Hello Paul,
The self-bailers are retractable shutes that face aft, sort of the opposite of scoops, so that when you are travelling fast enough, suction from the water passing beneath your hull draws out any water standing above the bailer (i.e. in the cockpit). They work very well but if you are moving too slow (you will find out how fast too slow is), there is insufficient flow to create that suction and then they just become two holes in your boat and let water in, so only open them when you are sure you are going fast enough that they will work and remember to close them when the wind drops off.
Be sure to close them as part of your coming ashore routine as the action of putting the boat on its trolley could damage the bailers if they are left down. Don’t ask how I know.