The influence of a sail upon the speed of a boat also increases with the angle that it makes with the center line of the hull. The more the yachtsman slackens out his sail, the more it will help the boat along. The reader will see that these two conditions interfere with each other, and therefore the trimming of the sails becomes a compromise. It is good for the young yachtsman to remember to sail his boat with the sails as slack as possible, as long as she keeps a good course. He should also remember not to overload her with sails, since the nearer to an upright position she maintains the faster she will go.
It is not possible to depend entirely upon the trim of the sails to keep a model in a given course. This is because the strength of the wind varies so that the sails are in balance one moment and out of balance the next. The sails abaft the mainmast overpower the sails before it when the wind increases. The result of this is that the bow of the boat will be repeatedly turned in the direction _A_, Fig. 146.
Some form of automatic rudder is therefore generally used to overcome this tendency of the yacht to "luff" in the wind. Fig. 147 shows the course of a yacht reaching from _A_ to _B_. The dotted lines show the course she should follow. The full line shows the effect of puffs of wind, which repeatedly take her out of her course. Many times she may completely turn around and make a similar course back to the starting-point, as in Fig. 148. There is also the danger of her being taken back when pointing directly against the wind--the wind will force her backward stern first for some distance, as illustrated in Fig. 149. She will do this until she manages to get around on one tack or the other.
The dotted line _B_ illustrates the course in which she would be driven under these conditions. It is not practical to sail a model yacht dead before the wind without an automatic rudder. With the use of an automatic rudder the erratic movements shown in Fig. 148 can be entirely overcome. The action of the rudder is such that every time the boat leans over to luff up into the wind, the weight of the rudder causes it to swing out, and thus prevents her from losing her course. As a different type of rudder is required, according to the course in which the yacht is sailing, the weight should be adjustable if the same rudder is used.