Amps and Volts...

[electric]

I am running a Oval Master CF 29 with a 10xl and a x440 prop today. With this set up using two 7.4 volt Lipo's I was pulling around 58 to 65 amps. I then put in some 11.1 volt lipo's 30c and the boat was out of control fast! It seems like everytime I put these batteries in things start breaking. At any rate, I was pulling like 130amps! Here is the question. Do I need to prop down as I raise the voltage? It sure appears so. I think I have reached the top end of this hull anyway since it is torqued over on one side the whole time and is difficult to keep on line.

[G Doggett]

As you increase the voltage you need to use a smaller prop as the rpm increases.
However 2 X 11.1 is way too much for a 10XL anyway on direct drive.
The 10 XL is rated at 1668 rpm per volt, with a maximum ampage of 66, so on your original set up on 14.8 volts it was trying to spin at 24686 rpm with the 40 mm prop which loaded it to 58 - 65 amps, which is about right.
With 22.2 volts it was trying to spin at 37029 rpm, which is close self destruction, and it pulled 130 amps while trying to spin the 40mm prop at that speed.
So if the rpm didn't kill your motor the excess ampage probably has.
How hot was the motor after this run ?
Graham.

[electric]

Chuckle. That explains why their was smoke comming out when I was done! Really, the smoke was because the shaft had worked lose and was slipping. The motor was ok temp wise it is was at around 140 degrees. I suspect the high amperage might have been caused by the drive shaft "shrinking" under the torque to the point it was binding.

So your reply begs the question. Why have a 120amp High Voltage ESC that can handle up to 50 volts when the motor can't handle it? Should I be considering a different type of motor and prop combination to handle these higher voltages? I was thiking along the lines of higher voltage equals more watts(Power) with less heat(amps). I did some reading on the motor and it states that is good to 50,000rpm. So if I lowered the prop this would lower the amp draw, but the rpm would stay at the level you calculated correct??

This equation does not seem to be working for me. Wish I had gotten a electrical engineering degree at this point.


I am looking for top end speed on this set up. I am thinking of buying the 33" Delta Force hull because the Oval Master just isn't doing well with this much going on.

[video200]

actually it states that the bearings are good for 50 000 rpm. dont relay say that the motor is up for the 50 000 rpm task.

[electric]

True, but I should be at around 37k according to Graham

[video200]

yes
1668 rpm X 22,2v = 37000 rpm.
still dont think i would reccomend it. spessialy not when you pull those amps.

[electric]

Yes, but if I put a "smaller" prop on shouldn't I be able to get the amps to drop to an acceptable level?

[video200]

i would think so. but you need to prop way down to get within the reccomended max 66 amp.
dl the FE calc and play with the numbers :)
according my my FE calc you can get @ 62 amps with a 435 and 54 amps with a 632

[electric]

Thanks, I will give this a try and see what the results are. So I go back to the original question, is anyone using more Voltage to get more speed? Why would Hydra sell a 50volt ESC if the motors can't handle it? I am guessing not many people have jumped up on the voltage level yet so there is not alot of experience in dealing with that?

[ReddyWatts]

You should use a lower kv motor with the higher voltages. This will keep your rpm down and allow you to use a larger prop. If you use a prop that is too small, it will cavitate and give poor performance.

[electric]

Always a challenge, that is what makes the hobby interesting. What lower KV motor would you recommend? I suppose the idea is I get a motor with more torque and a bigger prop and thus generate more speed?

Maybe it would be helpful to go at this another way. If I want to go in the 50+ mph range would someone some recommend a set up. I currently own a 9xl, 10xl motor, a 120amp/50volt hydra esc, and a set of 3s1p 30c 11.1 volt batteries. I also have several sets of 2s1p 7.4 volt batteries. What hull would you drop these in and what prop would you recommend?

[ReddyWatts]

If your lipos are all the same brand, mah and c rating, you could use a 2s and 3s in series to make 5s on the 10xl and a X440 prop for around 43mph.

[G Doggett]

Do you want a SAW set up or do you want to race for 3 - 4 minutes ?
The two set ups are quite different.
IMO the Ovalmaster 29 is a good hull on calm water, and as much as it hurts me to admit it,the Delta 29 - 33 is pretty hard to beat.
My Toysport Triton has hit 45 mph on GPS with 4S lipos, 8XL and M445 prop, but this is a fairly conservative set up so that it can last for several minutes while oval racing. The average current draw with this set up is 70 - 80 amps. The 9XL would be similar.
If you want SAW then takes Reddy's suggestion of using 5S and the 10XL and start with the X440 prop.
Check amps and temp and if OK try a slightly bigger prop until you reach your target speed or thermal in the attempt!
Graham.

[electric]

Graham. Thanks for taking the time to answer the question more completely. I suppose I am going to get a Delta 33 and then shoot for a simliar set up to yours. My brother has a Synchron and Triton, both good boats, but out of my price range. The exchange rate is making that boat expensive.