what size brushless motor for rc boat

What Size Brushless Motor for RC Boat? [Comprehensive Guide]

Introduction

Getting into the RC boat hobby can be exciting and super fun. But there’s one thing that every newcomer to this hobby struggles with. That is finding out the correct motor size for their RC boat.

Don’t get tensed at all. We’re here to help.

How to determine what size brushless motor for rc boat?

Firstly, the motor size depends on the dimensions of your boat. There are some ranges for choosing a motor according to the boat size. Then comes the KV rating of the motor. You must pick a motor as per the battery and rpm requirement. Lastly,  you should consider the relation between motor diameter and rpm.

This is maybe not enough to help you make your decision. Go through the whole article, and everything will be a lot clear.

3 Factors To Choose The Right Size Brushless Motor

There are different types of RC boats out there. Generally, the motor can be selected by trial and error process. But choosing a motor for a specific boat is tough. 

To make that easier, you have to understand the factors below.

Hull & Motor Size

The first thing you have to consider before choosing the motor is the hull size. The length and the diameter of the brushless motor depend a lot on your hull. 

There is no perfect motor diameter for any hull size. But there are some ranges that might help you choose your motor size. The range is reliable on both the length and width of the hull.

Let’s take a look at some of the ranges regarding hull and motor size ratio.

  • If the hull size is around 600mm, the can size of the motor should be 28-28mm.
  • In the case of a hull around 800mm, you should use a 36mm can size motor.
  • You’ll need a 40mm brushless motor for hull size 1200mm or so.
  • If the hull size is more than 40mm, the size should be in the 56-58mm range.

These ranges will make your motor list a lot narrowed down. With the rest of the factors, your choice will become even easier. However, a good hex driver set is needed to adjust the motor on the boat.

Motor KV Rating & Battery

KV is simply the rpm per volt of the motor. And rpm is the number your motor rotates per minute.

Suppose you have a 6s LiPo battery in your boat. 6s Lithium polymer battery has 6 cells, and each is 4.2 volts when fully charged. When the cells are under load, they are 3.9 volts. So if we calculate, the total voltage we get from a 6s battery is 23.4 volts. 

Now just multiply the KV rating of the motor with the total voltage. Thus you’ll find out the rpm of the motor. By this calculation, you’ll know how much rpm you’ll get from a particular motor. This will help you select a brushless motor according to your rpm requirement and battery voltage. 

However, if you’re confused between 2 motors with different KVs, go with the lower one. In that case, you can use a larger propeller for your boat. Nonetheless, remember to pick between JB weld and Gorilla glue if you ever stick anything with your boat.

Nevertheless, controlling the speed of your RC boat is vital no matter which motor you use. For that, ESCs come in handy. Here are some excellent quality ESCs recommended for you-

Product 1
Product 2

Motor Diameter & RPM

There’s another thing that you’ll have to keep in mind. It’s the relationship between motor diameter and rpm. The convention is that rpm usually decreases with the increase of the motor diameter. 

As for the hull and motor size, there are some estimated ranges for this as well. Let’s have a look at the ranges for motor diameter and rpm.

  • If your requirement is 40,000 – 45,000 rpm, the motor diameter should be around 36mm.
  • With a motor of 40mm diameter, you would get about 35,000-40,000 rpm.
  • In the case of 58mm diameter, the expected speed is 28,000 – 30,000 rpm.

An example will make everything more straightforward for you. Let’s say you have a 6s powered

‘Traxxas Spartan’ RC boat. Considering the size of the boat, the motor diameter should be around 40mm. 

A perfect motor of this diameter would be TP Power Brushless 4060 V1. The motor has 1650 KV. As we saw before, a 6s LiPo battery has a loaded voltage of 23.4 volts. 

To find out the rpm, you just have to multiply this with the KV. You’ll get 38,610 rpm. However, remember to always choose high-quality batteries for your boat. And, you can easily find out the perfect motor according to the boat size, battery, and rpm requirement.

FAQs

Question: Are motors with lower KV better?

Answer: The higher the KV is, the more resistance there will be. The higher resistance results in a slower motor. It means the turn rating has an inverse relationship to the motor’s power generation. So motors with lower KV tend to be faster.

Question: How can I choose the right ESC?

Answer: The current rating is the first thing you need to look at when choosing an ESC. When a motor spins, it draws current. If more current is drawn than what it can handle by the motor, it starts to overheat. Overheating will cause it to fail in time. In worst cases, the ESC might end up in flames.

Question: What do the 540, 380, and 280 size motors mean?

Answer: 540, 380, and 280 sizes determine the diameter of the motors. The 540, 380, and 280 motors have 36mm, 28mm, and 24mm diameters, respectively.

Last Words

That was all about what size brushless motor for rc boat. Hopefully, the decision regarding your brushless motor is much clearer now.

And if you still have any queries, feel free to let us know.

Wish you good luck with RC boat hobbies!

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