NAMBA Propwash October 2017.pub

to have a positive locking mechanism. The servos need to be securely mounted on a tray in the radio box. Of course the radio box needs to be securely mounted as well. Throttle servos don’t need either power or precision. Analog servos will work well. These days even standard size servos have more than enough power for rudders. Most rudder servos with metal gears and over 100 oz/in torque will be fine. Digital servos give precision response, but ultra speed isn’t needed. Since ruggedness is important, I wouldn’t use smaller than standard size servos. Outboards are a special consideration. Here the issue is the impact on the servo when the boat flips. In the past, ¼ scale servos were needed, both for torque and for gear size. Today they are probably overkill but still not a bad idea for 7.5 size and larger boats. In any case, don’t use metal servo arms on outboards. The plastic arm will act as a fuse. The splines will strip on overloads. Fixing the problem involves replacing the servo arm, not the servo. Gas outboards need an even heavier duty servo. The servo gearboxes sold by Servo City are often used. Even with careful attention to all the above, loss of control can still happen. Race organizers should always assume that someone will lose control of their boat. Pond safety systems like high banks or sports netting will help contain runaways. Spectator control is essential. Suppose that the gas cat pictured below had hit a person rather than a tree. Pieces of the sponson’s plywood are still embedded in the tree. What happens if something fails? Common fail safes cause the receiver to set the servos to a predetermined position if there’s no signal. The usual choices are throttle off and steering to either right or left turn. None of this works if there’s a power failure in the radio box or a servo failure. A somewhat drastic solution is to have two complete radio systems running from the same transmitter. Each system has a battery, switch, and receiver connected to its own servo. That way if one system fails either the throttle or the steering can be controlled. A Y connector can be used to hook the servos to both receivers for full control on failure. With this setup it’s important to test both systems independently by switching off each side, one at a time. Since servos usually fail one at a time, some control will be maintained with a single operating servo. All assembled systems need to be tested before running. These days an antenna off range check isn’t needed (or possible with 2.4 radios). The throttle and rudder response needs to be tested before launch. The engine should be running for the rudder check to confirm that ignition interference or vibration isn’t causing a problem. Servos are the last link in the control system. Their push rods or cables need to exit the radio box through a water tight connection. A rubber boot works with push rods. There are sliding seal systems as well. Cables exit through grease filled tubes. The linkages need to be stiff with no play at the connections. The connections at the cable or push rod ends need PROPWASH 8 October 2017 Pushrod Boot and Throttle Cable Tube O-ring Pushrod Seal and Secure Radio Box Mount Servo Arm with Clips on Clevises Servo City Geared Servo