Join Date: Jan 2000
Thanked 12 Times in 6 Posts
Rep Power: 17
Understanding Weather Helm
I was reading Tom Whidden's book, The Science and Art of Sails, and did not understand his explanation regarding the need for a few degrees of weather helm. He stated that there was an advantage to the lift of the keel and of the rudder being in the same direction. Why is this?
Mark Matthews responds:
Weather helm indicates that the forces acting on the sail are out of balance with the center of lateral resistance. Most of the time weather helm is referred to as a problem and the aim is to reduce the amount of effort required to steer the boat. A little weather helm, or the tendency for the boat to turn into the wind, is good in limited amounts since it gives the helm a positive feel when steering and allows the skipper to monitor how the boat reacts to changing wind conditions.
However, any time you're using both arms to fight the helm and the rudder is cranked over all the way to one side just to keep the boat going straight, not only are you exerting energy that may be needed later, but you are also slowing the boat down. Some skippers like the feel of a little weather helm because it can also indicate when to follow advantageous windshifts, or lifts, when heading upwind. To control excessive weather helm, first make sure that your sails are trimmed relatively tightly. Then start easing the traveler to leeward (if you have an adjustable traveler), or tighten the vang and ease the mainsheet. Angling the rudder a bit off center adds a little lift to the hull.
Water flowing past the hull, keel, and rudder of a sailboat is subject to the same basic rules as air flowing past the sails. The only difference between the sails and underwater appendages is that the latter are symmetrical while the former are asymmetrical. But the angle of attack (which we call "angle of incidence" for wind hitting the sails and "yaw angle" for water hitting the keel) solves the problem of getting "lift" from the keel. For a more thorough treatment of this topic, I'd refer you to Steve Colgate's article The Balance of Hull and Sails in the Learning to Sail section of SailNet.