If you like the idea of getting maximum efficiency out of a thruster that extends beneath the bottom of your boat, but would prefer it be out of the way while running, check out a retractable thruster. It can be mounted at a boat’s bow or stern and retracts into the hull beneath a covering plate. Because a retractable thruster when deployed extends deeper into the water than a tunnel-style thruster, it also tends to exert more influence in cases where a tunnel type that’s closer to the water’s surface would ventilate. Some manufacturers of retractable thrusters are Max-Power (www.max-power.com), Side-Power (www.imtra.com), and Vetus (www.vetus.nl). Retractable thrusters are generally more expensive than tunnel types with the Side-Power SR80/185T-12V starting at $11,300 on the IMTRA Web site. This particular thruster is designated for boats from 35 to 48 feet long.