Everyone who has sailed a few times will have experienced it. You are having fun sailing relatively fast. The boat is heeled more than it should and you are pulling the tiller to keep her on course. Suddenly… a gust! And then other weird things start to happen. You are not heeling anymore but struggling to control the boat and not be thrown overboard. For some unknown reason, the boat is now possessed by an evil and disobedient spirit and not under your command anymore. She wants to turn into the wind no matter how hard you try to stop her.
If you have not yet experienced this apparition before, I’m sure ‘It’ will come to you one day. So, what kind of supernatural force is taking over? Let’s call it the omnipresent weather helm! This is the opposite of the lee helm, an attempt of extra-terrestrial forces to turn the boat away from the wind.
Why does it happen?
What is the origin of such a mysterious entity? Some will say that there is nothing supernatural in it, and that it is the pure result of simple physics. This is the story that they will tell you:
When a sailboat is not heeling too much and the mast is almost upright, the hull cuts the water in a nearly symmetrical way. This symmetry produces similar hydrodynamic forces on both sides, and therefore there is no strong tendency to turn it to any side. However, this symmetry is lost when the boat is heeled. This occurs because a heeled boat has a lot more of the lee side of the hull submerged than of the windward side, and this produces a turning moment that pulls her into the wind. More importantly, when the boat is heeling only slightly, the centre of the aerodynamic force pulling the sails (centre of effort, CE) is almost along the boat’s centreline (only slightly to the lee side). However, when the boat is heeling heavily, the sails are over the water. If you think that the centre of this force is somewhere between the foot and the head of the sail, then it is easy to see that the more she heels, the further to the lee side the force will go. The increase in the arm of the force results in a turning moment that will swing the boat into the wind. A similar effect, but in the opposite direction, is produced by the drag force exerted on the keel, leading to an even stronger turning effect.
In addition, heeling will put the rudder in an awkward (non-vertical) and less efficient position, making it hard for you to steer the boat away from the wind.
Blah, blah, blah. Don’t listen to them! This is all about evil spirits trying to sink your boat.
How to avoid it?
There are few things that you can do to save you from the deep darkness of the weather helm. The most urgent action is to reduce heeling, and the best way to do it is to reef the main. Reef whenever you remotely suspect that you will need it. While on light racing yachts and dinghies heeling can be reduced by moving people to the windward side, this will make no significant effect on heavy cruising boats. In this case, the best salvation to the weather helm is: don’t let It come by having appropriate and well-balanced sail area. As soon as you start feeling ‘Its’ presence (you will feel the helm getting heavy, starting to be pulled by that invisible force), then it’s time to reef. Reefing will reduce the forces on the sails and heeling. It may sound counter-intuitive, but reefing an overly heeled boat can actually improve speed. This happens because heeling causes a lot of unnecessary drag that slows the boat down.
You should always start by reefing the mainsail, because its centre of force is behind the centre of lateral resistance (CLR, see a short description below if you do not know what it means). As the CLR acts as a pivot point, and because the centre of effort of the mainsail is behind the CLR, reefing the main will (in addition to reducing heeling) reduce the tendency to turn into the wind. On the other hand, reefing the head sail would probably make things worse, as it actually helps reduce the weather helm by pushing the bow of the boat away from the wind. In fact, if the boat keeps trying to turn into the wind after you’ve reefed the main and heeling was reduced, increasing the area of the headsail (or any sails forward of the CLR) can reduce the weather helm. By fine-tuning the areas of main and head sails you can achieve a perfectly balanced sail, in which case the helm feels very light. If you have not reefed and are suddenly hit by a gust, the best you can do is to spill the wind --let go the main sheet and/or steer into the wind. Dinghy sailors do this so frequently that it becomes an automatic reaction. Dinghies are extremely light and in general very susceptible to heeling (and capsizing!), so this is one of the first lessons dinghy sailors learn.
If your boat shows a consistent tendency to sail into the wind, you might need to reduce the mast rake. If you have ever sailed a windsurf you will know that you can steer it without a rudder by simply moving the mast aft or forward. Moving the mast forward will bring the CE forward of the keel and this will turn the windsurf to leeward. Bringing the mast aft will take the CE back and turn it into the wind. On a sailboat, you can obtain the same effect by tuning the mast rake –the angle between the mast and the vertical. Reducing rake will bring the CE of the main sail forward, which helps reduce the weather helm.
Finally, a tiny bit of weather helm is actually welcomed and considered safe by many sailors. If, for example, you are sailing single-handed and fall overboard, a boat that is possessed by the weather helm will turn into the wind and gradually stop. When the helm is perfectly balanced (neither weather nor lee helm), you also loose the feel of the helm, which is uncomfortable to many people. So keep in mind that we all like a tiny bit of evil, but do not let it take control!
Centre of lateral resistance
The centre of lateral resistance of a hull is a theoretical point that helps us to understand many aspects of the behaviour of a boat. The idea is that it represents a pivot point about which the boat will rotate when pushed. For example, imagine that a boat is docked with all lines on board. If you push the bow of the boat, the stern will probably swing back onto the dock. Conversely, if you push the stern, it is the bow that will come to the dock. However, if you push the beam at a particular point (the centre of lateral resistance, CLR), the boat will move sideways, and bow and stern will be moving away from the dock at the same speed. For sailboats with a fin keel this point is typically located somewhere at the keel, but this location will, in general, depend on the shape of the hull and keel.