The Centre of effort of a rig can be best described by stating that the entire force of wind power generated by a sail can be considered to act through one point – in the same way that the center of gravity of a body is the point around which that body balances. In this blog, we explain the dynamics of the Centre of effort, how you can change it and WHY it needs changing.
A badly tuned boat will produce rudder drag which will slow it down. As discussed in a previous blog the water is 1,000 times denser then the air, so drag from the hull is more important to eliminate than drag from the sail. We can therefore initially focus on tuning the sail to give the tiller feel that we know is fast – light, neutral tiller.
To achieve neutral (light) tiller feel, the centre of effort must be directly over the pivot point of our boat (usually the center board but not always). We do this by adjustment of. rig controls and mast rake, tensions etc.
Let’s divide the subject into two areas: 1. how does different locations effect our speed and height, and 2. how do we move it around
1. How does different locations of the center of effort will effect our boat? or in other words “what will we feel”?
When the wind is strong we either need to add more weight to keep the boat flat or change the leverage with the sail, We can’t add more weight or hike or wire further out then the maximum, luckily we can move the center of effort down to allow us a better leverage and make it easier for us to keep the boat flat, by bending the mast we will flatten the sail and move the center of effort down, it is just the opposite when the wind is light and instead of slumping in the boat and going slow we need more energy (a stronger engine) in that case we need to move the center of effort further up, we achieve this by straightening the mast hence making our sail deeper and moving the center of effort up.
If the boat feels like it keeps wants to luff up even though we keep the boat flat it means the centre of effort is located further back of the pivot point of the boat, in that case we would need to move the centre of effort forward in the boat, bending the mast, altering one of the sail controls or using a sail with a certain shape will help us to achieve this, again the opposite applies when the boat constantly wants to bear away, in that case it means the centre of effort is located forward of the pivot point of the boat and we need to move the centre of effort backwards to balance that force.
2. How do we move the center of effort around?
our sports is mainly divided into two types of sail boats or more to say mast types
Unstayed masts – relates to the type of boats with a “free” mast (no shrouds or spreaders) such as optimist, windsurfing, Laser and many other single handed boats, with those masts we generally have two main controls that bends the mast. The down-haul (Cunningham) which pulls the fabric of the sail down hence bending the mast and also moving the center of effort forward. The Kicker (Vang) which pulls the boom down and pulling the tip of the mast through the leech which in turn will bend the mast and move the center of effort down, but in this case will also close the leech and move the center of effort backwards. In some boats or surfboards we can move the position of the entire mast step forward or backward while sailing which in turn will move the center of effort forward or backwards, finding the balance between these controls is one of the major skills taught to our sailors in order to find the right balance between power (up/down) and rudder pressure/flow (forward/backward)
With stayed masts we determine the general bend of the mast by the tension on the shrouds. In classes such as 470 or 420 it is a balance between the tension on the shrouds and the angle of the spreaders, in 49er which also have caps and lowers it is the balance between the tension of the different wires that will determine how much the mast will bend (since the spreaders are fixed)
The other controls will also move the center of effort further up or down, back or forward. The Cunningham will generally have the same effect of further bending the mast and moving the center of effort down and forward
The kicker is a bit more complicated, for example in 470 for the initial pull of the kicker will tighten the leech and bring the centre of effort backwards. After you pull a lot, the bottom of the mast will bendit will bend, moving the center of effort down but still not closing the leech, moving the center of effort forward. The point of change of the kicker pull will be influenced by the use of chocks to prevent the mast from collapsing forward at the bottom part and will cause the kicker to bend the mast simply by forward load of the kicker pull.
The interaction with the jib and the shape of the sails will determine the position of the centre of effort, different boats, different masts, different sailors all have an impact on the desired centre of effort position but the principal remains the same, centre of effort moving up will power up the boat and moving down will depower your boat.
We can therefore state that the center of effort is the average of all the forces pulling the sail (in its different strengths and directions) usually it will be at the deepest part of the sail.
By using our controls, we can move it up or down, forward or backward, each position will have an impact on our speed and height as well as on the power of the sail, basically moving it down will reduce the power and will make it easy to keep the boat flat in stronger winds.
Moving it forward or backward, will also have a big impact on speed and height. Gaining the balance between the power you need and the balance to stop drag and give the tiller feel we know is fast is an art, developed through experimentation, experience and pure hard work.
There are many ways to achieve that balance and during our training sessions at Toplevel Sailing we teach our sailors (and coaches) how to feel the boat and adjust the controls to eliminate rudder pressure by moving the center of effort around. Off course in some classes there are various sail shapes that will also determine the position of the center of effort.