DIY Props for more power from DST-700 motors


#1

I am new to this issue. I want to build a hybrid ROV which would be bigger than the Open ROV (about 6 - 8 kg and with 6 motors).

I have searched for low kv motors, and it seems that the one used by Open ROV (DST-700) are the best choice in terms of price and power. From what I understand on Open ROV they run with the standard prop(60 mm x 32 mm pitch) on 9 V and 7 A at full power which gives about 6300 RPM and 60 W. The speed of water projected should be about 32 mm * 6300/60 m/s or 3,3 m/s.

I plan to run them on 3S 12 V Lipo or Liion batteries and to run them to at least 120 W (I am sure that with water cooling they can handle even much higher power). That would give about 7700 RPM and 12 A.

One of the biggest issues is to find adecuate propellers. I could not find bigger props than the 60 mm. with small pitch and LH RH versions.

I have seen that DIY props cut from bronze are not too complicated to produce. So I am thinking of going this route.

The Prop Parameters:
I want to have the same maximum vehicle speed and therefore same water jet speed expelled from prop, i.e. 3.3 m/s. That means that the pitch should be (7700/60)/3.3 = 26 mm.

I want to go with two blades instead of three blades, for the sake of fabrication simplicity. I assume that in order to match the three blade version power, I need to increase the prop “wash area” by 50%. Furthermore, I want to increase the total power by 100 %. That means that the original prop area of 28 cm (30 mm radius2 *3,141) should be now 28 * 1,50 * 2 = 84 cm. However, since it will run on 7700 instead of 6300 RPM, a reduction of this area by (6300/7700) * (6300/7700) = 0,67 is required. Therefore, the final prop area should be 84 cm * 0,67 = 56 cm2 or a prop with a diameter of 84 mm.

So I assume that a 84 mm diameter by 26 mm pitch prop will work well with DST-700 motors at 12 V power and current of 12 A.

Can someone confirm if my calculation is roughly correct and whether this is really a good idea to do bronce props oneself? I really cannot think of where they can be obtained.


#2

Hi @michail:

Good luck on your project to make a custom prop! We’d love to hear how things manage to work out for you.

There are a couple of misconceptions in your original post. The Kv rating of the motor refers to its unloaded speed (or, more correctly I believe, the voltage it generates when being driven as a generator), but the motor is not being run in an unloaded condition. While I don’t have the data in front of me now, I believe the props turn at somewhere around 4000 rpm when running at full power in a bollard-pull condition (the vehicle is not moving). I believe 7A is about the correct current draw.

If you don’t have the software tools to model the motor and the propeller, you might want to just set up a test tank using something like an aquarium and just experimentally run the motor with different props. You might find that the current motor and prop at 12V is actually pretty close to what you want to achieve.

-W


#3

Actually, you are right, Kv rating is for no load condition, usually, the revolutions under load (for air at least) are about -10 -20 % less. I am very surprised that OpenRov runs on 4000 revolutions. Could it be that the original props are not really adecuate?

Luckily, it should not really change the logic of my calculations (or so I hope), because I am using relative values. Basically, by increasing voltage from 9 to 12 volts one should decrease the pitch by 9/12 to get the same water speed. And prop size changes should give the same power increase because I am working relative to original props.

The other way to go would be to design the prop parameters from scratch, using some software; for air, I always used ecalc which is quite good, but it does not have options fro water… I will see if I can find something on the internet…


#4

Is this helpful at all ?

http://www.csgnetwork.com/marinepropcalc.html