Model Boat Electric Motors: Brushed and Brushless
Model boats use two kinds of electric motor: brushed “can” motors, which are simple and smooth at low speed and suit scale and slow boats, and brushless motors, which are far more powerful and efficient and power almost every fast boat. The right choice depends on the hull, the speed you want and the battery. This guide explains sizes, kV, cooling and how to match motor, prop and ESC.
By Kyle HilsonUpdated: October 20269 min read
Brushed or brushless
| Brushed | Brushless | |
|---|---|---|
| Wires | 2 | 3 |
| Efficiency | Lower; brushes wear | High; little wear |
| Low-speed control | Very smooth | Good with sensored or quality ESCs |
| Power for size | Moderate | Very high |
| Typical boats | Scale boats, tugs, ships, slow sport boats | Fast sport and racing boats, larger scale boats |
| ESC | Brushed ESC | Brushless ESC, water-cooled for speed |
For a scale model that should cruise realistically for a long time, a good brushed motor is often the better choice. For anything that planes, brushless is the standard. The RC boats guide explains which hulls plane and which don’t.
From the workbench: most of my hobby time goes into boats of the NAVIGA Mini Eco Expert class, an electric racing class, and I make many parts for my boats myself.
Brushed motor sizes
Brushed can motors are named by size numbers that roughly follow the can length and diameter, such as 385, 540, 600, 700 and 775. Bigger cans handle more current and swing larger props. For scale boats, motors are often combined with a reduction gearbox or belt drive, so a fast-spinning motor turns a larger, scale-sized propeller slowly and efficiently.
- Fit suppression capacitors across the terminals to reduce radio interference.
- Use a flexible coupling between motor and shaft to absorb misalignment.
- Run new motors in at low voltage for a few minutes so the brushes seat.
Choosing a brushless motor
- Inrunners (rotor inside the case) are standard in fast boats: compact, high rpm, easy to water-cool with a jacket.
- Outrunners (rotating case) have more torque at lower rpm; they suit some scale boats, usually with a gearbox, and air boats.
- kV is rpm per volt. Higher kV suits fewer cells and smaller props; lower kV suits more cells and larger props. kV says nothing about power handling, which depends on motor size.
- Size code: boat inrunners are often named by diameter and length in millimetres (for example 36 × 56); larger means more power handling.
For how motors, ESCs and batteries work in general, see RC basics.
Matching prop, battery and ESC
- Start from the hull maker’s recommendation for motor size, kV and cell count; it saves a lot of trial and error.
- Choose an ESC rated for the cell count and well above the expected current, with water cooling for fast boats.
- Start with a small prop, run for a short time, and check the temperature of motor, ESC and battery.
- Increase prop size in small steps only while temperatures stay acceptable.
Watch out: boats load the motor continuously, unlike cars and planes. A prop that is one size too big can overheat a motor or ESC in under a minute. Change one thing at a time and check temperatures after every change.
Cooling and mounting
- Water-cooled motors need a pick-up behind the prop or on the rudder, a jacket around the motor and an outlet through the hull.
- Route the cooling water through the ESC plate first, then the motor.
- Mount the motor firmly and align it carefully with the shaft; misalignment wastes power and wears the coupling.
- Drain the cooling system after running so water doesn’t stay in the motor jacket.
Wiring the motor, ESC and receiver is covered in how to wire an RC boat.
FAQ
What is the best electric motor for a model boat?
For scale and slow boats, a quality brushed motor, often with a gearbox. For fast boats, a water-cooled brushless inrunner sized to the hull maker’s recommendation.
Can I use a car motor in a boat?
Car motors can work in boats if they are cooled properly. Boats load motors continuously, so add a water-cooling jacket and keep the prop small at first.
How do I reverse a boat motor?
Swap the two wires on a brushed motor, or any two of the three wires on a brushless motor. Make sure the prop matches the rotation direction.
What does kV mean for a boat motor?
The rpm the motor turns per volt without load. Combined with the battery voltage it sets the prop speed; the prop size must be chosen to keep current within limits.
More on this topic: hull types, drive systems and running safely are in the complete RC boats guide.
About 20 years in RC. Mostly boats in the NAVIGA Mini Eco Expert class, with many self-made parts; also a King Motor Baja 5B, a home-built electric plane, home-built nitro planes he has flown, and an RC helicopter. More about SwellRC
