The Gift

Around Christmas 2024, I had the idea of building something special for my wife's cousin's upcoming birthday. He was turning 11 in May 2025, and I thought a remote-controlled boat would be a fun and educational gift — something he could actually play with and that I could build from scratch.

I started acquiring parts in early 2025 and assembled the boat incrementally, testing each module as it came together. On May 16th, I delivered the finished boat alongside a Raspberry Pi as his birthday present.

The Concept

The boat is controlled via Bluetooth from a smartphone app running in gamepad mode. It uses differential steering — two independent geared motors, each driving a paddle wheel on one side. To go forward, both motors spin. To turn, only one motor runs. The body is made of plastic and styrofoam, with all the electronics housed inside a waterproof plastic enclosure.

Electronics and Evolution

The electronics went through two major revisions as I discovered the limitations of the initial design.

Version 1: 9V Battery

The first version used an Arduino Pro Micro, an HC-05 Bluetooth module, an L298N H-bridge motor driver, and a 9V battery. I also added two LEDs — one to indicate the board was powered on, and another to show the Bluetooth connection status.

However, the 9V battery simply could not deliver enough current to drive the geared motors with any meaningful torque. The motors would barely spin under load.

Version 2: 18650 Battery

To solve the power problem, I replaced the 9V battery with a 2200 mAh 18650 lithium cell. This required adding a TP4056 charging module (for USB recharging), an LM2587 step-up boost converter (to raise the voltage), and swapping the bulky L298N for a compact DRV8833 motor driver.

The Software

The Arduino code receives single-character commands over a Bluetooth serial connection: F (forward), B (backward), L (left), R (right), and 0 (stop). The smartphone runs a Bluetooth gamepad app that sends these commands when the directional buttons are pressed.

Two challenges came up during development. First, both motors starting simultaneously at full PWM would cause a current spike large enough to trigger a brownout — the step-up converter could not handle the inrush current of both geared motors at once. The solution was implementing a soft-start PWM ramp, gradually increasing the duty cycle instead of jumping straight to the target value.

Second, the initial ramp implementation used a separate blocking for loop for each motor, which meant motor 1 would complete its entire ramp before motor 2 even started. This caused the boat to veer off course when going straight. The fix was writing a dual-ramp function that increments both motors' PWM values together in the same loop:

void analogWriteRampaDupla(int pino1, int pino2, int valorFinal, int tempoStep = 5) {
  for (int v = 0; v <= valorFinal; v++) {
    analogWrite(pino1, v);
    analogWrite(pino2, v);
    delay(tempoStep);
  }
}

The source code is available on GitHub, including the main control sketch (DRV8833 version) and individual test sketches for the Bluetooth module and motors.

3D Printed Parts

Paddle Wheels

The paddles had to be large enough to reach the water from their mounting position, but light enough not to overwhelm the geared motors' limited torque. I settled on an 8-blade radial design modeled in Fusion. I printed the first paddle myself, but ran out of filament for the second one — my brother-in-law Diego kindly printed it for me.

Shaft Coupling

The geared motors have a square output shaft, but the paddle wheels need a round bore. I designed a coupling adapter that transitions from a square cross-section to round, allowing the paddles to mount securely on the motor shafts.

The Result

The boat was delivered on May 16th, 2025, along with a set of printed instructions featuring a QR code to download the Bluetooth control app and the connection credentials. A fun project from start to finish — and hopefully a memorable birthday gift.