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February 9, 2011

Helm Balance – Center of Effort, Lateral Resistance, Centerboard, Mast Rake

Slight Weather Helm

Often when reading about sailboats, one encounters criticism with respect the excessive weather helm as a negative characteristic of a vessel’s sailing performance. With a little research, weather helm is revealed as a tendency of the sailboat to turn windward necessitating a constant and opposite application of pressure on the tiller or wheel. Then opens a whole science related to helm balance. Properly designed cruising sailboats exhibit a slight weather helm. The alternatives are as bad as excessive weather helm. Neutral helms do not provide feedback to the captain while lee helm vessels can accidentally jibe, a dangerous maneuver, or get knocked down by nasty and unexpected puff. Let’s take a look at the ways to balance the helm which include mast location, bow sprites, reefing, mast rake, centerboards, weight distribution, and heeling.

Background
Before starting this discussion, we need to understand more deeply the cause behind an unbalanced helm in terms of a vessel’s center of effort and center of lateral resistance. The center of effort (CE) is in simple terms the geometric center of the sail plan while the center of lateral resistance (CLR) is the geometric center of the underbody. Weight, volume, and forces play roles too, and these two locations are constantly changing. A good way to estimate the locations is to loft paper cut-outs of the sailplan and underbody below the waterline and then balance these separately on pins. The balancing points are the estimated center points. Imagine the CLR as the pivot point for the vessel and the CE as the driving force location acting on the sail plan. If the CE leads the CLR, a sailboat has lee helm; if the CLR leads the CE, she has weather helm.

To make it more confusing, yacht designers normally talk about CE in terms of leading CLR, i.e. lead percentage, which would result in lee helm. Off the design board all vessels appear to have lee helm instead of the slight weather helm which is optimal. How do we rectify practice and theory? Well, it turns out that while underway, the CLR moves forward a considerable amount relative to the CE. Therefore, the lee helm designed vessel ends up in practice with weather helm.

Ways to Balance the Helm
Below are sections that describe common producers to balance the helm. Some relate to difficult and permanent modifications while others are temporary changes done while underway. The first listings of mast location, bowsprits, reefing, and mast rake change the CE while centerboards, weight distribution, and heeling longitudinally move the CLR.

Extending a Retractable Bowsprit Downwind for Lee Helm

  1. Mast Location: Traditionally the primary cause of excessive weather helm was too far aft mast locations. In the CCA and IOR days large J-dimensions were the rage. The large foretriangle pushed the mast location, sail area, and therefore CE far aft of the CLR. The wind would catch the tail end of the vessel and pivot her about the CLR into the wind. For example, the Valiant 40 has too much weather helm. On the V40 pilothouse versions, Perry moved the mast forward to balance the helm.
  2. Bowsprits: Similarly a bowsprit is added to many designs that have too much weather helm. Then the headstay is secured to the bowsprit which enlarges the headsail area moving the CE forward. To further balance the helm on the V40, Perry designed the V42 which has the same underbody with a two foot bowsprit. Another example is the Corbin 39. Some recent yachts have extendable bowsprits that tie into the bow chock such as the Jeanneau SO 39i. This provides a way to balance the helm with out installing a permanent sprite. It is handy for a beam reach with a gennaker offsetting weather helm as needed and providing clear air.
  3. Reefing: Reefing the main or furling the jib as well as the opposite of putting up storm jibs, etcetera are versatile ways to move any which way the CE. This ability is the reason why ketch and schooner rigs came into vogue and why double headsail sloops are the primary offshore cruising vessel of the day. For instance by furling the main, the CE adjusts forward; by furling the jib, the CE moves aft. The trend towards simpler rigs dependent on large furling mains has meant that the common way to handle weather helm due to high winds is by reefing the main appropriately.
  4. Mast Rake: The highest in modern performance such as the deep bulb keeled Farr 40’s excessively rake their masts to maximize directionality and speed. Any kind of helm imbalance causes the captain to compensate by angling the rudder. This angling is like a break slowing the down the vessel. When sailing upwind, Farr 40’s rake their mast aft which moves the CE aft. They seek to minimize any tendency to fall off, i.e. lee helm. They can point extremely high up to 25 degrees and make more direct progress. Offwind the crew can rake the mast forward to move the CE forward and minimize weather helm. By raking the mast, racers effectively balance the helm without changing sails.
  5. Lowering the Centerboard Upwind to Increase Weather Helm / Pointing Height

  6. Centerboards: Why does a centerboard allow a yacht to point higher? One reason is that by lowering a centerboard you move the CLR forward and reduce lee helm, i.e. the tendency of the vessel to fall off. Imagine the board swinging down and forward. The forward action shifts the longitudinal area (and weight) of the hull forward. Clearly the balancing point, CLR, must be further forward to compensate for the shift in area and weight forward. This change minimizes lee helm and leads to higher pointing.
  7. Weight Distribution: The Tayana 37 design is notorious for wicked weather helm. Part of the issue is a water tank secured in the bow. This foreward distribution of weight moves the CLR forward, far forward of the CE. Owners remove this tank to restore rig balance. Similarly you can move around cruising weight or on a smaller vessel crew to adjust the helm balance. Weight moved foreward increases weather helm while too much weight aft leads to a sailboat squatting and having lee helm.
  8. Heeling: Heeling changes the underwater hull shape and the CLR. Heeling leeward causes the CLR to move aft, and the vessel to turn into the wind. Reducing heeling reduces weather helm. On smaller vessels like Lasers crew can hike out to balance the helm.

Conclusion
I covered the ways to balance your helm whether via moving the CE due to mast location, adding a bowsprit, reefing, or raking the mast; or moving the CLR by lowering a centerboard, moving weight longitudinally, or increasing/decreasing heel. A sailboat’s center of effort and center of lateral resistance are two characteristics that affect helm balance. Yacht design has many other calculations such as the center of buoyancy, center of flotation, fineness coefficient, prismatic coefficient, comfort ratio, and capsize screening formula. All these combine to determine the sailing performance of a yacht. Please feel free to comment below on your experiences and advice with respect to helm balance.

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