# Tether drag on ROV

I am working on the tunnel inspection ROV design. We will use a tether as a cable. Can you help me calculate the force that will affect the ROV of the tether cable? cable length will be 7000m. You can specify in the suggestion for calculation.

Hi GĂ¶rkem! Rafael from Blue here.

The main force that will affect the ROV will probably be the drag of the tether cable. Because the cable suffers from a drag force, it will be in an â€śarchâ€ť position from the boat to the ROV, and this drag will also affect the ROV.

To calculate for the drag of a cable, we would usually use the drag coefficient of the cylinder, hence 1.2, but I found a paper (â€śMeasurement and Analysis of Hydrodynamics of ROVâ€™s Tether Cableâ€ť - Kojima et al) that better estimates the drag coefficient of tether cables in the water as being around 2.0

With this value in mind, you can calculate for the drag using the drag equation. Keep in mind that the area in the formula is the area of the cable facing the water current, which can be approximated by the length of the cable times its diameter, and that you need an estimative of the velocity of the water current.

As an example, if you were using a cable with 7.6mm diameter, 7000m length, and a water velocity of 0.5m/s, we would have around 13KN of drag. Keep in mind that this is for a very low velocity of the water. With 2m/s it goes to 200KN.

Those are estimatives, but it can be seem that a 7Km cable generates a lot of drag.

Also, keep in mind that our tether interface board has a maximum rating of 2km.

Edit: Iâ€™ve just talk to @adam and he informed me that the Tether interface board has a rating of 300m and the Slim board a max of 200m. Beyond that, bandwidth drops off quickly.

@gorkemengin The smaller the tether diameter the less the drag, so you want to use the smallest diameter tether that you can. We carry a smaller-diameter tether here. If you want to go up to 7km though, you will probably need to go with a fiber optic cable. There are some discussions on this forum about fiber optic tethers.

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.

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Assuming you mean for drag of the tether in open water (e.g. not against some other surface/object), itâ€™s not something weâ€™ve measured (that Iâ€™m aware of), but this resource specifies that

The C_d for cables ranges from 1.2 for unfaired cables; 0.5â€“0.6 for hair-faired cable; and 0.1â€“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

This post from February is perhaps also worth noting

I couldnâ€™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 Iâ€™m unsure how that number was arrived at. Itâ€™s interesting that it was published 22 years before the first resource I linked you to, but wasnâ€™t accounted for - perhaps the authors of my link were unaware of that paper, or perhaps those findings have since been disputed (I havenâ€™t looked into it enough to know).