@Fishercat
Thanks so much for your clarification, Fishercat.

It's very useful for me and all naval modelers.

As I said, I prefer to take measurements (when possible) and read the factory data (if available). Those formulas are purely indicative.
However, considering a higher load current (working current) is a more conservative approach. We'll adjust the ESC, battery, and cables to that current; if the motor draws less, so much the better; it means these devices will be stressed less.
However, this has nothing to do with what you're saying, and what you say remains valid. In fact, if we can adjust the load and therefore the motor current (like choosing the propeller), it's best not to raise this value too much.
For the 60 A calculation, I actually used 20%, which gave me 12 A. It might still be a bit high.
In conclusion, it's best to size the ESC and battery for the most critical situation, but, if possible, let the motor operate under moderate stress.
We should also consider the motor case temperature in case of doubt.
I'm referring to brushed motors.

@AlessandroSPQR great stuff.

I would add one thing - a lot of hobby motors can't be run even at 25% of stall current continuously without risking overheating. When I see something like "max power 47 watts" on a 12v motor - that low figure tells me that they're protecting the motor. That's also where the "prop no larger than can diameter" guideline comes into effect.

Oh, I almost forgot.
I’m writing another message because I’ve already edited the previous one too many times to refine the translation, and I don’t want to make a mess of it.

Actually, in model boating, there is another state that exists between the "no-load" current and the "operational" (working) current.
It’s not particularly useful for practical purposes—and you’ll never see it mentioned in a motor’s manufacturer specifications—but it’s worth noting to avoid any confusion.
This state occurs when the propeller spins while not submerged—that is, when the model boat is tested "dry," out of the water.
The propeller spins freely in the air without the resistance of water; consequently, the current draw will be lower than it would be when the propeller is moving through water. However, this draw will still be higher than the true "no-load" current (i.e., with the motor’s pinion gear completely unencumbered), due to the friction within the propeller shaft assembly.

While it isn’t necessary to know the current draw in this specific state (propeller spinning out of the water), I’m mentioning it to help you avoid potential mistakes.
If you want to determine the "no-load" current draw (I0), be sure to measure it using an ammeter wired in series before installing the motor into the model—not after.

Although I stated that "it is not necessary to know the current draw in this condition," it is also true that if the current draw in this state is nearly identical to the true no-load current, it means you have built a highly efficient drive system. An efficient system with minimal friction will consume less electrical current.

@Karoq

Hi Karoq, perhaps I missed something or misunderstood—if so, please excuse me and disregard my comment.

I believe Fishercat, Toradog and DJW have already provided you with a lot of useful and precise information, but I would like to clarify one fundamental point.

When analyzing the characteristics of an electric motor (specifically brushed motors), in addition to other manufacturer specifications, you should consider three current values:
- No-load current (also referred to as "idle current").
- Load current (or "operating current"). When referring to the maximum value for which the motor is designed to operate continuously, the term "rated current" or "continuous current" is used.
- Stall current.

• No-load current (I0): This is the current drawn by the motor when it spins freely without any mechanical load applied to the shaft. It accounts for overcoming internal friction (in bearings and brushes) as well as iron losses. This is the lowest current value encountered during normal operation.
• Load current (IL or In ): This is the current drawn when the motor is subjected to its intended working load (nominal torque). This value varies based on the effort required from the motor and serves as the reference parameter for sizing cables and thermal protection components for continuous use.
• Stall current (IS): This is the maximum current drawn when the motor shaft is mechanically locked and unable to rotate (RPM = 0). In this state, the back-electromotive force (back-EMF) is zero, the impedance is at its minimum, and the motor acts essentially as a simple resistor—generating the maximum possible torque, but also the maximum amount of heat. If this condition persists, it can burn out the motor.

You should not take the no-load current into account, as you will never operate your model under those conditions. In the water, the current draw will always be higher (I personally measure current by placing an ammeter in series with the circuit while testing the model boat in a bathtub at various speeds, from minimum to maximum).

You should never use the stall current (which I suspect you might be doing now) to size your ESC and batteries—unless you are using a specific calculation or algorithm that treats it merely as a reference parameter. Stall current is harmful to the motor; you should never reach it, and if it does occur, it must be interrupted immediately via a fuse or some other protection system.

The current you should focus on is the "load current" or "operating current."
Often, however, this value is not provided alongside the other two.
In such cases, you can either apply empirical formulas based on the other two values ​​(no-load current and stall current) or conduct practical tests—which, in my opinion, is the best approach.

What formulas can be used to determine the operating current?
As I mentioned, I prefer to run a series of tests under real-world conditions (with the propeller in the water at maximum speed) and then add a bit of extra tolerance.
Nevertheless, here are a few formulas:

If you do not know the torque of your load but want to determine the current at which the motor will perform best, you can mathematically calculate the "Maximum Efficiency Current", Ieff_max.
In the context of RC modeling, this is the true "ideal operating current"—the point at which the motor wastes the least amount of energy as heat, thereby offering maximum runtime and longevity. It is calculated by taking the geometric mean of the no-load current and the stall current:
Ieff_max = the square root of (I0 × Is)
• Practical example: I0 = 1.59 A; IS = 60 A.

• What does this mean? To ensure this motor operates at peak efficiency, you should select a propeller or gear ratio that results in a current draw of approximately 9.8 Amperes under load.
1.59 A × 60 A, all under a square root, equals 9.8 A—approximately 10 Amperes.

An alternative, simpler, and more empirical method:
If you are merely sizing cables or a battery and wish to avoid complex calculations, there are two widely used rules of thumb in the field of RC modeling for estimating the recommended continuous operating current:
• The 20–25% of stall current rule: A safe continuous operating current—one that prevents the motor from overheating—is estimated to be around 20% to 25% of the stall current. For instance, if the stall current is 60 A, the motor can safely operate continuously at approximately 12 Ampere.
• The "triple no-load" rule: For standard, high-efficiency brushless motors, the nominal operating current is often between 3 and 4 times the no-load current. If a motor draws 1 A at no-load, its typical operating current under an optimal load will be approximately 3 A – 4 A.


I have excluded any formulas that require knowing the operating torque or the stall torque.

While I don’t have as much practical, hands-on experience with brushless motors as I do with brushed motors, I believe that—despite some minor differences—the same principles apply to both.

If you’re interested, some time ago I compiled a collection of information and experiences (from myself and other model boat enthusiasts) regarding small, permanent-magnet brushed motors. Among other things, it includes the method described by RoyCV, which I have quoted verbatim, citing him as the source.

Here is the link.

https://model-boats.com/wiki/147457

@Karoq if you show a pic of the prop, I can estimate the pitch.

Hi Dave
The stall current at 60 amp is "Extrapolated " according to the data sheet, ( Guesstimated ? ), I was a little sceptical about that and for Max power it says 30amp current. So if I take it easy on the throttle then I think 2 esc and 2 batteries should do. If not then I'll ugrade the batteries .
I have some lithium batteries 18650 doing nothing at the moment .
Don't know the pitch of the props and the description dosent give any indication and they do counter rotate.

@Karoq Hi, I found this info on the HS540, most of the specifications you mentioned line up, but that 60A stall load sounds very high for a motor of this size. I agree that if you have two motors drawing even 1.9A no load then that single NIMH battery could well be struggling once you put the boat in the water and start to load the motors up.

HS540 High Torque DC Motor 6-12V — Switch Electronics | Your One Stop Component Shop https://share.google/oqTbEpmwQOb1GmSDX

Tricky to say re the props without knowing the pitch, sounds like those motors like to rev given a 20,000rpm no load top end. You may need to experiment to get the best result, so if the boat is heavy a smaller, lower pitch prop will allow the motor to rev with manageable current draw, up the diameter or the pitch and the motor will load up more. And as you have a pair of motors the props should ideally counter rotate, left and right handed.

I found a datasheet for my motors ( voltage 7 volts ) and I think the batteries may not be up to the draw, the motors no load is @ 1.59 amp. Stall speed is something like 60 amps .
Props are 36mm so they should be OK?

I'll double up on the batteries and the ESC and see what happens , thanks for the suggestions

@Fishercat Thank you for the information.

@Chum444 sure. This is what I have. It will tell you what your amp draw is in real time and (at least until you unplug it) tell you how many amp hours of power you've used. It uses the same connectors as my LiPo battery packs so I didn't have to wire it up.
https://www.amazon.com/dp/B0DZNBRLJX

This larger one, that requires you to add your own connectors also gives you peak watts - which can be super-handy. I just didn't need it as I don't do models I can't hold on to, throttle it up, and read the display:
https://www.amazon.com/HTRC-Precision-Analyzer-Battery-Voltage/dp/B06XPVKY13?th=1

Both of these are generic and can be had from a number of folks with bewildering names at a slight variety of prices.

@Fishercat Care to share name, model # of the multimeter you are using? Sounds like a very useful tool.

First, many NiMH batteries of 4000mAh are only good for around 4 amps of draw before voltage drops. Some can't do that much. Cells designed for high drain could do more like 10 amps or more. But, I don't think either of us has a good way of knowing how high drain your battery is designed for.

Those motors will draw at least 4 amps EACH at full throttle, and that's assuming that your props aren't too big for them. And, a starting surge would be higher.

So, you may simply not have enough battery. You would likely have to try a LiPo or two or three of the packs you are currently using in parallel. Pretty much any LiPo that size should be good for at least 40 amps of draw. That's why more folks don't use NiMh and LiFeP04 packs - they can't keep up with motor draw unless you have good size packs and lightly loaded motors.

As to the motors, I saw similarly labeled motors on Ebay - labeled as 20,000 rpm no load and 1.9 amps no load at 12v. I also saw a UK vendor for a similarly labeled motor indicating that the motor is capable of putting out 47 watts of power. If that's at 12v it's about 4 amps. That's not to say that the motor CAN only draw 4 amps. That's just as much power as it should be allowed to draw before one worries about overheating. Stall current would be higher.

I don't know if motors with that label are generic or come in different windings. So, I don't know if your motors will behave that way - but there is a reasonable chance that they will.

What size are your props? A motor with that high a no-load speed is going to have fairly low torque, and I would not run it with any prop larger than the diameter of the motor can - 35mm. A larger prop, or even a particularly high pitch prop that size could cause it to overheat, as motors get much less efficient as they approach peak power output. Please note that if it's over-propped, running it on only 7.4v won't save it. It can still overheat. If you have larger props, you need bigger motors (or brushless - but the darn things whine like crazy).

As other folks have suggested, it would be good to test the amp draw. I don't test a motor at stall. I bought a little amp meter on Amazon that plugs in between my battery and ESC, using the same connectors as are on my battery packs. I read the draw at full throttle in a large plastic "test tank" - (storage bin for Christmas wrapping paper that can easily take a model that is over 1M in length). Some of the examples of such meters on Amazon will retain a memory of the maximum amp draw as well, so can be tested on the water.

As to your ESC - while the specs on the generic Chinese units ARE often absurdly over-rated, 4 or 5 amps per motor at full throttle is not unreasonable, and if the unit is good and doesn't need to be reprogrammed for your radio, it should be OK. If in doubt, you can always pick up an Mtroniks Viper - albeit for quite a bit more money for the capability. Personally, I am moving to Sabertooth 2x12RC units designed for robotics. They handle 2 motors and don't whine at low speeds like the Vipers. I don't know if they're available in the UK, however.

In general, I've had pretty mixed luck with the generic ESCs - some are great, but some have unexpected issues like needing to be programmed for my equipment or not have a usable neutral in the center.

My motors are described as HS540 6-12v which I think it will run on a voltage between 6 and 12 volts.
The ESC I dont think will be 320 amps it is just named that in the description , nothing to indicate on the esc what rating it is , I'll try the procedure suggested by ToraDog and work from there

Hi Chum The ESC is supposed to handle two motors , it has the two red and two black wires but I can wire in another the same to see if that makes a difference.

Both you and DJW have given me ways forward and things to try out

thanks

As I follow up, it should be remembered that a "540" motor refers to the can size of the motor. Not, the speed, torque, current draw, or voltage.

First, your information is rather thin. Also, you indicate that your ESC is a 30 amp unit, then you indicate that it is a 320amp unit. ??? Which is it. If the latter, as Chum says, beware..
The capacity of the ESC is irrelevant unless you know the current draw of the motors. 540 motors come in all sorts of different sizes. Also, what voltage are your motors? You have indicated that you are using a 7.2 volt battery, so are your motors 6 volt? if so, then they are drawing twice the amperage of a 12 volt motor.
I would start at the being. Determine your true motor specs., voltage and current draw. The latter can be done if you have specs for the motor. If not. hook up an ammeter in series with one of the motor leads, and apply the appropriate voltage to the motor while holding the shaft. DO THIS ONLY FOR A MOMENT!!! Long enough jus to get a reading on your meter. That reading will be your STALL current draw. The maximum draw of that motor. That number times two should be exceeded by the maximum current rating of your ESC. By how much is up to you.
If your motors are 12 volt. you need a 12 volt battery supply.
Make sure you shafts run free. No two motor and shaft arrangement ever turn exactly the same. Don't worry about that.

I would begin by testing the esc with just one motor attached. If that works satisfactorily the implication would be that the esc cannot handle the combined current requirements of two 540 motors. I’m told that the Chinese esc’s current capacities are highly overrated.
You may need an individual esc for each motor. If you do find this to be the case don’t forget to disconnect the red wire in the receiver plug on
one esc. Otherwise they will both be trying to control both motors. You want one “master” esc (red wire connected in the receiver plug) & one “slave” esc (red wire disconnected in the receiver plug).

The alternative to an esc for each motor is to install an esc with a higher current rating. One of my boats (my avatar) has a 45A esc controlling twin Johnson 540s. Works quite nicely.

The esc worked as far as I could figure . ebay china special, its a 320A brushed high voltage esc with cooling fan builtin on top .It finds the centre position automatically, thats the sound when first switched on, a high-frequency drive system with overheat protection, low voltage protection and uses a low resistance field effect tube. No other instructions were included.

This is the first testing in water and the shafts seem OK to me ( a novice)

I did notice one motor seems to start to run before the other . So I suppose that that shaft might be tighter than the other , I'll check out that one.

I do have another esc same type and make ,so worth trying that out .

Thank s for your pointers

Hi, could you specify the type of ESC being used..? Other thoughts would be has the ESC been set up and working well before, or is this first testing..? Also, are you sure the shafts are running free..?