Hello! I am new to ROVs and I am looking for a simple explanation of the advantages of a thinner diameter tether. I understand the less drag the further the ROV is away from shore. I also understand the longer my tether is the more power loss I will have sending power to my ROV. Is there an easy way to illustrate this? If my ROV has a 4.5mm tether that is 300m meters in length and I have another ROV with a 10mm tether and is 300m in length, how do I quantify performance issues, loss of power to the ROV? Any suggestion would be super helpful. Thank you in advance for your help!
This is a complex problem that is best demonstrated and understood by undertaking some practical testing. Practical tests are also fun.
The theories that apply to hydrodynamic forces on cables require careful modelling of geometry and the application of factors for drag that are in turn governed by the types of flow around the tether (see this old but still relevant paper on the subject of submerged cables).
The simple equation for drag on a tether is given at Equation 1 on Page 9 (1) of the document however this equation becomes more complicated if you want to calculate an accurate result for an operational scenario…hence the benefit of practical testing with a spring scale or a digital load cell attached to a tether.
For example the force on a 1.0m long 4.5mm diameter tether being dragged sideways through seawater at 2.5m/s is approximately 1026/2 x 1.1 x 1.0 x 0.0045 x 2.5^2=15.9N (assuming a coefficient of drag of 1.1, which is the highly variable factor). Please note the force is a function of velocity squared so it increases rapidly for any increase in current velocity.
In general, if you increase the tether diameter to 10mm, the force becomes 15.9/4.5 x 10=35.3N (approx 3.6kg). This is not entirely accurate because there is a complex relationship between drag, speed and geometry that typically results in a larger coefficient of drag for a larger diameter…until supercavitation occurs…which is a whole other conversation that falls well outside the boundary condition for typical ROV tethers hahaha.
These forces ignore the much smaller but still relevant force created by the length of tether being dragged lengthwise through the water at each end of the loop (ie, the tether that is not perpendicular to the current). A tether typically adopts a catenary shape in the water as soon as the slack is taken up…so this further complicates the equation.
I could go on and on and on…so in general, keep your tether as thin and as smooth as possible and minimise the length of tether exposed to perpendicular current.
This is super helpful! Thank you for taking the time and answering my question.
You are welcome,
Post your results if you do any practical testing.