TheFridge189

Hello rcuniverse. I think this is going to be a very stupid sounding question, but I need some advice! So I have an rc boat with a traxxas 380 vxl motor, and a aquacraft 45 amp brushless esc. On land, the motor screams and is very very fast. However, when I put it into the water, the motor turns very slow, and the ESC wont let me hold throttle for more then a second. I even tried lifting just the prop out of the water, and the motor was super fast again. So I slowly lowered it iinto the water, and It went from a big roost, (Only part of the propeller submerged) to little ripples in the water (Prop fully submerged). Im guessing this is a sign that I need a smaller prop? But I find the ESC not letting me hold the throttle for more then a second odd, or maybe its just a feature of the esc to let idiots like me know the prop is too big haha. MY PROP SIZE isGrimRacer Metal Prop
40mm Diameterx 53mm Pitch
2 Blade Propeller for 3/16" prop shafts.
Well anyways, any help would be much apreciated!!!
Thanks
~Nikko

Diesel6401

My Feedback: (1)

Need more info:

clockworkclock

How long have you had that setup for?
If the motor is spinning Extremely fast when you take the load off the motor it is possible that the rotor has been demagnetized.
Which means you got your motor too hot at some point in time.
This means stupid amounts of rpms with no torque. So once you put prop in water it would slow right down.
Also as far as what the other guy said, when you run your brushless motor unloaded, I though the only danger was getting too many rpms, then the rotor would fracture and break apart.
Is that the damage you are referring to? Because then you can just try it on lower volts and the damage would not be an issue. Also most rotors will only break apart at extremely high rpms,
my neu motor is rated upto 60k rpms so it can probably go a little higher then that.
Then again I hevent been in brushless very long so what do I know ^ . ^
I still suggest you try to find out if your rotor is demagnetized.
If you take your motor out, hold the can and pinch the rotor with your fingers, when you apply throttle the motor should be able to spin free, or give you a really hard time trying to keep the rotor still.
If it doesn't then chances are your rotor is demagnetized. Also DONT DO THIS VERY LONG, ONLY A SECOND OR TWO! IF you do it too long you risk damaging your esc and motor.

Clock.

Von Ohain

There is no danger associated with running a brushless motor unloaded.
If your motor can break from excessive rpm when running unloaded, your problem is that you have choosen a motor with too high Kv rating.
The only danger with watercooled setups is running them dry for EXTENDED PERIODS, in wich overheating is a real risk.
This has, however, nothing to do with running it unloaded.
And this is never a problem when just dry testing your boat for just a few seconds.

Most motors is rated for around 50-60k rpms, and will endure substantially more than this.
A unloaded boat motor will never ever run as fast as this, simply because a properly selected boat motor will land you somewhere in the ball park of 30k rpm LOADED.
If 30k rpm loaded gets you over 60k rpm unloaded, your problem is a very poor setup, and ruining it immediately is the BEST thing you can do, since that gives you a real excause to buy a better setup

TheFridge189

The hull is 72inches long. I know all of you people are going to say i'm crazy for putting this motor in this boat, but it was all I had lying around and i'm not ready to spend $800 plus on an 1/5 scale brushless setup. I know my motor is not demagnitized, and I know I didnt hurt it by running it unloaded because I was aware of the dangers of running the motor unloaded, and I onnly did quick jabs of full throttle, and about 1/2 throttle for 1-2 sec unloaded. I never went flat out for like 5 seconds like I think what you guys thought I was doing. I am running the boat on 14.4v, and I know my motor works fine. I know on real boats in this type of scenario you would change the pitch, which could make sense for me as the prop sits fairly deep in the water. Anyways, there is some new information for you guys to ponder.
Thanks!
Nikko

Von Ohain

I'd say your prop is too big, and your motor is too small.
I run 40mm surface drive props on a 540 can size (36mm) high performance brushless motor (Leopard).
40mm surface drive is big even for a 380 can size (28mm ?) high performance brushless motor, even more so for a poor bog standard Traxxas, running submerged drive!

You could get away with running a large prop on a small motor, by choosing a low Kv motor, spinning slowly.
But that doesn't sound like its the case with you, since it spins vigoriusly when unloaded.

I'd say get a proper setup, and if you can't afford a large enough electric setup, make it a gas boat
Very large electrics is expensive, cumbersome, and makes little sense to me.

TheFridge189

Yes I fully agree, and I will eventually put an rcmk motor in the boat. So I tried a new prop and it worked better. However, the esc is acting VERY weird. For some reason when under load (In the water) the esc won't let me do throttle for more then about a second. When on land, I can give throttle for as long as I want. Then, sometimes when I wait untill the esc lets me give throttle again (When in water) I will give it throttle and sometimes the esc will beep, and shut off. VERY weird. Any ideas? I even just read the manual for the esc and it failed to mention any of these probloems. It is the same esc as used in the sv27.
Thanks
Nikko

shenlonco

My Feedback: (2)

I recall those traxxas 380 brushless motors are very high KV and also what battery type and voltage are you using/
i would suggest going with a very small prop 30mm and even much smaller.

TheFridge189

I am running two 6 cell nimhs in series. 14.4v. The prop I put on is probobly around 23-35mm. I remeber it was designed for 380 motors, just to put that out there.
Thanks
Nikko

shenlonco

My Feedback: (2)

Well that's why it won't work the motor is 4000KV so X your 14.4v = 57,600 RPM way to much for a boat? what size is the hull?
you want to be in the 30,000RPM range or a little more so if you run one 2s lipo or even one 7.2 or 8.4 NIMH pack it would work better also you need a pretty good pack for it also.

Von Ohain

I think the ESC is entering some failsafe mode from too high current, too high temperature or that the low voltage cutoff engages from battery voltage dropping so much under excessive load.
I believe this problem will correct itself when you get a smaller prop and get current draw back to acceptable levels (if the ESC has not sustained damage already).

Diesel6401

My Feedback: (1)

WHAT you put that motor in a 72" boat, what did you expect would happen? I'm surprised your didn't blow something up. That motor wouldn't work in a 22" hull let alone 72". You really need to take some time to learn what your doing. Your going to hurt someone or yourself being careless. Also DO NOT RUN THE MOTOR UNLOADED, IT WILLLLLLLLLLLLL damage your motor. Brushless motors NEEED a load on them.

It you can't pay to run the boat the correct way,then put it away or sell it. Just throwing random motors into stuff is going to lead to a disaster

blizard05

A 380 motor in a 72' boat,thats like puting a Zen in a F250

shenlonco

My Feedback: (2)

Yep right on and i did not see it was being used in a 72" boat that's crazy!!
First way to big of hull
second to much KV and way to much voltage and to small of a motor that's for sure!

Von Ohain

I don't think there is anything constructive coming out of yelling at him.
Nobody has ever gotten hurt from running a underpowered RC model.
Not knowing how all the internal works does neither present any safety risk.

In much the same way as your local priest doesn't know how the internal combustion engine works, but hes a way way safer driver than the weekend warriors that know their cars in and out.

I think its better to just stay quiet rather than start yelling. The best thing you can achieve is to scare him off the hobby, and how helpful is that?
We have all been beginners once. Show them patience if you want to help them, and if you haven't got the patience, don't bother at all.

Thats just my thoughts.

And no BLDC motor will get damaged from running unloaded.
I hold a B. Sc. in electrical engineering, if you don't trust my word, I'l give you the f**kton of equations to prove me right.

BLDC motors can get damaged from running without cooling, but thats a entirely different matter all together.

shenlonco

My Feedback: (2)

I still say he has to much voltage for that set up he needs to run a 2S lipo or a good 7.2 or 8.4 NIMH pack to at least get the boat moving at 14.4V it puts to mucjh of a load on the motor.

Von Ohain

Which is easily compensated for by selecting a smaller prop, until amp draw is down to acceptable levels.

If you also remember to take Voltage drop into account when calculating loaded RPM, you get to a number that isn't too bad actually.
40320 rpm, with a load constant of 0.7.
High, thats true, but for a small prop its not excessive.

And the load constant of 0.7 is a thumbrule for a typical setup.
A highly strung setup with a high Kv motor will have even poorer load constant, landing his loaded RPM even lower than this estimated 40320.

With a load constant of 0.6, he'll land on 34560 rpm, which is pretty good rpm actually.

The only problem I see with this setup is the poor load constant, which has arisen from a highly strung drivetrain.
Not ideal, but the guy has no money for a new setup, and he want his boat on the water, so lets do whats best possible out of the situation.

Any dumb engineer can buy his way out of a problem with money and fancy equipment.
The true genious solves his problems with as simple means as possible, of what he has lying around.

And yes, sure this won't be any racer. It will barely be limping with that small motor, but then again, if thats all the gear he can afford, then its all he can afford, and thats the end of it.

shenlonco

My Feedback: (2)

I made lots of brushless boats he needs to run lower voltage and a small prop or it will just cut out like he described all the time due to thermal shut down.

Von Ohain

Do you agree that 34 000 rpm is not excessive?

If you can answer yes to that question, you have also agreed that 4000 Kv is not too much on 14.4 V battery voltage.
Given that you believe in maths, of course.
If you don't believe in maths, then theres not much I can do to persuade you.
Witchcraft isn't my speciality, and you need witchcraft to not make my equations correct here.

TheFridge189

Thank you Von!First of all, I understand that this motor is underpowered. However, I also know from my expeirence in another 28 inch boat, this motor on 14.4v has pushed it to upwards of 35mph. SECOND of all, If you guys are just going to say "You're an idiot go spend upwards of a thousand dollars to get this boat running" I would apreciate it if you left this forum. See what you guys don't understand is how much fun I have had building this boat all myself, and I havnt spent gobs of money on it.. Not to state the obvious, but it's clearly you guys who don't understand the true fun in this hobby, and youare too blinded bymoney to see what this hobby is truly about.Thanks for the little advice you gave me.With bthat nonsense out of the way, I am still not exactly convinced on how lowering the voltage may fix the problem.Could someone explain to me their logic behind this? Ithink the problemis strictly drive train/ pitch issues.Tommorow I willgrease up my flex shaft even more and go from there.
Thanks
Nikko

shenlonco

My Feedback: (2)

As i stated above here it is again when making a boat you figure RPM out like this as your goal to start with (it's not 34,000 RPM ) It's 57,600 RPM TOO MUCH! and for a 380 hidious! you know how big of a brushless motor you would need to turn a prop with that much RPMs?

Well that's why it won't work the motor is 4000KV so X your 14.4v = 57,600 RPM way to much for a boat? what size is the hull?
you want to be in the 30,000RPM range or a little more so if you run one 2s lipo or even one 7.2 or 8.4 NIMH pack it would work better also you need a pretty good pack for it also.

shenlonco

My Feedback: (2)

been there done it before with your set up 57,600 RPM you would need a super small prop and when you do do this then your boat won't move because of it's size and the prop then will cavatate and just never get the boat up and on plane.

Now you lower the voltage the RPMs of your motor get almost half less you then can use and find a prop that will push your boat.

Just go look at any brushless boat that runs on two 2s lipos and see there motors KV rating for your self they are NOT 4000KV for this reason!
they use 1800KV maybe 2200KV motors...just look at aquacrafts site and see what they use in the 2 battery pack brushless boats.

Von Ohain

Quod erat demonstrandum

shenlonco

My Feedback: (2)

Don't know what i am talking about ha?
here are some 2 battery pack boats with what KV motors they use.

I need no say more!
CASE CLOSED!
if you don't want to take my advise then so be it.

Super V 27
36-56-1800kV Brushless Motor
Designed for marine applications, the 6-pole 36-56-1800kV brushless motor delivers smooth, consistent power – along with higher efficiency, longer run times and less maintenance.

montley crew
â–*Water-cooled 6-pole 1800kV brushless motor!

UL1
The 6-pole, 36-56-2030kV brushless motor is an AquaCraft design created exclusively for the UL-1 and high-speed hauling. Includes gold-plated, 4 mm motor plugs to boost efficiency.

proboat mystic cat
Programmable 60-amp (4S compatible) water-cooled ESC
Powerful 1800Kv water-cooled brushless motor

Pro Boat Stiletto
Motor Size: 1800Kv brushless water-cooled

proboat black jack 26
Motor Size: A3630-1500 water-cooled brushless

proboat miss geico
1500kv water-cooled brushless motor provides mind blowing speeds and maintenance free operation

TheFridge189

So if I used this same setup, but with an lower kv lets say aquacraft motor, It wouldn't have these problems at all?
Thanks Shenloco!
Nikko