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T200 Thruster Configuration

Hi. We are designing a long range tunnel inspection ROV and using a total of 12 T200 Thrusters. 4 of these Thrusters are mounted in the front of the vehicle at a 30/45 degree angle and 4 are mounted in the reverse direction at the back side. 4 of them are mounted vertically on the top of the ROV, two on each side of the ROV. A vectored thrust configuration is basically used. I wanted to ask about this configuration as I have concerns about the huge amount of tether drag ( almost 5km length) and the ROV drag itself. I tried to design a hydrodynamic shape to reduce the to total drag force. I am also not very sure about the Thruster positions. I mean if I place the Thruster further along the yellow panel, how will it affect the thrust force? Does the distance between the vertical thruster makes a difference? Any help about the thruster positioning will be greatly appreciated as I am new to this.
If someone can also help me calculate the Total Forward and Vertical Thrust force of the ROV, it will be greatly helpful. I made the calculations myself but I just want to be sure that what I am doing is correct. I have attached the picture of how the Thrusters are attached to the side panel. An identical side panel is attached to the other side of the ROV and the components are attached in between these panels.
Any help, tips and suggestions will be greatly appreciated in terms of design, stability, maneuverability or anything that you could think of.
Thank you.

Hi @Tamim, sounds like an interesting project :slight_smile:

This will depend on tether thickness+weight, the smoothness of the outer jacket material, and what it’s against (e.g. floating in water should have less drag than if it’s against the edges of the tunnel).

That depends mostly on the shape, and motion speed. Depending on the precision requirements of your use-case it may be worth doing some fluid dynamics simulations to determine the most drag-inducing areas and smooth them out where possible.

Sorry, not sure what you mean here. It’s not clear which thruster(s) you’re referring to, or which direction you would be moving them ‘along the yellow panel’.

Distance between thrusters is mostly important for efficient rotation (away from the center of mass → more rotation torque from the same thrust), along with whether they can interfere with each other (if they’re very close together they can "steal each other’s water’’ in a sense, or you could have one changing the direction of the flow from another one, which reduces the resultant force in the desired direction).

That’s reasonably simple to do, but depends a lot on your configuration. See our T200 technical details for information about the performance depending on thruster supply voltage, forward/reverse thrust levels, power usage for a given voltage and control signal, and other relevant information.

The force caused by a single thruster (assuming an unrestricted water flow in and out) can be represented as below:


where

\begin{align} \color{blue}F_{\uparrow} &= F_T \cdot \cos({\theta})\\ \color{red}F_{\leftarrow} &= F_T \cdot \sin({\theta}) \end{align}

and F_T is the thrust as specified in the technical details, for your operating power and control signal.

When you have mirrored thrusters left to right the horizontal thrust forces will cancel out when you’re going forwards, so you’ll have a net forwards thrust unless you have more drag going one direction than the other (depends on your ROV shape and balance).

Also take care to consider the propellers you’re using - if your propellers are mirrored between the front and back then your thrust forces will be from 4x forward and 4x reverse thrusters, but you may wish to set up the propellers so that all 8 thrusters are using ‘forward thrust’ when the ROV is going forwards. and ‘reverse thrust’ when the ROV is going backwards, which would make going forwards faster than going backwards.

Have a look at these two threads - they go into quite a bit of detail:

Hi Eliot. Thanks a ton. This was a great help. I also have one last question. As you can see from the picture above, the front lateral thrusters are placed at a distance of lets say 100mm from one end of the ROV while the lateral thrusters at the backside are closer to the rear end of the ROV ( less than 100mm) so we can say that they are not exactly symmetric in terms of the center of mass of the ROV. Will this affect the ROV in any way?

Glad to hear it :slight_smile:

The center of mass is determined by the mass of all the components - without knowing the mass and location of each component in your ROV design it’s not possible to say for sure where the center of mass is. As a trivial example, you could have a very heavy weight at the back of your ROV, in which case the CoM would be much closer to the rear than the middle or front.

If the rear thrusters are in fact further from the CoM than the front thrusters, the torques generated by the thrust force from the back will be larger. That shouldn’t affect moving forwards/backwards, but when trying to move directly sideways the ROV will tend to rotate by leading with the back, and if you’re trying to turn the back will turn a bit faster than the front.

I don’t imagine that will cause many issues in a tunnel inspection, but it’s at least worth knowing about :slight_smile:

Thank you so much Elliot. Do you by any chance know the Drag coefficient of the Blu ROV Tether cable? I am trying to estimate the tether drag that we will have for the ROV.

Assuming you mean for drag of the tether in open water (e.g. not against some other surface/object), it’s not something we’ve measured (that I’m aware of), but this resource specifies that

The C_d for cables ranges from 1.2 for unfaired cables; 0.5–0.6 for hair-faired cable; and 0.1–0.2 for faired cables.

Our tether is primarily cylindrical and has no fairing (aerodynamic shaping/tassels that reduce drag), so I suppose 1.2 is the number to go with, which also matches the tether drag example provided on that page :slight_smile:

This post from February is perhaps also worth noting

I couldn’t find an openly available download for that paper, but this google books preview includes the introduction and the results. Without reading the full paper I’m unsure how that number was arrived at. It’s interesting that it was published 22 years before the first resource I linked you to, but wasn’t accounted for - perhaps the authors of my link were unaware of that paper, or perhaps those findings have since been disputed (I haven’t looked into it enough to know).