An improved propeller for the vertical thruster


#1

Hi All:

Here at OROV HQ we've been working on optimizing the motors and propellers for future versions of OpenROV. Here's a little peek at what we've been doing, so you can modify your OROV similarly if you desire.

One known issue with the current Graupner props is that they are not nearly as efficient in reverse as they are in the forward direction. While this is not a major problem with the horizontal thrusters, it is a problem with the vertical thruster. The vertical thruster is already challenged in the reverse (lifting) direction, as it has to push water through a very constrained cavity. When this is coupled with the poor reverse efficiency of the current Graupner prop, the vehicle has very little in the way of spare lifting capability. If the ROV happens to be negatively buoyant, then it may be difficult to even lift the vehicle off the bottom.

We've been testing other propeller shapes on our thrust test jig, which can measure the thrust produced while actually mounted in the ROV. Here's a photo of our rig:


What we found is that the Graupner's two-bladed racing propellers, while not quite as efficient going forwards (descending for the vertical thruster), are significantly more efficient in the reverse direction than the current props.

Here's a photo of the propeller we've settled on for now with the current 2213 motor, the Graupner 2303 series:


This propeller is available in a number of different diameters. 55, 57, or 60mm diameters should work well. Any of these choices should give about a 3X-4X (!!!) improvement in lifting thrust on the ROV. The difference is truly startling.

This propeller (the 57 mm is shown in the photo) is powerful enough in reverse that, if the motor c-clip is not installed, the propeller will pull the motor bell off of the motor. So it is essential that the c-clip be reinstalled on the motor after the propeller is installed.

So here's the pieces you need to upgrade your ROV:

- New propeller: We use Cornwall Model Boats for buying small quantities of propellers. Here are their 2303 series racing props.

- New motor: The 2213 motor is available from HobbyKing.

- Spare c-clips: These are necessary if you're only swapping the motor bell and not the whole motor. Available here.


- C-clip removal tool: This will save a lot of time. I bought these cheap ones on Amazon, then ground the end down to fit the small motor C-clips.



Here's the steps you can use to upgrade your unit:

- Remove the current vertical prop/bell, then remove the current motor from its base. It may be easier here to remove the whole motor and base, then reinstall the base with the allen screws pointed in a direction that makes them easily accessible.


-Epoxy the new propeller on the bell of the new motor. If you want you can leave the bell on the new motor base, and then splice the whole new assembly into the old wiring, or you can remove the c-clip, remove the bell from the motor, and just install the new bell/prop onto your old motor and re-install a c-clip.


- Once you have the new motor/prop combination together, insert it into the base that is already mounted on the ROV, and tighten the allen screws.


Once you have made this modification, you will be startled at how much better the vertical performance of the ROV is. I have been driving my ROVs for the last month with this new prop and some custom modifications to the depth-hold software, and the combination of the two makes the ROV truly a joy to drive.

-Walt


#2

Hi Walt
I just tried the new propeller (i used the 55mm).
I actually see more lifting, thats great, however I noted a much bigger rotation-moment for the entere Rov. My heading-hold mode seems not to work as it should, so it is extremely difficult to keep the direction, when I used the vertical thruster. Did you observe the stronger rotation as well?
Hub


#3

Hi Hubertus:

Yes, I did. I'm not sure that there's any fundamental way to get around the torque. A bigger ROV might use two vertical thrusters to cancel out the torques. For a smaller ROV, this might not be practical. So.....

- You can try to offset it open-loop in the software. Have it so that when a large amount of diving or lifting force is applied, the horizontal motors provide a counteracting yaw. I'm going to try this in the next couple of weeks.

or,

- Have good heading-hold software, and always use it when diving/ascending. The current heading hold software was just a proof-of-concept and can definitely be improved. It's on our to-do list, but I can't tell you when that's going to happen. Ideas and patches from the community are certainly welcome.

-Walt


#4

Why not you simply try a vertical plane working as a tail ?


#5

Reviving this old conversation to ask about getting one of the new 2 bladed vertical props from somewhere else? The Cornwall site has a minimum purchase and I am being cheap! I see Hobby King seems to have something really similar but I would hate to get the wrong one. I have a 2.5 ROV, if that helps.