The rudder is a simple but important part of the model yacht. Because of its simplicity, the importance of rudder design can easily be overlooked. The question is, what makes the rudder so important? Well, the obvious answer is the rudder controls the direction the model yacht sails in. But there is more to it than just that. This article will explain the other important facts and how they relate to the model yacht.

Yachts have an intentional imbalance between the center of effort of the sails and the center of lift of the keel. For most model yachts the sail's center of effort is aft the keel's center of lift. This gives the yacht a weather helm and is an asset in sailing efficiently. Without a counteracting force, the yacht's stern will swing to leeward as weather helm turns the boat into the wind. To stop this from happening, the rudder must generate enough lift to hold the stern to windward and keep the yacht moving on a straight course. The key phase here is lift that is generated by the rudder. If a rudder creates too little lift, the yacht will not perform well. It is not the size of the rudder that influences performance, it is the shape that makes the difference.

On most model yachts, the rudder is placed well aft of the keel. The hydrodynamic behavior of the rudder as it travels through water is similar to that of the keel. Because the rudder's behavior is like the keel's, there is an added twist. The angle of the rudder to the water can at times get large. Normally, the inflow of water ranges from 3 to 10 degrees while the keel is between 2 to 8 degrees. These higher angles change the parameters of design and influence the foil section and aspect ratio.

The rudder sits in the outflow of the keel and is called upon to provide lift at very small angles of attack and not stall when required to prevent a broach at high angles of attack. Because of these demands of the rudder's foil section needs to produce a large lift force and function at high angle of attack.

The rudder for the most part is a foil. And because it is a foil shape, lift is created. The secret for a rudder to have good lift is the foil thickness. If a rudder foil shape is too thin or too thick, the lift is effected, (see Example 1 above). In Example 1 (published in Abbott and Von Doenhoff's Theory of Wing Sections), a foil thickness between 11 and 14 percent produces higher lift coefficient at sailing speeds. So if a rudder is too thin and drag is less, so is the lift. On the other hand if a rudder is too thick, lift is less, but drag is increased causing a slower speed. For a good idea of a foil shape see Example 2.

For all model yachts, the rudders are fully immersed types. Because of this, a fully immersed rudder enjoys high lift values at the top of the rudder because of the "end plate" effect, (see Example 3). Another reason for the high lift at the top is because that is where most of the foil shape is located. Where as at the tip, the foil is thinner and the water vortexes off the tip, and lift is lost.

That is about it on the design of model yacht rudders. It all comes down to using the right foil thickness. Too thin and out of shape and the rudder will not create enough lift. Too thick and the rudder creates too much drag and lowers the amount of lift it generates. I hope this article has explained what makes a difference in rudder design to produce a winning model yacht.