ships.js
// <img src='images/ships/3.png' class='ship' id='s6'>
function ships() {
  var sn=1;
  for (var fn=1;fn<5;fn++) {
    var s1="<img src='images/ships/";
    var s2=".png' class='ship' id='s";
    var s3="'>";
    document.write(s1+fn+s2+sn+s3);
    sn++;
    document.write(s1+fn+s2+sn+s3);
    sn++;
  }
}
index.html
<script src='ships.js'></script>
<body>
  <div id='main'>
    <div id='squares'>
      <script>
        squares(); seed(); do_counters();
      </script>
    </div>
    <div id='shapes'>
      <script>shapes();</script>
    </div>
    <div id='ships'>
      <script>ships();</script>
    </div>
  </div>
</body>