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.