The Spare ‘S’ Mark can be used anywhere on the course as an additional mark of the course, in order to make the course more interesting. However it is most often used as the mark at the opposite end of the start/finish line from the Committee boat. It is particularly useful to use the S mark at the end of the start/finish line if there are a lot of boats on the water and a long start line is required. Other opportunities for using the S mark in this way might be when you want to start in a corner of the lake and it is much easier to move the S mark than it is the other marks.
If you are using the S mark as part of the start/finish line then the start/finish tile should be placed below the S mark tile on the committee boat. In the following example S marks the start line. Note that in this instance on the second lap the S mark has to be kept to the right but it is possible for the boat to choose a route that either
a) Goes between the committee boat and mark S (red arrow) OR
b) Goes behind the committee boat and then tacks to head towards (green arrow) H
On the final lap the (the requisite number of laps completed or shortened course) the boat must pass between the committee boat and S in order to finish (red arrow).
According to our Sailing Instructions the mark S can also be used to form a GATE that each boat MUST pass through on each lap.
“A tile showing ‘LINE’ may be shown on the course board as the start and finish, and if so then an imaginary line between the committee boat and a buoy marked ‘S’ or a danbuoy forms a gate and every boat must pass upwind through the gate at the end of each lap.”
In this case putting the ‘LINE’ tile below S mark indicates the GATE and all boats, on every lap, must pass between the committee boat and mark S (red arrow). This is very useful for large events where a long start line is needed and makes it easier for the Race Officer to count the number of laps.