.page3_ship{

background:url(../image/boat_02.png) 0 center no-repeat;
background-size: 486px 385px;
position:absolute;bottom:0;left:50%;margin-left:-121.5px;width:243px;height:385px;}

当两个类名同时存在时生效
.page3_ship.animate{
-webkit-animation: 1s animate_page3ship steps(2) infinite;
}

规定动画各个阶段的状态和动画名字
@-webkit-keyframes animate_page3ship{
0%{background-position: 0 0;}
100%{background-position: -486px 0;}
}

添加aimate动画