Calculator Inputs
Example Data Table
| Case | Power/engine | Engines | Propulsors | Diameter (m) | ρ (kg/m3) | η | Factor | Estimated BP (tf) |
|---|---|---|---|---|---|---|---|---|
| Harbor tug | 1500 kW | 2 | 2 | 2.40 | 1025 | 0.55 | 1.08 | ~40.7 |
| Work boat | 900 kW | 2 | 2 | 2.00 | 1000 | 0.50 | 1.00 | ~22.1 |
| Utility tug | 1800 BHP | 2 | 2 | 2.20 | 1025 | 0.52 | 0.92 | ~29.3 |
Example results are generated by the same equations as the calculator.
Formula Used
This tool estimates static thrust from an actuator-disk relation, then converts thrust to bollard pull:
- A = π (D/2)² (propulsor disc area)
- P_eff = P_shaft × η (effective power per propulsor)
- T = ( P_eff × √(2 ρ A) )^(2/3) × C (thrust per propulsor)
- BP(kN) = (T × N) / 1000, and BP(tf) = BP(kN) / 9.80665
η is an overall efficiency, while C is a correction factor for nozzle and interaction effects.
How to Use This Calculator
- Enter power per engine and select the correct unit.
- Set the number of engines and the number of propulsors.
- Provide propulsor diameter and water density for your site.
- Choose an overall efficiency that matches your drivetrain and propulsor.
- Select a propulsor type, then keep auto factor or set manual.
- Press Calculate Bollard Pull to view results immediately.
- Use the CSV or PDF buttons to export your latest calculation.
Professional Guide to Bollard Pull Planning
Bollard pull is the static pulling capability of a tug or workboat measured at zero speed. It is widely used for early-stage selection, estimating escort capability, and comparing propulsion packages. Because the vessel is not moving, the propulsor operates in a heavily loaded condition and the result is sensitive to power delivery, propulsor size, and losses through the drivetrain and hull interaction.
This calculator provides a practical planning estimate using an actuator-disk relation that links effective power to thrust through water density and propulsor disc area. You can enter power per engine, the number of engines, and the number of propulsors to reflect twin-screw or multi-thruster arrangements. The diameter value drives disc area, so use the manufacturer’s nominal diameter for propellers or the effective diameter for ducted units.
The overall efficiency input consolidates real-world losses such as gearbox and shaft losses, electrical conversion losses for diesel-electric systems, and propulsor/hull interaction effects. For planning, values around 0.45–0.60 are common, but the correct number depends on the installation. The correction factor lets you apply a separate multiplier for nozzle benefits, wake effects, or empirical adjustments based on experience.
For preliminary sizing, run three scenarios: an optimistic case, a most-likely case, and a conservative case. Keep power fixed, then vary efficiency and the correction factor to create a practical range. If your vessel operates in rivers or warm, low-salinity areas, density may be closer to 1000 kg/m3, which reduces thrust slightly. When you have more propulsors than engines, the tool assumes power is shared evenly, so verify your power distribution strategy. Record each run with the export buttons so design assumptions stay traceable.
Example: a harbor tug with two engines rated at 1500 kW each, two ducted propulsors of 2.40 m diameter, sea water density of 1025 kg/m3, efficiency of 0.55, and a factor of 1.08 yields an estimated pull near the value shown in the example table. If you reduce efficiency to 0.50 or select a lower factor, the predicted pull drops noticeably, helping you understand how uncertainty in inputs impacts the outcome.
Use this tool to compare options consistently, then validate with vendor curves and certified bollard pull trials. For contract or compliance decisions, always rely on tested performance and applicable measurement standards; use estimates only during early project planning phases.
FAQs
1) What does bollard pull represent?
It is the maximum steady pulling force a vessel can exert at zero speed, typically reported in kN or tonne-force, and used to compare tug capability under static conditions.
2) Why does propulsor diameter matter so much?
Diameter sets disc area. For the same effective power, a larger disc area reduces loading and increases achievable thrust in static conditions, which can raise the estimated pull.
3) What should I use for overall efficiency?
Use a planning value that reflects your propulsion train and interaction losses. If you have trial data, tune efficiency so the calculator matches measured pull for a similar vessel.
4) When should I change the correction factor?
Adjust it when you need to account for nozzle effects, unusual wake conditions, or empirical corrections from experience. Keep changes modest unless supported by test results or supplier data.
5) Can I use BHP instead of kW?
Yes. Select BHP and enter the rating per engine. The calculator converts horsepower to watts internally before applying efficiency and thrust estimation.
6) How accurate is this estimate?
It is a planning estimate, not a certified value. Real pull depends on propeller design, nozzle geometry, hull form, towing point, and measurement method. Always confirm with vendor curves and trials.
7) Why are my results different from a brochure?
Brochure figures may use different ratings, environmental conditions, or test procedures. Ensure you match power definition, density, and efficiency assumptions, and consider that certified trials can include correction methods.