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 freerunning speed as the governor limits its
RPM and, the propeller blade sections near the tip may operate at
a negative angle of attack when freerunning, causing face cavitation
and blade vibration (singing).
In general, the propeller should be
selected for a tradeoff between the thrust needed at towing speed
and the top freerunning speed. An increase in one leads to a reduction
in the other  increasing pitch make the freerunning 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, 34 knots for continuous towing,
and 912 knots at freerun.
3. A compromise design speed can
be found at about half of the freerunning speed.
4. Efficient bollard operation should
produce about 30 lbs (130 N) of thrust per engine brake horsepower.
5. Use an equilibriumtorque 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 freerunning 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 freerunning 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.
