By TEBAOFLEX Jyen. Originally published on March 17, 2026.
A tow cable’s depth and top tension depend on its wet weight, deployed length, relative water flow, towbody forces and surface geometry. A single drag coefficient or vessel-speed figure cannot establish the performance of a complete tow system. The cable and handling equipment also need adequate mechanical and electrical qualification.
Use relative flow and distinguish force directions
In a simplified steady model, normal drag per unit length can be written fn = 0.5 × rho × Cn × D × Vn². Here Vn is the flow component normal to the cable, D is a stated reference diameter and Cn must correspond to that geometry and operating regime. Tangential drag uses its own coefficient and reference-area convention.
If all other assumptions remain unchanged, doubling normal flow speed multiplies this drag term by four: a 300% increase, not a 400% increase. In service, the cable shape, flow components and coefficients can change, so tow tension does not necessarily follow that simple ratio.
A limited straight-segment equilibrium example
For a straight segment at angle theta above the horizontal in horizontal flow, let F0 = 0.5 × rho × Cn × D × V² and let w be submerged weight per metre in N/m. Neglecting other normal forces, the simplified balance is F0 × sin²(theta) = w × cos(theta). This describes a local idealization, not the complete towbody boundary-value problem.
With q = F0/w, the positive solution is cos(theta) = 2q / [sqrt(1 + 4q²) + 1]. For hypothetical rho = 1,025 kg/m³, Cn = 1.5, D = 0.0175 m, V = 3.09 m/s and w = 7.8 N/m, q is about 16.47 and theta about 14.05 degrees. These assumed coefficients are not measured TEBAOFLEX performance.
A complete calculation must resolve the distributed forces and cable shape together with towbody drag, lift and weight. Top tension is not obtained by simply adding every horizontal and vertical force as a positive scalar. Additional payout can increase depth as well as layback; it does not universally stop adding depth at one critical angle.
Vortex shedding is a screening calculation, not a noise guarantee
The estimate f = St × Vn/D relates shedding frequency to a Strouhal number appropriate to the flow regime. For illustration, St = 0.20, V = 3.09 m/s, theta = 45 degrees and D = 0.0175 m give approximately 24.97 Hz. The angle in this example is assumed independently of the preceding equilibrium example.
Whether vibration develops strongly depends on structural modes, tension, damping, current profile and coupling. A frequency estimate alone cannot prove lock-in, a particular drag increase or an acoustic noise level. Moving the estimate outside one sensor band also does not guarantee fatigue safety.
Select fairings with the cable and winch
Manufacturers such as Fibron offer different fairing systems for seismic lead-in cables. Their function and handling requirements are design-specific. Do not assign universal coefficients of 0.35 or 0.65 to every ribbon or hairy fairing, or assume all fairings pass through existing sheaves and level-wind equipment.
- Obtain tested drag and vibration behaviour for the relevant orientation and speed range.
- Check attachment, bending, spooling, abrasion and recovery arrangements.
- Assess changes in effective diameter, buoyancy and mechanical loading.
- Confirm connector, termination and optical/electrical performance under the combined duty.
Separate analysis, qualification and operational evidence
DNV-RP-C205 provides guidance on environmental loading of marine structures; citing it does not certify a specific tow cable or a claimed depth improvement. Request the design basis, qualified operating envelope and traceable tests. Report any predicted depth or tension with its assumptions and uncertainty, and distinguish it from an actual sea trial.
Sources and further documentation
Discuss your application
Send the operating conditions, installation drawing and required acceptance evidence to TEBAOFLEX. WhatsApp: +86 185 9591 3361. Request documentation for the exact offered construction before ordering.