How To Size An RC Boat Motor: A Helpful Guide

RC boats are becoming increasingly popular worldwide. With high speeds, these radio-controlled boats offer a lot of thrill for anyone who wants to try them out. However, achieving your desired speeds and getting your RC boat to work properly requires the right motor size. If you’re wondering how you size an RC boat motor, here’s how!

The length of the hull is a key factor in understanding what size of motor you need to push the weight of your RC boat. RC boat hulls that have a diameter of about 20 to 34 inches typically use a motor with a diameter of 36mm.

In this article, we’ll help you understand exactly how you need to size your RC boat motor. We’ll dive into the specifics and even help you choose the best motor for your boat.

How To Select the Right RC Boat Motor?

Selecting the right size motor for your RC boat is essential in making sure it works properly. The right size motor will ensure that your boat functions at the speed you want. 

If you want tips to make your RC boat go faster, please check out my article covering this topic here

Your RC boat motor has many specifications that need to be considered, including the type of motor as well as the speed and cells needed. 

However, the first step in selecting the right motor for your RC boat is to size the motor diameter according to the size of your boat.

How to Size the Motor Diameter?

It’s essential to choose a motor that fits your RC boat properly.

The size of your hull is the main determinant in choosing the motor diameter. It allows you to understand what size of the motor is required to push the weight of your RC boat.

Measuring the length of your hull is the first step in choosing a motor for your RC boat.

After finding out the length of your hull you can choose an appropriate size for the motor of your boat. Although there aren’t motor sizes for every hull length, there are ranges that can help you choose the right motor.

Let’s take a look at some motor ranges:

Hull LengthMotor Size
Around 600mm 28-29 mm
Around 800 mm36 mm
Around 1200 mm40 mm
Greater than 1200 mm56-58 mm

Source: https://rchobbytips.com/what-size-brushless-motor-for-rc-boat/ 

Understanding Motor Specifications

RC boat motors have many different ratings and specifications that determine their performance. After selecting the motor diameter you have to take a look at KV ratings.

The KV rating on the motor is an important specification that determines the RPM (revolutions per minute). This is basically the speed of your motor.

To determine the KV rating you need you first have to know the number of cells you’re going to use. Most motors use Lithium-Polymer (LiPo) batteries.

The table below shows the different KV ranges you can choose for different amounts of LiPo cells.

LiPo CellsKv Rating
2 Cell2700 to 4725
4 Cell1350 to 2365
6 Cell900 to 1575
8 Cell675 to 1185

Source: https://www.radiocontrolinfo.com/rc-electric-boats/brushless-boat-motor/ 

Things like hull design, weight and battery specifications all cause variations in KV and performance. 

Note: If you’re unsure about what KV to use, always choose the lower option as it is the safest.

How the RPM Changes With the Motor Diameter

RPM or ‘revolutions per minute is the speed of your motor. This can be determined by multiplying the KV with the battery voltage.

For example, a 1500 KV motor running on a 10V battery will have 15,000 rpm. 

It’s important to know the relationship between rpm and cap size as it can help you choose the motor you require. 

Generally, as the diameter of your motor increases, your rpm decreases. 

If you know the range of rpm you require, you can choose a motor diameter that matches your need.

RPM RequiredMotor Diameter
28,000 to 30,00036 mm
35,000 to 40,00040 mm
40,000 to 45,00058 mm

Source: https://rchobbytips.com/what-size-brushless-motor-for-rc-boat/ 

Understanding RC Boat Motor Specifications

The motors for RC boats use a standard number scheme that allows you to understand their body size and KV rating.

For example, let’s consider a 3655-3000 KV motor. The name of the motor can be broken down to understand its specs. 

The first part, 3655, can be broken down into two pieces: 36 and 55. The 36 represents the diameter of the motor i.e. 36 mm and the 55 represents the length of the motor, i.e. 55 mm in this case.

The second part, 3000KV, represents the KV rating (ratio of rpm and voltage) with no load. 

5 Reasons Why You Should Choose a Brushless Motor

There are two types of motors for RC boats: brushed and brushless motors. 

The brushless motor doesn’t have brushes inside the DC circuit. These brushes help the brush motor run as they’re necessary for the flow of the current.

As technology is advancing, brushless motors are becoming more and more common. They have several distinct advantages over brushed motors.

1. Battery Efficient

A brushless motor is more efficient than a brushed motor. 

With similar power outputs and speeds, a brushless motor will give you around 1.5 times more run time than a brushed motor.

2. More Power

The brushless motor has more power than the brushed motor. This means that for the same size of motor you will get more output from the brushless motor than you would from the brushed one. 

You could also say that a brushless motor would give you the same amount of power in a smaller size. This can allow you to choose a more powerful motor for a smaller RC boat. 

3. Low Maintenance 

Brushed motors have a lot of internal friction. This is caused by the constant pressing of the brushes against the commutator inside the motor. Heat and friction caused by this contact can wear away the internal parts of the motor.

For a brushed motor you may have to periodically replace the brushes or even replace the entire motor if there is too much damage.

On the other hand, Brushless motors have no parts that are in direct contact. Lack of internal friction and heat means that there is less wear and tear on the internal motor. 

Therefore, a brushless motor requires less maintenance and can operate longer.

4. Better Protection From Water

Brushless motors offer more protection from the elements. The motor is completely sealed and no water or dust can get inside, preventing any damage to the internal components of the motor.

In comparison, the brushed motor has many holes to increase cooling. This can allow any particles or sediments to get inside and damage the motor. 

5. Less Radio Interference 

Friction generated by the components can create radio interference. This can prevent you from running your RC boat properly. 

A brushless motor reduces this interference in your radio receiver allowing you to drive your boat without any issues.

Disadvantages of Brushless Motors

Despite being the newest technology out there, brushless motor boats have some issues that also need to be considered.

1. Brushless Motors Aren’t Self-Contained

The brushless motor has a controller that is necessary for operating the motor. Without the controller, the motor is completely useless. 

This controller determines when to change the direction of the wires inside the circuit of the motor. This controller is external and on RC boats it is known as the Electronic Speed Control (ESC).

2. Brushless Motors Are Costly

The brushless motor costs a lot more than the brushed motor. 

The newer technology in brushless motors means that they are quite pricey as compared to their brushed counterparts. So, you might end up spending a lot of money on one motor.

2 Types of Brushless Motors

Brushless motors typically have two types depending on the internal circuit of the motor. 

In brushless RC motors, there are fixed electromagnets and permanent spinning magnets. The setting of the two magnets determines the type of the motor.

1. Inrunner Brushless Motor

Inrunner brushless RC motors the electromagnets are in the shape of a cylinder around the outside of the motor. The permanent magnets rotate around the inside of this cylinder.

Inrunner motors look more like conventional motors. They also have a higher rpm due to smaller rotating magnets.

2. Outrunner Brushless Motor

In outrunner motors, the electromagnetic windings are fixed in the center and the permanent magnets spin around them. 

They have a rotating outer casing with permanent magnets on the inside part of the rim. They have a lower rpm and therefore turn more slowly. However, they have greater torque.

The outrunner motor is the preferred choice as it provides greater torque and therefore has increased efficiency and performance as compared to an inrunner motor. However, their design makes it difficult for water-cooling features to be integrated into the motor. 

Our Top 4 Picks for Brushless Motors

We’ve told you the different factors to consider while choosing a motor for your RC boat. Now let’s get into some of our top picks. 

1. Turnigy XK2845-3650KV Brushless Inrunner

The Turnigy XK2845-3650KV Brushless Inrunner motor is one of our top choices for RC boat brushless motors. 

Turnigy ensures that every motor part is flawless. They test each motor module at 14 different stages. So, you’ll receive consistent quality in Turnigy motors.

This motor uses precision bearings designed for high speed as well as Nickel-plated sintered neodymium rotors. This gives it increased efficiency and high power in high-temperature conditions as well.

The motor has a 3650 Kv rating with 4 poles and a maximum current capacity of 48A. You also get 16.8V/4s Lipo of max voltage.

You can get the Turnigy XK2845-3650KV Brushless Inrunner for $25.69, available at hobbyking.com

2. Dynamite A3650-2000Kv

This water-cooled motor has a 2000KV rating. The motor features a 6-pole design with 5.5 mm motor bullets. The plugs allow you to connect seamlessly with existing ESCs without any hassle.

This brushless motor offers reliability combined with high power and great speed allowing you to race your RC boat without any hassle.

You can get the Dynamite A3650-2000Kv here on Amazon. 

3. XTEAM 3665 Brushless Motor 

The XTEAM 3665 is a beast you can find in various KV levels. The motor is appropriate for boats of sizes 650-800mm. 

The 3665-2700kV brushless motor provides great speed, torque, and stable output power to your RC boat. However, you can go for higher kv levels if you want more speed, acceleration, and torque.

This fine metal motor features a 4-pole design, ensuring a smooth and constant power delivery. The brushless motor also features advanced processing technology that regulates heat and increases motor durability. 

The XTEAM motor is a fast and reliable motor that can allow you to drive your RC boat with ease. 

You can get the XTEAM 3665 here on Amazon.

4. SSS Motor 4082 2000kv

The SSS Motor 4082 2000kv Brushless Motor comes with neodymium magnets and high-quality bearings to maximize power and torque.

The motor is hand-wound to increase efficiency up to 90% and contains end bells that are assembled with screws for both strength and ease of maintenance.

You can get the SSS Motor 4082 2000kv here on Amazon.

If you would like to know more on brushless motors for your RC boat plus more of our picks of the very best brushless motors for your RC boat. Please check out my article for best brushless RC boat motors here

How To Choose an Electronic Speed Control (Esc) For Your Motor?

Choosing the right motor isn’t the end of your journey. Your brushless motor requires an ESC to function properly.

The ESC (electronic speed control) decides the speed of the brushless motor. A good ESC will integrate seamlessly with your motor to provide the best experience while you drive your RC boat.

So let’s look at the different factors you need to consider while choosing an ESC for your RC boat motor.

I have many articles regarding ESC’s. You may want to check this one out: How To Tell If Your ESC Is Bad

Consider the Current Rating

The first thing you should consider when choosing an Electronic Speed Control for your RC boat motor is the current rating. This is also known as the ampere rating. 

When motors spin, they draw current. However, if they draw a current that is more than the ESC current rating, then your ESC will overheat and may get damaged. 

Motor size, KV ratings, and the size of your propeller can affect your ESC drawings. Therefore it is important to choose your ESC current rating after selecting a suitable motor for your RC boat.

A simple rule is to select an ESC with at least 5A capacity more than the highest rating for the motor. 

There are two ratings for an ESC, Burst, and Continuous. The burst rating is the highest current that the ESC can withstand for a short “burst” of time. The continuous current is the maximum amount of current the ESC can handle continuously.  

Consider Input Voltage Rating

The maximum amount of voltage that your Electronic Speed Control can handle without getting damaged is known as the input voltage rating. 

You have to make sure that your ESC is compatible with the LiPo battery that you are using. For example, some ESCs can support up to a 3S battery voltage while some support 6S. 

A battery voltage that is too high can damage your ESC as well as the motor on your RC boat.

Consider the Weight and Size of the ESC

The weight and size of your electronic speed control also depend on its current rating. 

ESCs need a certain weight and size to increase performance as well as retain their cooling capabilities. Lighter ESCs may have issues with heat dissipation that may cause them to overheat.

Generally, single ESC units weigh around 4 to 6 grams.

How To Connect ESC to Your Motor?

Connecting your Electronic Speed Control device to your motor is simple. 

Both the ESC and the motor have three wires, these need to be connected together in the right order for the motor and the ESC to work.

The middle wire of the ESC needs to be connected to the middle wire of the motor. 

Then, if you want a clockwise spin, you need to connect the right wires with each other and the left wires with each other. To get a counter-clockwise spin in the motor you will simply reverse the direction of the wires i.e left to right and right to left. 

FAQs

Can Brushless Motors Work Without ESC?

No, brushless motors require ESC (electronic speed control) to work. ESC keeps the motor rotating and ensures that there is power by switching the current between the wires of the motor. This controls the AC to DC conversion in the motor. Without the ESC the brushless motor would not be able to function.

Are Motors With Lower KV Better?

Motors with lower KV are faster than motors that have a higher KV. The higher the KV, the more resistance there is. More resistance ultimately results in the motor slowing down due to friction. This lowers the power generated by the motor.

Final Thoughts

Choosing the right motor for your RC boat is necessary for optimal performance. You can choose the motor diameter that suits you by measuring the size of your hull and considering your rpm requirements. Good luck finding the right motor and safe boating!

I hope that this article has helped you in some way. Please check out more of my content covering all aspects of RC cars, planes, boats, drones, and helicopters at goodiesrc.com

Paul Good

This awesome hobby has something for everyone, whether you like to build your RC from scratch and keep modifying till you've got it to just how you want it, to track racing. You might be out with a boat, or perfecting your skills with a plane. Are you taking some cool pics with your drone? I'm sure my site will help you on your journey.

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