Sizing a Model Boat Motor

Sizing a Model Boat Motor is an often problematic step in the building process. Learn how to figure your power needs for any powered radio controlled boat.

Sizing a model boat motor seems to be one of the most difficult parts of building a radio controlled model boat. Most often it requires a trial-and-error approach, and the starting point is a complete guess.

Even if this effort eventually lead to an optimized solution, it will have come at a price, both financially and in time spent.

There is a better way, however. Not unlike the way full-sized ships are designed, only in reverse.


Scale Speed

Just a quick mention of the term Scale Speed. You may ask: How do we scale speed? The simple answer is, we don't.

For a scale model boat it's highly impractical to convert a speed from one scale to another, and try and size a power train to fit that number. I don't even know how to do that.

Instead, it is better, and a whole lot easier, to aim for the model having the same appearance on the water as the full size boat or ship. So we're aiming for creating the same wave pattern as the original. This may sound hokey, but that is essentially how scale speed is defined.

To get there, it is best to start with the power rating of the full size vessel and recalculate the power to the reduced scale.


Sizing the Power Needs of a Full Size Ship

When designing full-sized ships, scale models are first made and pulled in a test tank. By measuring the drag the model experiences at different speeds, and factoring in the scale of the model, a relationship can be established between power and speed.

It has long been established that the relationship between the full size power need of a vessel, and its scaled down model is the scale factor cubed.


Sizing a Model Boat Motor

The method I use, calculates the power requirement from a full size boat of comparable design. Historic warships are easy - typically the exact performance is well documented.

Civilian ships and work boats can be difficult and may require that you use data from a tug to figure the performance of a trawler, for instance.

To illustrate, here are a few examples.


Example 1: Model Boat Motor for a PT-109 at 1:32 scale

As the first example, we may as well use the <a href="http://www.building-model-boats.com/rc-pt-boat.html">RC PT boat</a> that I'm building.

Here I'm building a semi scale model of the famous PT 109 at 1:32 scale. These boats were built by Elco and had successively larger engines as the war progressed, and the boats got heavier and heavier.

RC PT boat moving at scale speed.
pt boat model at scale speed

For the sake of this exercise, let's pick the mid-war version at Three Packard V-12s at 1,500HP each. That's a total of 4,500HP.

The scale I've chosen is 1:32. Cubed, that is 1:32,768.

The power needed for the model PT 109 is: 4,500HP/32,768=0.137HP

One HP = 746W, so 0.137HP = 102W

Now there is only one little detail left - efficiency. The specified power on the full size engines is in mechanical Horse Powers on the motor output shaft.

I can almost guarantee that the full size power train is more efficient than that on my model. Therefore, take the calculated number as a minimum number. So either live with the fact that your boat will be a little slower, or add a factor of safety of about 25 to 50%.

To select motors, we have to decide if we're going to use a single or multiple motor set-up and divide the total power with the number of motors used. So here the options would be:

  • A single motor at 100 to 150W
  • Two motors at 50 to 75W
  • Three motors at 33 to 50W


Example 2: Artesania Latina Hellen RC Trawler at 1:20 Scale

This came in to the Q&A section as a question from David Thomson of Toronto, Canada. He bought the kit and was wondering how to power her as a radio controlled model boat.

The trawler Hellen by Artesania Latina.
model of trawler hellen by artesania latina

In the original question, he was asking if he should go with a Graupner 400 or 500 motor.


Here are her particulars:

Displacement 6kg (about 12lbs)

length 858mm (33 3/4 inches)

beam 240mm (11 5/8 inches)


Artesania Latina apparently didn't provide any help whether it being a suggested model boat motor size or any hint as to how the original boat was powered.

Given we have nothing to go by, this method is not as precise as if you have the original motor spec. However, I think we have a good shot at finding the exact motor size needed without any additional iterations.

In this case I went to a book called "100 Boat Designs Reviewed". This is essentially a sales catalog for the boat plans offered through Wooden Boat Magazine.

One boat that caught my eye is called "Dynamo" designed by the legendary William Gardner - and she looks a lot like a fishing boat.


Dynamo's particulars are:

Length: 11.7m

Beam: 3.4m

Displacement: 15 tons

Powered by a Six-Cylinder Chrysler Crown @ 70 - 135hp


Using the numbers from "Dynamo" and scaling them down to the size of the "Hellen" we get this:

Length: 858mm

Beam: 249mm

Displacement: 5.9kg

Power: 0.053HP = 40 Watts (based on 135hp at full scale)


As you can see, the length, beam and weight are all right on. The 40W power indicates that the Graupner 400 should be sufficient. It is rated at 35 to 75W.

If the 500 motor was selected, the boat would have much more head-room, but may run the risk of having too much power. This can be cured by choosing an appropriate gear ratio.


Model Boat Motor and Gear Box

Most scale models do better with a gear box to swing a scale size propeller. The working point of the motor will be more favorable.

The most notable difference with a gear box is that the motor efficiency will be much closer to optimal. As a result the motor will run cooler and wear less, leading to a longer motor life.

The gear ratio needed depend on the properties of the motor and the type of propeller, pitch and diameter. Usually the ratio ends up in the 2:1 to 6:1. There is some testing required here.

Graupner makes a range of nice motors, gear motors and accessories, but are a bit pricey in my opinion. Something I've started to look into are geared motors for model airplanes.

A GWS gear motor for model airplanes can double as a cost effective model boat motor.
model boat motor

So far I've found the GWS motors exceptional value for money. For instance, the GWS EPS-150 can be found online for less than $10. It comes geared and with double ball bearings on the output shaft.

The output shaft is a threaded M3 shaft. You may need to get creative to transfer the torque to your propeller shaft.

The mounting may need some mods too, but I'm sure it can be worked out.

These are relatively small motors. If you have a big boat, obviously you'll need a bigger motor. If the GWS motors are too small for your model boat, the Graupner model boat motor line are your next best option, in my opinion. They're more money, but are well engineered and will work well.

If your boat is too big for the motors offered by Graupner, you may have to resort to what I call "the Robot Wars Territory". Some of the best value for money are replacement motors for brand name cordless drills, such as DeWalt, Makita and the likes. They are available as spare parts and can be found online or from your local repair shops. They make extremely powerful model boat motors of up to 2HP.

One thing to be aware of though. The more extreme performance of the motor, the more likely it is designed with a timing angle in the commutator. Say what??? Well, the effect is that the motor will not be equally fast when run forwards vs. backwards. This will cause problems for model boats with multiple motors running opposite directions.


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