Mooring Line Load from Wind and Current
Wind and current drag on a moored vessel, the load per line on an equal-sharing assumption.
Example
You enter
- Projected wind area above water (sq ft) 12000
- Wind drag coefficient 1
- Wind speed (mph) 50
- Submerged lateral area (sq ft) 3000
- Current drag coefficient 1.2
- Current speed (knots) 3
- Number of mooring lines 6
- Horizontal line angle off perpendicular (degrees) 30
- Most-loaded line's share of the total (%) 70
- Safety factor on breaking load (UFC 4-159-03: 3.0 minimum for wire rope and synthetic line) 3
You get
- Wind force 76,800 lb at 73 ft/s
- Current force 91,836 lb at 5.1 ft/s
- Total load 168,636 lb
- Which governs 19.5787
- Per line, sharing equally 32454.1
- Required mbl (lb) 97362.3
- The most-loaded line 136307
- Worst to equal ratio 4.2
- What the lead angle costs 13.3975
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeWind and current drag on a moored vessel, the load per line on an equal-sharing assumption, and the load on the most-loaded line at an entered share. The worked case has the current beating a 50 mph wind on a quarter of the area, and one line at 70% carries 4.2 times its equal share.
Formula or tablewind F = 0.00256 Cd A V^2 (A in sq ft, V in mph); current F = 0.5 rho Cd A V^2 (rho 1.99 slugs/cu ft, V in ft/s at 1.688 ft/s per knot); per line = total / (count x cos angle); required MBL = line load x safety factor.
Source and editionStandard drag relations. The terminal's own mooring analysis, the line manufacturer's published minimum breaking load, and the vessel's mooring arrangement govern.
Free accessPublic drag relations; the drag coefficients come from published guidance for the hull form.
What governsVessel master governs. Verify against the stability letter and USCG-approved loading manual.
API field nameswind_area_ft2, wind_drag_coefficient, wind_speed_mph, submerged_area_ft2, current_drag_coefficient, current_speed_knots, line_count, line_angle_deg, worst_line_share_pct, safety_factor, wind_force_lb, current_force_lb, total_force_lb, current_over_wind_pct, load_per_line_lb, required_mbl_lb, worst_line_load_lb, worst_to_equal_ratio, angle_cost_pct
- Equal sharing an assumption a real arrangement does not honour, and the error is always unsafe: load distributes by stiffness, length, and geometryspec-v1834 method
- Vertical angle not modelled; a line with significant vertical angle is partly pulling the vessel down rather than holding it inmooring analysis