Bollard Pull Calculator

Preliminary bollard pull estimate for tug operations. Enter engine power, number of engines, propulsion type, and K-factor. Returns bollard pull in tonnes/US tons plus effective thrust in kN.

Open propeller K

0.011–0.012t/kW

Kort nozzle K

0.013–0.015t/kW

HP → kW

0.7457kW/HP

Tonne → kN

9.81kN/t

Units Switches engine power and pull units

Engine power (HP) Continuous rating per engine

Number of engines Twin screw most common on harbor tugs

Propulsion preset Updates K-factor below

K-factor override Override for yard standard or measured back-calc

Bollard pull

37.8US tons

Open propeller, K = 0.0115

Total engine power

4,000HP

K-factor used

0.0115t/kW

Effective thrust

336.4kN

Metric

34.3tonnes

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Tip K-factor captures the real-world propulsion efficiency that horsepower alone cannot. Calibrate K against known sister vessels or measured trials before relying on an estimate for charter-level decisions.

Tip Why the 0.0115 factor? It's not arbitrary — it's the product of unit conversions and physical efficiencies:

Decomposition BP_tonnes = HP × 0.0115 = HP × (0.7457 kW/HP) × (η_prop) × (K_t coefficient) / (g × 1000) ≈ HP × 0.7457 × 0.55 × 0.65 × 0.0386 ≈ HP × 0.0115

where η_prop is propulsive efficiency (~0.55 for an open prop, ~0.62 for a Kort nozzle), K_t is the thrust coefficient at zero advance (~0.5–0.7), and the 0.0386 factor lumps gear & shaft losses with the kg → tonne conversion.

Warn Static bollard pull is typically 5–15% below this estimate for an open prop, and 10–25% above for a properly-designed Kort nozzle. Wake fraction, water depth, and tunnel obstruction all matter — IACS Recommendation No. 90 and DNV-ST-N001 (Marine Operations) define the certification test in 70 m+ open water at 5 minutes sustained.

Worked example

Twin 2,000 HP engines, open propellers, K = 0.0115.

1. Engine power per unit (HP → kW) 2,000 × 0.7457 = 1,491.4 kW 2. Total installed power 1,491.4 × 2 = 2,982.8 kW 3. Bollard pull (tonnes) 0.0115 × 2,982.8 = 34.3 t 4. Conversions US tons: 34.3 × 1.10231 = 37.8 tons thrust: 34.3 × 9.80665 = 336.4 kN BP = 34.3 t ≈ 37.8 US tons ≈ 336.4 kN

Propulsion K-factor reference

Propulsion type	K-factor range (t/kW)	Notes
Open propeller	0.011–0.012	Conventional twin-screw tugs without nozzles.
Ducted / Kort nozzle	0.013–0.015	Nozzle improves static thrust; common on towing tugs.
Azimuth / Z-drive	0.013	Strong maneuverability; exact value varies with propulsor design.
Tractor tug	0.014–0.016	Escort/harbor-assist designs optimized for static pull.

Common mistakes

Using maximum intermittent instead of continuous rating. Bollard pull tests and tow certifications run at continuous power. Peak-rating inputs overstate capability by 5–15%.
Generic K for unique propulsors. Z-drive and voith-schneider tugs deviate from the open-propeller rule. Use vendor or trial data when available.
Ignoring hull clearance and fouling. A clean hull in trial condition delivers more pull than the same boat after a season. Back-calculate K from sea trials, not brochures.
Treating the estimate as certified. Tow approval agencies require instrumented bollard pull trials. The calculator is for planning and tender screening only.

Warn This is a preliminary sizing tool. Contracts, regulatory filings, and escort-capability certifications require a certified bollard pull test per the class society's procedure (ABS, DNV, LR, etc.).

FAQ

What is bollard pull?

Bollard pull is the static pulling force a tug produces while secured to a fixed bollard. It is commonly reported in metric tonnes or US tons and is one of the main performance ratings used when comparing tugs for towing or harbor-assist work.

How is bollard pull different from engine power?

Engine power is energy available at the machinery. Bollard pull is the thrust that actually reaches the towline in a static condition. Propeller type, nozzle use, drivetrain losses, and hull interaction determine how efficiently power converts to pull.

What affects K-factor for a tug?

Propulsion arrangement, propeller diameter and pitch, gearbox ratio, nozzle geometry, hull-propeller interaction, and machinery condition. The empirical shortcut works best when K is calibrated from similar vessels or measured trials.

Is measured or theoretical bollard pull more accurate?

Measured. A certified bollard pull test with calibrated load cells captures losses the formula cannot. Empirical estimates are useful for concept comparison and early sizing, but they are not a substitute.

What bollard pull rating do I need for towing?

It depends on tow displacement, required control margin, weather envelope, and operation type — harbor docking, coastal towing, escort work, and deep-sea tow are all different. Operators select tugs on bollard pull plus maneuverability, winch capacity, and safety margin.

Sources

DNV DNV-ST-N001 — Marine operations and marine warranty IACS Recommendation No. 90 — Bollard pull tests ABS Guide for Vessels Operating in Low Temperature Environments SNAME Propulsion performance papers

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