail plan. On the keel is labeled the CLR or center of lateral resistance which is the turning axis for the boat. The CE is calculated for each sail by drawing three lines; one from each corner to the middle of the opposite side and where the lines intersect is the CE. Then, the CEs are combined to find the overall CE. The CLR is calculated by the designer but you could figure it out by pushing on the side of the boat at different places until you found the spot that, when pushed, would produce a perfect sideways movement.
To have a perfectly balanced sail plan it would seem that, as in the weather vane above, the CE and the CLR should line up. What actually happens is the sailboat will automatically, because of the shape of her hull, develop weather helm as she heels. The CE will then need to slightly 'lead' the CLR to create a boat with initial proper balance.
So what does all this mean? A boat that has proper balance between weather and lee helm will be easy to steer and will also make her way efficiently through the water. Imagine steering a boat with no detectable helm in other words she has a perfect balance; there would be no 'feel' to the steering wheel or tiller. For a couple of reasons it is good to have a slight amount (about 3°) of weather helm. This gives the needed 'feel' to wheel or tiller, the rudder works most efficiently, and, in the case of the helmsperson falling overboard, the boat would have a tendency to round up into the wind.
What's wrong with too much weather helm? Think about this: how do you counteract the tendency for the boat to turn into the wind? What do you do when you want the boat to go in a certain direction? You turn the rudder. So, the more weather helm the more you need to turn the rudder. Not only is this quite tiring but it is slowing the boat down. When you drag a large appendage sideways through the water it takes a lot of force to make it move. This force could be going into driving the boat faster but you are now using it turning the rudder sideways slowing you down.
Properly balanced. How do you reduce weather helm to the correct amount? Too much weather means that the CE is too far aft, how can we bring the CE forward? If we made the main smaller by reefing it that would bring the CE forward. If we move the traveler to leeward that also brings the CE of the main forward thus bringing the combined CE forward. We could rake the mast forward by tightening the forestay and easing the backstay. We could induce mast bend by tightening the backstay.
You might not think we could adjust the CLR of the boat but, on smaller boats, it's pretty easy to do. Too much weight in the bow creates weather helm by bringing the CLR forward. Move weight aft to decrease weather helm.
When you are making adjustments try to keep in mind where the CE and CLR are and how you want to adjust their relationship.
What might we use lee helm for? We know lee helm means the boat will naturally want to sail itself downwind. How about if our destination lies downwind and we'll be traveling on a run or close to it? We might very well choose to be lazy and just use a large sail forward such as a cruising spinnaker. Maybe not quite so fast as if we also used the main but the boat will be easy to control and sail with no possibility of an accidental jibe.
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