This belted rack from that same printer skeletonized makes the perfect powered ballast system. To control it all you need it a reversing brushed dc speed control plugged into the Rx.that runs the dc motor on the belt pinion. If the supply voltage is too high it would be too fast for a person to control. But the gyro could do it. because it has and optical shudder, like I built ,but with a micro aperture and a positional ribbon it tracks. It can know where it is so u might even be able to hack it in as a digital servo. It has the feed back suilt in. This can fit in between the rails in the bottom on the boat. If you don't want extra ballast weights you make a ballast tray to hold the batteries or other rc equipment.