This article was inspired by a recent experience at our pond involving a motor boat with twin brushed motors, twin NiMh batteries, and initially, a single ESC unit powering the two motors.

In addition, the model also used a “Power Board” (such as the type supplied by Component Shop) with multiple fuses and a Voltage Regulator to provide a 5 volt output to run the radio control systems.

The initial setup featured a dual 20 Amp ESC connected to the two motors together plus 10 Amp fuse protection for each motor.

On accelerating towards full power in the water, the motors would “hiccup” like a bucking bronco with seemingly random speeding up and down but feeling restricted in its top speed too. The 10 Amp fuses would quickly blow. This gave about 30 seconds of sailing before stopping due to the fuses blowing. Replacement with 15 Amp fuses, still under the ESC rated power of 20 Amps, hence “safe” for the ESC, resulted in a similar debacle with both 15 Amp fuses blowing after a very short time.

My recommendation was to measure the current actually drawn by the motors at full speed and from there, you can make informed choices about the wire amp rating needed, the value of the fuses required, the amp rating of the ESCs needed, and crucially, whether the motor being used is appropriate to the boat’s desired performance and endurance (ie battery life).

Without that amps value, you are just wildly guessing as to what is needed to run the motor successfully.

All measurements need to be under load in the water. Taking an amps reading with the motor spinning freely on the work bench gives a totally misleading impression as to the amps drawn.

Please note that when looking at motors to buy, that an amps drawn may be quoted when free running, and this is far less than reality when under load.

For example, this motor draws 3 amps at 12 volts when spinning freely, but 14 times that when under load! And the propellers are only modest three bladed plastic ones with a diameter of circa 35-40mm.

After this, the owner took the boat away and returned a few days later with a pair of different ESCs in, each rated to 25 amps, and larger fuses, 20 amps each. Ie within the ESC limit of 25 amps for safety.

Into the water, and again, similar problems, though the boat would run if the throttle was kept only partly open at say up to 70% of maximum. Feeling optimistic, the throttle was taken to 100% and fuses were blowing again.

So, bring boat in, out of water, stare at the electrics and one of the ESC smoked itself after a blown fuse was replaced!

I then insisted that the amps drawn by the motors must be checked next, otherwise, the owner is simply going to blow up ESCs, and probably the Power Board (as that too has a rated maximum amperage handling ability), not to mention melting wires.... The owner concurred.

A Watt Meter was attached into a simple circuit – Motor at one end, Watt Meter in middle, and Battery at other end. Boat into water, ie under proper load, and connect.

The result was a spectacular result of 43 amps drawn by a single motor. Insanity for the speed being provided. Together the motors would draw over 80 amps!! The motors each had a suppressor block attached and the possibility of some current leaking through that was suggested by a club member. This was considered but the wiring on the suppressor was very thin (as it does not have to carry any significant current at all when working properly) and if any large amperage was flowing through it, it would have got hot, blackened and smelt burnt. It did not.

Further, in support of the “accuracy” of the 43 amps measurement, the wires to the motor were becoming uncomfortably hot (ie you would not want to hold them with your fingers) in a few seconds after startup. The wire was circa 14 AWG (rated to carry circa 32 amps continuous). This supports the over 40 amps measured in my opinion. And so of course the wires are too thin for that current draw too!

On Youtube, I found an “Unboxing video” of this particular make of motor and an ESC made by the same business sent to the Youtuber by the business. The implication was they were a matched pair in terms of amps. The ESC was rated to 50 Amps! This rather adds weight to our findings I suggest.

There was some disbelief expressed at the lakeside that this ordinary looking modestly sized brushed motor could really be drawing such a large current. It is a personal record as to the highest current I have measured! The evidence suggest it really does.

Further, there was no significant resistance to turning the propeller shaft so that was not causing a high current draw.

The owner is now considering replacing the motors with some which are more appropriate – ie draw modest amps for a reasonable performance that he will be happy with.

For those familiar with my canal narrow boat Isis, that has prodigious speed and uses a motor that runs on 12 volts and draws a mere 7 amps flat out.

In conclusion, all parts of the power delivery system need to be correctly specified to avoid cooking and loss of speed, and loss of money....

Battery - Fuse - Wiring - ESC - Wiring - Motor

And also consider the type of battery needed. High currents imply use of LIPOs, modest currents imply NiMh or lead acid.

Happy sailing!