Methods of wiring a bow thruster

Confederation Marine Modellers

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           Most bow thrusters consist of a tube into which is inserted an impeller, driven by a single motor, to force water from one end to the other. The tube is permanently glued into a hull with the ends open to the water. The control of the motor must allow rotation in either direction, to push the model to either side, and be selectable. There are two basic ways to do this, with a few variations. The two basic ways are: one, use a speed control or two, use switches.

1. Control by Speed controller.          .

            The same type of speed control that is used for the boat’s propeller motors can be used for a bow thruster. This is the simplest method of wiring and controlling a thruster. Control of the speed is not really necessary, but ​the use of a reversible ESC makes the bow thruster easy to wire and control. You could have one radio channel dedicated to the bow thruster direction, but it’s easier to use a ‘Y’ connector to drive it from the rudder control as in Figure 2. You may want to add a power on-off control if you don’t want it to operate every time the rudder moves. This optional switch will require a separate radio channel to operate it. Remember that the bow thruster has almost no effect if the boat is moving at a reasonable speed, so without an on-off switch you are adding an unnecessary drain on your battery

2. Direction Control by switches.
2a.  The switches can be push buttons, limit switches, or lever switches, operated by a servo, either directly by the rudder servo, or by a separate servo.
2b.  With separate radio-controlled electronic switches.

           There are two simple wiring arrangements in the 2a category, of similar wiring complexity. One is the use of a double-pole centre-off reversing switch operated by a separate servo, as in Figures 3a and 3b. This separate servo can either use its own radio channel or be Y-connected into ​

          The second is the use of two standard limit switches wired as shown in Figures 4a and 4b.

          Whether you use the rudder servo, or a separate servo connected to the rudder channel, you arrange the switches so that they operate when the rudder reaches its extreme port or starboard position. When the rudder is centred, both switches are open and the thruster is turned off. When the rudder servo moves over hard to starboard, it closes the contacts on one ​switch and the thruster moves the boat to starboard. Likewise when the servo moves to the port position.
          Figure 4 shows the wiring in the form of a schematic; figure 4b shows the actual wiring connections to a typical lever-operated limit switch. If you use two batteries in your boat, and have or prefer to use simple on-off switches, you can wire them as shown in Figure 5. As shown for limit switch arrangement, when the rudder is centred, both switches are open and the thruster is turned off. When the rudder servo moves over hard to starboard, it closes the contacts on the starboard switch, so the thruster is powered from battery A, and moves the bow of the boat to starboard. When the rudder moves hard to port, the starboard switch opens and the port switch closes, and the thruster is powered in the opposite direction by Battery B.
          You can use the voltage of both to power other equipment such as the boat’s propellers, as in Figure 7. If batteries A and B are each 6V, you will have 12V to supply other functions.​

​          Fuses are always recommended in wiring from a battery, but especially in each of these arrangements. If the two switches are operated at the same time, either by mistake, a sticking switch or other fault, there is a short circuit through the two switches.
          For arrangement 2b, you use two radio-controlled switches. One must be a double-pole type, but the other can be single pole. Wiring for this is shown in Figure 8. The double-pole switch is always set to operate the thruster in one direction or the other. The thruster only operates when the single pole switch turns the motor on.
           There are a number of companies supplying all-electronic single pole switches which can be used for the on-off control. Double-pole receiver-controlled switches are harder to find, but available from Harbor Models and Dimension Engineering.
There is one electronic device which combines the functions of the two switches in one. This is the P91 Hi-load Twin Switcher from Action Electronics. It retails at £30 (approx. Cdn$57). Refer to the maker’s website for wiring data.

A note about wiring schematics.

          The information above shows wiring diagrams known as schematics. This is the most convenient and useful format for this information because it is independent of the hardware (switches, push-buttons etc.) used. Hardware, and individual model boats, vary widely. If you are unfamiliar, or uncomfortable, with figuring out the actual wiring from the information in these schematics, there are many club members who will be only too willing to advise.
           Figures 3b and 4b show the actual switch wiring connections for the setups shown in Figures 3a and 4a. Figure 3b shows the wiring which is a common arrangement for reversing the direction of motor rotation. In this case, ‘supply’ is the battery and ‘load’ is the thruster motor. This is the view of the terminals on the bottom of the switch.Figure 4b shows the wiring for the setup shown in Figure 4a using two limit switches.

           This is a common style of switch but not all limit switches have their terminals arranged as shown. There will always be three terminals labelled C (common), NO (normally open), and NC (normally closed.) If your switches have a different arrangement of terminals, simply connect the wires to the terminals labelled C, NO and NC as indicated in Figure 4b.

Note: The word ‘normally’ used above describes the condition of the switch contacts when the switch is not operated, for example, when it's lying on a bench.

Using two water pumps.
          A cheap and simple alternative to a commercial bow thruster is to use two water pumps. Two water pumps can be arranged to take water from a common suction hole in the bottom of the hull. One pump is arranged to discharge water on one side through the bow thruster opening in the hull, and the second pump discharge on the other side.With this arrangement, any of the electrical schemes that uses double-pole double-throw switches, as in Figure 3, 4 or 8, will work. Each water pump motor is connected to one pair of switches. For example, figure 10 shows the wiring of two radio-controlled switches when used to operate two separate water pumps.
          In theory, an ESC can control two water pumps as a bow thruster. The two pumps would be wired in parallel to the ESC. One of the pair of wires to each pump would have a diode connected into it. This will ensure that only one pump operates when port or starboard movement is selected.