Brushless Motor Requirements for Fast Electric Boats
By Chris Breen
I thought I would write a few notes in response to the frequent questions we get such as “What motor kV will suit the boat I am half way through building?” Or “what ESC should I buy?” Or, “I have bought a ready-to-run 4S boat, can I run it on 6S?” and so-on. As the issues are quite complex, I have tried to simplify things down a fair bit for new-comers.
What kV for my boat?
The kVs of the motors in our boats range from kVs in the several thousands down to kVs in the high hundreds, so the confusion is easy to appreciate. The kV is directly related to the cell count you plan to run.
It needs to be clearly understood that the motor kV number you need to choose is only half the question. You also need to understand the motor power rating you need to choose. If the motor cannot provide enough POWER, you boat will not perform even though the kV may be right. Torque is also important and is related to power but it’s more difficult to understand than power, so let’s just consider Power. Power is simple. It is just volts x amps.
A (simplified) definition:
kV is the revs/minute that the motor will theoretically spin at, for each volt the battery delivers. So a 1300 kV motor on a 6S battery will theoretically spin at 1300 x 3.7 Volts/cell x 6 cells or about 29,000 rpm.
Another (very simplified) definition:
Purely by coincidence, our boats perform best at around that same 29,000 rpm.
So, simply dividing 29,000 by 3.7 volts per cell and then by the number of cells planned (2, 4, 6, 8, or 10) will give a very good first estimate of what kV you should be choosing.
Doing this we get the following first estimate kV figures;
10S 800 kV
8S 1000 kV
6S 1300 kV
4S 1950 kV
2S 3900 kV
Unfortunately the theoretical rpm is forced down by the load on the motor and this effect gets worse as the kV of the motor gets larger. Allowing for this, we typically find kVs around the following numbers work well.
10S 850 kV
8S 1100 kV
6S 1500 kV
4S 2050 kV
2S 5000 kV
Ok, that’s the kV half the question answered. Now what about power? The issue is partly waste heat generation and whether the motor can tolerate that waste heat well enough to deliver a reasonable life time.
Now, without going into the whys and wherefores, the power requirements of our common boats are something like the following:
1/8 scale hydro (10S) about 2.3 kilo watts (2300 watts) (yes, that’s right, about the average radiant bar heater you have in your lounge, on FULL...)
Q hydro (6S) about 1.7 kW (1700 watts)
P Hydro (4S) about 1.1 kW (1100 watts)
N hydro (2S) about 600 watts
So we need to choose motors of the kV AND with AT LEAST the power ratings listed above, and preferably a bit spare.
Just to illustrate, that 1500 kV Q hydro motor will be about 85% efficient, meaning about 250 out of those 1700 watts going into the motor will turn into waste heat, very efficiently warming the motor up.....Yes, that’s a couple of those big heat lamps you probably have in the bathroom. Quite a big motor is therefore required to both deliver the needed power to the propeller and safely absorb the waste heat. This is why you see quite meaty motors in the bigger boats, and water cooling in pretty well everything.
What ESC do I need?
Big, is good.
The ESC needs to be able to accept the nominal voltage of the battery pack, and the amp draw. ESCs are usually specified as a maximum cell count or voltage (ie 4S, 6S, etc), and a continuous amp rating eg 180 amps or 240 amps or whatever.
Again without going into the whys and wherefores, the amp draw in our common boats is around the following: Note that amp draw is heavily influenced by the propeller choice as we will see in a moment.
1/8 scale hydro (10S) about 60 to 90 amps (approx)
Q hydro (6S) about 90 to 110 amps (approx)
P Hydro (4S) about 50 to 80 amps (approx)
N hydro (2S) about 80 to 130 amps (approx)
Interestingly, as can be seen, ALL our boats can use an ESC rated at around 100 to 150 amps or more and the smallest (2S) boats are likely to have the highest amp draw and therefore need big ESCs...
Most of us a quite conservative and choose ESCs rated substantially higher than the figures above. This gives us a good margin for current spikes on take off, props getting tangled in weed, carelessly testing a prop that is too big, and so on
Caution, many ESCs cannot accept voltages above 5 or 6S. Boats using higher cell counts must use ESCs specified as “HIGH VOLTAGE”. Unfortunately many ESCs also need a minimum voltage to “arm” properly at switch on, so check carefully that the ESC you are choosing will arm if you are going to use it in a low cell count battery boat.
Can I run my 4S boat on 6S??
Well, you probably can if you are very careful....
Let’s take the 4S UL1 Hydros from Aquacraft. Standard, these run a 2030 kV motor, 4S batteries, 60 amp ESC and an X440 propeller. The FE calculator software predicts this set up will pull about 55 amps and run at about 75 kph. What happens if you change to 6S without any other adjustments. FE Calc predicts a new speed a little over 100 kph, but amps have jumped to 105 and probably the ESC has exploded, the motor is suitable for frying bacon, and the batteries have puffed. But, if we had substituted an X432, FE Calc tells us the speed is only slightly slower at about 95 kph, but amp draw is back to a safe 45 amps. Unfortunately a X432 prop on a UL1 MIGHT just cavitate, only a test will be conclusive. Don’t just install 6S and expect the stock equipment to survive. The rule is “if you increase the cell count, you down-size the prop”.
I hope this is helpful, but remember these notes are greatly simplified and should be used as a guide only.