Retractable thrusters, as the name suggests, may be retracted into the hull when not in use. This eliminates additional drag, which is something that sailors among others are often concerned about. It also solves limitations due to draft, for example, if the bow section of a boat is too shallow and therefore any tunnel would be too close to the waterline to be effective. More space is required in the hull by a retractable thruster, however, along with the need for a mechanical installation. On Side-Power thrusters, electric actuators are used to extend and retract this kind of thruster.