Balance Your Boat
Categories:
Skipper’s Corner | Hulls
by Herman Kraus
One of the most frequent questions heard at the pond is "why does my boat keep trying to turn all of the time?"
There are two primary answers for this question; (1) the rudder is not centered, or (2) the boat is out of balance. An off-center rudder is normally due to one of two causes. There may be a mechanical misalignment between the servo and the rudder, or the trim control on the transmitter control stick is offset. Both of these problems are easily seen and corrected. The problem of boat balance is detectable only by holding trial sailing sessions. When a boat is in balance it should track on a straight close-hauled course with no corrective inputs from the skipper. This article addresses the techniques for optimizing balance, note that I said "optimizing", because the balance is strongly influenced by existing conditions of wind and waves. Let us start with a few definitions.
It is commonly accepted that the aerodynamic forces generated by the sail plan can be concentrated in a single point called the center-of-effort (CE). The submerged lateral-profile of the boat acts to resist the sideward movement (leeway) induced by the sail plan forces. The sum total of this resistance can be concentrated is a single point called the center-of-lateral-resistance (CLR). Mast-rake is defined as the fore/aft deviation of the mast from the vertical, and is measured in a direction aft of the vertical. (Note that the vertical referred to is the horizontal plane with the boat sitting on its designed waterline, and not the vertical to the deck.) So much for definitions -- lets get practical.
Picture a common playground seesaw with only one person on it. When the person is directly over the pivot point the plank is horizontal and has no tendency to tip one way or the other. When the person moves away from the pivot point the position of the plank changes and the side supporting the person tilts down, (We could achieve the same effect by moving the pivot point away from the person but it would be slightly more difficult.) Now let us replace the plank with a top view of the boat and make the top surface of the plank the windward side. Replace the person with the CE and the pivot point with the CLR. Now the dynamics of boat balance should be apparent. When the CE is over the CLR there is no tendency of the boat to turn. If the CE is moved toward one end of the boat then that end will turn away from the wind. Thus, if the boat exhibits a tendency to turn into the wind ("weather helm") this turning tendency can be corrected by shifting the CE forward. Similarly, a tendency to turn away from the wind (“Leeward helm”) can be corrected by shifting the CE aft. Sounds simple in theory, but how is this accomplished in the real world?
The location of the CE is not at the geometric center of the sail plan as commonly believed, nor is the CLR at the geometric center of the submerged lateral-profile of the hull. The CE can "travel" within a large area of the sail plan depending upon how each sail is set (i.e.: tensions along each sail edge) and the trim of each sail relative to the other. The CLR location is also quite mobile with its initial location being dictated by the underwater shape of the hull (i.e.: full keel, fin keel, etc.), and it has relatively large excursions for various angles of heel and fore/aft trim. When the boat is upright in the water both the CE and CLR are on the fore/aft centerline and the waterline entry at the bow is symmetrical. A heeling boat presents an asymmetric waterline entry, the CLR moves slightly to windward and the CE moves well to leeward (often winding up outboard of the hull). These heeling conditions result in a torque acting to turn the boat to windward. Since sailboats sail with some angle of heel 99.9% of the time it is not surprising that the CE winds up forward of the CLR when a boat is balanced. This fore/aft displacement is called the "lead". When conditions change the amount of heeling changes and adjustments must be made to restore balance. When balancing a boat you do not care where the CE or the CLR is located; rather, you care about their positions relative to each other. Remember the seesaw, as long as the person was over the pivot the plank remained balanced. So, as long as the CE is positioned properly with respect to the CLR the boat should remain balanced
So how do you go about balancing your boat? First make a trial sail and observe the performance on a close hauled course with your hands off of the transmitter. If the boat turns toward the wind then the CE should be moved forward. The easiest adjustment, which moves the CE forward, is a decrease in the amount of mast-rake. Make a small decrease in mast-rake (a little bit can go a long way) and then another trial. Repeat this process until the boat is performing the way you want. Of course if the first trial results in the boat turning away from the wind you must increase the mast-rake. Make only small adjustments so you do not overcompensate and become confused. It is better to creep up on your final setting rather than to try a giant leap, which may, in all probability, overshoot. Later, as you gain more experience, you can try other adjustments such as "tweaking" the set of the sails, changing the trim of the sails, etc. Be certain when attempting a variety of adjustments to make them one-at-a- time so you will know how each affects the balance, Also keep in mind that different wind strengths will call for readjustment to keep the boat as closely balanced as possible.
Because every boat is different (e.g.: old sails versus new sails, rough bottom versus smooth bottom, different fore/aft distributions of internal equipment, etc.) it is up to you to experiment and find the optimum settings for your boat. What works for one person may not necessarily work for another. As an example in a wind of 6 or 7 knots I feel that my boat balances best when sailing with a heel of 10 degrees. Other boats may perform at their best with only 5 degrees of heel while yet another may require 15 degrees. Observe the top performers at the pond or at regattas, when they bring their boats in they are making adjustments - watch and learn! Also remember, when you are making balance adjustments keep the picture of the seesaw in mind and you will always know which way to go.
One of the most frequent questions heard at the pond is "why does my boat keep trying to turn all of the time?"
There are two primary answers for this question; (1) the rudder is not centered, or (2) the boat is out of balance. An off-center rudder is normally due to one of two causes. There may be a mechanical misalignment between the servo and the rudder, or the trim control on the transmitter control stick is offset. Both of these problems are easily seen and corrected. The problem of boat balance is detectable only by holding trial sailing sessions. When a boat is in balance it should track on a straight close-hauled course with no corrective inputs from the skipper. This article addresses the techniques for optimizing balance, note that I said "optimizing", because the balance is strongly influenced by existing conditions of wind and waves. Let us start with a few definitions.
It is commonly accepted that the aerodynamic forces generated by the sail plan can be concentrated in a single point called the center-of-effort (CE). The submerged lateral-profile of the boat acts to resist the sideward movement (leeway) induced by the sail plan forces. The sum total of this resistance can be concentrated is a single point called the center-of-lateral-resistance (CLR). Mast-rake is defined as the fore/aft deviation of the mast from the vertical, and is measured in a direction aft of the vertical. (Note that the vertical referred to is the horizontal plane with the boat sitting on its designed waterline, and not the vertical to the deck.) So much for definitions -- lets get practical.
Picture a common playground seesaw with only one person on it. When the person is directly over the pivot point the plank is horizontal and has no tendency to tip one way or the other. When the person moves away from the pivot point the position of the plank changes and the side supporting the person tilts down, (We could achieve the same effect by moving the pivot point away from the person but it would be slightly more difficult.) Now let us replace the plank with a top view of the boat and make the top surface of the plank the windward side. Replace the person with the CE and the pivot point with the CLR. Now the dynamics of boat balance should be apparent. When the CE is over the CLR there is no tendency of the boat to turn. If the CE is moved toward one end of the boat then that end will turn away from the wind. Thus, if the boat exhibits a tendency to turn into the wind ("weather helm") this turning tendency can be corrected by shifting the CE forward. Similarly, a tendency to turn away from the wind (“Leeward helm”) can be corrected by shifting the CE aft. Sounds simple in theory, but how is this accomplished in the real world?
The location of the CE is not at the geometric center of the sail plan as commonly believed, nor is the CLR at the geometric center of the submerged lateral-profile of the hull. The CE can "travel" within a large area of the sail plan depending upon how each sail is set (i.e.: tensions along each sail edge) and the trim of each sail relative to the other. The CLR location is also quite mobile with its initial location being dictated by the underwater shape of the hull (i.e.: full keel, fin keel, etc.), and it has relatively large excursions for various angles of heel and fore/aft trim. When the boat is upright in the water both the CE and CLR are on the fore/aft centerline and the waterline entry at the bow is symmetrical. A heeling boat presents an asymmetric waterline entry, the CLR moves slightly to windward and the CE moves well to leeward (often winding up outboard of the hull). These heeling conditions result in a torque acting to turn the boat to windward. Since sailboats sail with some angle of heel 99.9% of the time it is not surprising that the CE winds up forward of the CLR when a boat is balanced. This fore/aft displacement is called the "lead". When conditions change the amount of heeling changes and adjustments must be made to restore balance. When balancing a boat you do not care where the CE or the CLR is located; rather, you care about their positions relative to each other. Remember the seesaw, as long as the person was over the pivot the plank remained balanced. So, as long as the CE is positioned properly with respect to the CLR the boat should remain balanced
So how do you go about balancing your boat? First make a trial sail and observe the performance on a close hauled course with your hands off of the transmitter. If the boat turns toward the wind then the CE should be moved forward. The easiest adjustment, which moves the CE forward, is a decrease in the amount of mast-rake. Make a small decrease in mast-rake (a little bit can go a long way) and then another trial. Repeat this process until the boat is performing the way you want. Of course if the first trial results in the boat turning away from the wind you must increase the mast-rake. Make only small adjustments so you do not overcompensate and become confused. It is better to creep up on your final setting rather than to try a giant leap, which may, in all probability, overshoot. Later, as you gain more experience, you can try other adjustments such as "tweaking" the set of the sails, changing the trim of the sails, etc. Be certain when attempting a variety of adjustments to make them one-at-a- time so you will know how each affects the balance, Also keep in mind that different wind strengths will call for readjustment to keep the boat as closely balanced as possible.
Because every boat is different (e.g.: old sails versus new sails, rough bottom versus smooth bottom, different fore/aft distributions of internal equipment, etc.) it is up to you to experiment and find the optimum settings for your boat. What works for one person may not necessarily work for another. As an example in a wind of 6 or 7 knots I feel that my boat balances best when sailing with a heel of 10 degrees. Other boats may perform at their best with only 5 degrees of heel while yet another may require 15 degrees. Observe the top performers at the pond or at regattas, when they bring their boats in they are making adjustments - watch and learn! Also remember, when you are making balance adjustments keep the picture of the seesaw in mind and you will always know which way to go.