Bollard Pull

A HydroComp Technical Report
Report 110

Product link: NavCad, PropExpert

Bollard pull is a traditional design criteria used for the selection of propellers for tugboats. It is the theoretical thrust achieved at zero speed of advance and full engine RPM. Bollard pull is an abstract, somewhat academic, state that cannot be achieved in real operation. This is due to two things - propellers accelerate water as they spin so they never really see water at zero speed, and engines are unable to reach full RPM at towing speeds due to a declining torque vs. RPM relationship.

The bollard condition, however, is often used as a 'merit criteria' for towing applications. In other words, even though bollard thrust can never be achieved, it provides a simple means to compare one propeller against another in a towing scenario.

Propeller selection

Selecting propellers for a required bollard condition is a study in compromise. The natural temptation to select a propeller for nominal zero speed (and its associated small pitch) risks serious performance consequences. The vessel pays a penalty in free-running speed as the governor limits its RPM and, the propeller blade sections near the tip may operate at a negative angle of attack when free-running, causing face cavitation and blade vibration (singing).

In general, the propeller should be selected for a trade-off between the thrust needed at towing speed and the top free-running speed. An increase in one leads to a reduction in the other - increasing pitch make the free-running operation more efficient, while a reduced pitch improves towing thrust. Therefore, the propeller should be selected for a design speed somewhere between the two. Some typical guidelines are:

1. Good bollard performance is often found with propellers at about 0.6 P/D.

2. Typical analysis speeds are 0.1 knots for the bollard condition, 3-4 knots for continuous towing, and 9-12 knots at free-run.

3. A compromise design speed can be found at about half of the free-running speed.

4. Efficient bollard operation should produce about 30 lbs (130 N) of thrust per engine brake horsepower.

5. Use an equilibrium-torque towing analysis to provide achievable thrust at bollard (as opposed to theoretical thrust).

A recommended initial propeller selection procedure for bollard is:

1. Set analysis speeds to 0.1 knots for bollard and a suitable free-running speed (for example, 9 knots).

2. Enter the known effective power of the tug. Set to zero if this is not available.

3. Choose the design speed to be 50% of the free-running speed.

4. Set the propeller's design load to be 80% of the required bollard thrust.

5. Run a towing analysis with a suitable cavitation criteria (e.g., Manen line).

6. Evaluate the results, and modify the diameter and pitch as needed to meet the requirements.