What is weather helm and how to fix it in a catamaran?
Although most sailors understand whether helm and how to fix it, the underlying causes of weather helm in a catamaran and the reasons why common fixes work are complex and often poorly understood. Yet, understanding weather helm in depth is critical for sail trimming and boat balancing. In this article, I explain the basic processes at play that produce weather helm and cover some misconceptions about the role of the sails on this issue.
What is weather helm?
Weather helm is the common phenomenon in which the boat tries to turn towards the wind when underway in moderate and high winds. The opposite, lee helm, is when the boat naturally tries to turn away from the wind. Lee helm is significantly less common in catamarans (you will see why below).
Although you often hear that a little bit of weather helm is good, too much weather helm means that the helmsman would need to keep the helm turned too much to leeward in order to keep going straight. This causes the rudders to be too open, which act as a brake slowing down the boat significantly. Too much weather helm slows you down because it requires too much rudder to keep the boat straight. Too much rudder = slower boat.
Now, to understand why weather helm happens we need to understand the concept of Center of Effort. The boat’s Center of Effort (CE) is the location, on an aft-forward plane, where the force of the wind is concentrated pushing the boat sideways to leeward. On a “perfectly balanced boat” the center of effort is located such that the entire length of the boat is pushed equally to leeward. The picture below presents an example of a perfectly balanced boat such that the lateral force is equally distributed across the length of the boat on the aft-forward pane. This lateral force is then converted into forward force by a combination of the boat’s lateral resistance (keeping the boat from moving sideways) and the sail lift.
Weather helm happens when the CE moves aft of the midship. As the CE moves aft, the distribution of the lateral force gets uneven, such that the boat experiences more lateral force aft as compared to forward. Essentially, there is too much lateral pressure on the back of the boat. This pushes the stern of the boat to leeward, which makes the boat pivot and turn the bows towards the wind. This is essentially the same effect that makes a dinghy turn to port when the motor pushes the stern of the dinghy to starboard.
Thus, the underlying cause of weather helm is having the Center of Effort too aft of the midship of the boat. But what causes the CE to move aft?
The culprit is most often an imbalance between the main and the jib (i.e., an unbalanced sail plan). When a catamaran has too large of a mainsail in relation to the size of the jib and/or the mast is too aft, the center of effort will move aft increasing the weather helm effect. Most catamarans have very large mains creating a significant sail imbalance and thus significant weather helm. Catamarans also have very large coachroofs that are often located aft of midship. This adds windage on the back of the boat, especially when the cockpit is enclosed, which further pushes the CE aft. This issue is further maximized in the new Lagoon catamaran designs (Lagoon 42,46, and 52) because the mast was moved aft to allow for a self-tacking jib (although there are other benefits of aft masts). The large size of the main, plus the location of the mast, plus the large windage back of the midship if the cockpit is enclosed, means that there is simply too much lateral force on the back of the boat.
How to solve weather helm?
The solution is to move the center of effort forward towards the midship. You do this by powering the jib and depowering the main. The two most effective ways to depower the main is to reef or to increase twist so that the upper main is depowered. See my previous trimming article with tips on how to increase twist. In short, you increase twist in the L42 by raising the boom. To do this, you need to increase the distance between the traveler and the desired boom location (angle of attack). The longer the distance, the more you will need to ease the mainsheet to maintain the boom location. This will raise the boom and increase twist. In contrast, if the traveler is very close to the desired boom location, the mainsheet will be very tight. This will tighten the leech, reduced twist, and can overpower the main. In sum, the easiest way to reduce weather helm is to increase twist by trimming the main such that traveler is significant to windward of the desired boom location. In our boat, the traveler is usually 1m (3ft) to windward of the boom and the boom is always quite high.
However, in very high winds this may not even be enough. For example, a Lagoon 42 owner recently described significant weather helm in 40 knots of wind when sailing with no main and a reefed jib. In this case, even taking the main down completely did not move the CE forward enough. The likely culprit, in this case, was the windage created by the enclosures of the cockpit, which essentially overpowered the jib. Although my instinct would have been to open the jib more, I realize that a better solution may have been to take down the helm and cockpit enclosures. That may have been enough to allow the jib to move the CE forward.
Isn’t some weather helm good?
Yes. You often hear that some weather helm is good and too much is bad. Likewise, you often hear that you almost never want to have lee helm. But why?
A common explanation is that weather helm gives the helmsman feedback on the rudders while having no wether helm does not provide any feedback. This is true, but it is not what makes some weather helm desirable. Another common answer is that weather helm makes the boat safer because if you fall off the boat the boat would start turning into the wind and eventually go into irons and stop. This is also true but it is not really the reason why weather helm is good from a performance perspective. Instead, there are two reasons why some weather helm is good, especially on catamarans.
In most catamarans, the mainsail is much larger than the jib. For example, the Lagoon 42 standard main is about 40% larger than the jib. Thus, some weather helm will be present in the L42 by default simply because of the size discrepancy between the jib and the main. Eliminating weather helm completely likely means that the main was depowered too much and thus the speed may be suffering. The use of a well-powered main produces enough forward force to compensate for the rudder brake that results from mild weather helm. That is, the forward force of the properly powered main overpowers the minor brake created by the rudders. However, if the weather helm is too severe, the rudder-induced brake eliminates any benefit of the overpowered main and thus the speed would suffer.
There is another advantage of weather helm when going to windward. The direction of the rudder when turned slightly to compensate for the weather helm creates “lift” to windward as the rudder acts as a wing using the water as air. The water rushes on both sides of the rudder and this creates minor windward lift and thus a bit more speed. However, I doubt this effect is felt by anyone other than elite racers. A good explanation of this effect can be found here (although they should have shown the direction of the water so that it is easy to see why the lift generated is to windward).
In sum, some weather helm is good. Too much will slow you down. We aim for the center mark on our helm to be around 20-30 degrees turned to leeward. However, if the weather helm is so severe that the centerline needs to be turned past 40 degrees we see a significant drop in speed. In such cases, we depower the main just enough until the rudders stop working so hard.