ns out his jib and foresail, or
the smaller he makes these sails, the less their power will be to turn
the boat in the direction _B_. On the other hand, the larger they are
and the more tightly they are pulled in, the greater will be their
power. When the mainsail and all of the sails abaft the mainsail are
slackened out and the smaller they are made, the less their power will
be to swing the boat in the direction _A_.

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.

[Illustration: FIG. 146]

[Illustration: FIG. 147]

[Illustration: FIG. 148]

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