Opinion

What is a Tug's Bollard Pull and How İt is Calculated?


by Capt.Alpertunga Anıker - published on 24 November 2022 5042 -

The Bollard Pull value, which can be interpreted as the maximum thrust that is developed for a specific amount of time by the propulsion systems of any vessel to the fullest , when it has zero speed in the forward direction, is the criterion that allows us to understand how suitable a tugboat is for a port maneuver or a towing operation. 

Although the Bollard Pull value can be calculated in a simulation environment with some accuracy, the most realistic method is to use the standart procedures, which involve connecting a pier bollard with a towing line & mechanism on the tug and calculating the force or tension developed in the connection using a dynamometer either with load cell type or mechanic type. The first factor affecting the bollard pull is the total horse power (BHP) value of the tug's machinery. However, other factors affecting this measurement value are listed below.


  • Propeller Type
  • Whether the propellers are housed in specially designed shrouds (Kort Nozzle )
  • Underwater area of the tug and its shape
  • Whether the submerged part of the tug, and especially the hull, is clean.
  • How rough the sea is at the place where the measurement is made, so whether the tugboat makes a port or starboard roll (rolling) or fore and aft movement (pitching).
  • Sea water temperature at the measurement location, as machine performance is affected by the temperature of the cooling water.
  • The temperature of the engine room is also a factor affecting the performance of the machine. An engine room with a higher than normal ambient temperature adversely affects the performance of the engines.
  • There should be no significant current at the location where the bollard pull test will be performed. If the current in the region is above 1 knot, it will not be possible to obtain a healthy measurement value .
  • If there is a wind of more than 15 knots at the bollard pull test location, a healthy measurement value cannot be obtained.
  • Another issue that significantly affects the bollard pull test result is that the propeller blades are clean and smooth. This is also important for newly built tugs.
  • In an ideal bollard pull test, the tug should have almost no trim . However, a stern trim of 1/100th of the length of the tug may be permitted.
  • Mechanical or load cell powered electronic dynamometers are generally used in the bollard pull test. For an accurate measurement, the dynamometer must be calibrated before the test.
Pulling force or bollard pull value  describes the towing ability of a tugboat when its position is stationary, that is, when it is not making way. As the tug makes  way, the towing force decreases due to the underwater resistance of the tugboat.We can say that there is an inverse proportion between the speed of the tug and its towing capability.

Bollard pull tests are done under the supervision of class Institutions. It is essential to ensure that the bollard pull test protocols are followed appropriately since this value is crucial to the marketing of the tug:
There must be a water depth of at least 20 meters at the measurement site and the length of the rope must be long enough not to allow  the water returning from the dock to reach the tugboat. From the horsepower (BHP) of the tug's machine, the approximate bollard pull value  of the tug in question can be calculated. There are some formulas we can use for this:

Tug equipped with fixed pitch propeller: (freewheeling)  :        

 BHP x 0,9 x 1,10 / 100 = (t)

Tug equipped with fixed pitch propeller and kort-nozzle:                   

 BHP x 0,9 x 1,20 / 100 = (t)

Tug equipped with controllable pitch propeller: (freewheeling)      

BHP x 0,9 x 1,25 / 100 = (t)

Tug equipped with controllable pitch propeller and kort-nozzle :     

 BHP x 0,9 x 1,40 / 100 = (t)

The calculated figures must be taken as rough estimates and may vary based on the tugboat’s building specifications. This is even more true nowadays since there are so many various propulsion systems, each of which has the potential to produce a variable amount of bollard pull while having almost identical engines.

If we explain these formulas with an example:

Let's calculate the towing power of the tugboat BORA EKŞİ with fixed propeller pitch, propeller housed in kort nozzle and with a total machine power of 2572 BHP.

To put it in the formula:

Tug equipped with fixed pitch propeller and kort-nozzle :                   

 Bollard pull in tons = BHP x 0,9 x 1,20 / 100 = (t)

=  2572 x 0,9 x 1,20 / 100 = 27.8 ton

Figure -1: A legged jetty being used in a bollard pull test
Figure -1: A legged jetty being used in a bollard pull test
Figure -1: A legged jetty being used in a bollard pull test
Figure -1: A legged jetty being used in a bollard pull test
The certified bollard pull value of the tugboat BORA EKŞİ is 30 tons, according to the Bollard Pull Test Certificate. In the formula, a value of 27.8 tons was obtained, which is very close. The value recorded on the dynamometer in accordance with the regulations during the bollard pull test, which was conducted under the supervision of the class institution, is, of course, the most important factor here.

The trial location should be chosen such that there are no piers, docks, jetties, breakwaters, or other obstructions too close In order for the propeller wake to pass through or beneath the jetty.The optimal location for the bollard should be at a suitable spot on a legged jetty without walls. If a solid pier is utilized, the bollard should be positioned so that it offers a clear run for the propeller wake.
Figure -2: The tow angle to be adopted with solid piers
Figure -2: The tow angle to be adopted with solid piers
Figure -2: The tow angle to be adopted with solid piers
Figure -2: The tow angle to be adopted with solid piers
Since this is not the case for some wall-shaped solid piers , the water returning from the pier wall may prevent obtaining an accurate value in the bollard pull test.

In order to fix the tow rope, if there is no place like a legged jetty, a method should be applied as shown in Figure-2 below, where the propeller wake of the tug will not be interrupted and will not return.

In addition, while choosing a place to fix the tow rope, care should be taken to ensure that the tow rope is as parallel to the horizon as possible during the test. However, this cannot always be achieved in the most ideal way due to the fixed heights of the piers and quays.

Besides, the position of the tugboat while performing the bollard pull test should not be in a very shallow water depth.
Figure-3: Minimum Depth Required  / Tugboat BHP
Figure-3: Minimum Depth Required / Tugboat BHP
Figure-3: Minimum Depth Required  / Tugboat BHP
Figure-3: Minimum Depth Required / Tugboat BHP

The underwater depth should not allow a considerable water circulation around the tugboat. In this regard, there are detailed calculations that fall into the expertise of naval engineering. However, without describing these detailed formulas at length, the graphs thus obtained  give us an idea. The said ideal depth should be provided in a circle with a radius of 100 meters, with the tugboat being the center. The total horse power (BHP) of the tugboat’s engines determines the ideal depth at the bollard pull test location (Figure 3).

As mentioned above, if there is a current of more than 1 knot in the area where the bollard pull test is performed, it is not possible to make a correct measurement. In places where the tidal current is significant, it is preferred to start the  test 1-1.5 hours before the high water time. As can be seen in Figure-4, the current may only be allowed to come from a direction close to the stern of the tugboat.

Figure-4: Bollard pull test performed in current conditions
Figure-4: Bollard pull test performed in current conditions
Figure-4: Bollard pull test performed in current conditions
Figure-4: Bollard pull test performed in current conditions

Measurements made in the current from other directions will not be healthy.

1 knot of current coming from the head negatively affects the bollard pull by approximately 4%.

The graph showing the determination of the minimum tow length to be used in the bollard pull test according to the value of the total engine power (kw) of the tug is shown in Figure-5 below. As can be seen in the figure, a tugboat with a total of 4000 kW  engine power should require a minimum tow length of 250 meters when performing the test.

Figure-5: Minimum tow length required according to tug's engine power
Figure-5: Minimum tow length required according to tug's engine power
Figure-5: Minimum tow length required according to tug's engine power
Figure-5: Minimum tow length required according to tug's engine power

In the bollard pull test, the tow rope should be maintained in the fore and aft direction as much as possible. Measurements are made in three categories:  80%, 100% and 110% (Overload):

The bollard pull value measured when the tug starts towing with all its engine power is called Static / Maximum Bollard Pull. However, due to the tug's propeller cavitation, rising temperatures, etc. It is impossible to maintain this value for a long time . The value that matters most to us  is the one that the tug can apply continuously for about 10 minutes after losing the first peak value, which is called as Continuous Bollard Pull.

The comparison between Maximum Bollard Pull  and Continuous Bollard Pull is shown in the graphic below ( Figure-6 )

Figure-6: Maximum Bollard Pull  /  Continuous Bollard Pull
Figure-6: Maximum Bollard Pull / Continuous Bollard Pull
Figure-6: Maximum Bollard Pull  /  Continuous Bollard Pull
Figure-6: Maximum Bollard Pull / Continuous Bollard Pull
Figure -7: A Bollard Pull Certificate belonging to a test carried out at UZMAR Gölcük Shipyard
Figure -7: A Bollard Pull Certificate belonging to a test carried out at UZMAR Gölcük Shipyard
Figure -7: A Bollard Pull Certificate belonging to a test carried out at UZMAR Gölcük Shipyard
Figure -7: A Bollard Pull Certificate belonging to a test carried out at UZMAR Gölcük Shipyard
Continuous Bollard Pull, is the average of the values taken continuously in a 5-minute period, provided that it does not exceed 30 seconds. This measurement can also be made in separate periods depending on the conditions, for example, 3 times in 2 minute periods. As a result, the said value is determined by taking the general average of the measurements obtained. Apart from these two definitions, there is also a value defined as Effective Bollard Pull, which expresses the pulling force that a tugboat can apply in open sea. However, this value cannot be measured in the bollard pull test and is accepted as  75% of the Continuous Bollard Pull value.

when the ideal water depth and the tow length cannot be achieved, it is possible to obtain the closest results by applying corrections to the bollard pull test measurement values by making use of some available graphics as shown below (Figure – 8)

Figure -8: Bollard pull test  correction diagram
Figure -8: Bollard pull test correction diagram
Figure -8: Bollard pull test  correction diagram
Figure -8: Bollard pull test correction diagram

To give an example:

While the tugboat should be tested at a location with a depth of 20 meters under the keel , only 18 meters depth is possible due to the circumstances. In addition, 300 meters long tow rope should be used, but 240 meters long tow rope could be used in the test.

         LT   =  300 mt  (Required tow line lenght)

          L    =  240 mt  (Available tow line lenght)

          DO  =   20 mt   (Required depth under the keel)

          D    =  18 mt   (Available depth under the keel)

Based on these values:

L / LT   value is 240 / 300 = 0.8,    D / Do value is 18 /20 = 0.9.

Suppose a bollard pull value of 30 ton was measured with the available rope length and water depth.

With  L / LT ( 0.8 )  and  D / Do ( 0.9 ) values in hand, we find the correction coefficient as 1.02 in the left column. We multiply this coefficient with the value measured in the test and thus find the expected  bollard pull test value  under ideal conditions.

We find the value 30 x 1.02 = 30.6 tons.

In addition to  port maneuvers, tugboats are also used for towing idle ships, barges ,floating docks, etc.. While providing such services, we know that various classification societies use some different calculation methods, more or less detailed than the other, in order to figure out the engine power a tug should have. However, a formula that gives a very rough idea about it without going too far is as follows.

BHP = ( Dx2 / 3 ) × ( v² ÷120 )

D =  Displacement of the vehicle to be towed (t)

 v = Required tow speed ( knots )

Figure-9: A Bollard pull test performed at Gölcük Uzmar Shipyard
Figure-9: A Bollard pull test performed at Gölcük Uzmar Shipyard
Figure-9: A Bollard pull test performed at Gölcük Uzmar Shipyard
Figure-9: A Bollard pull test performed at Gölcük Uzmar Shipyard

For example, let's calculate how much horsepower the tugboat should have in order to tow a 6000 ton barge in the form of a ship with a tow speed of 9 miles:

Lets put these values in the formula

BHP = ( 6000 X 2 /3 ) × ( 81 /120 )

BHP = ( 4000 ) × ( 0.675 ) = 2700 HP

The horsepower required was found to be 2700 HP for a conventional tugboat with freewheeling  propellers (not in kort nozzles). Dividing this value by 100, we find the required pulling force, which equals 27 metric tons.

However, tugboats with propellers housed in kort-nozzles are approximately 10/20 percent more advantageous than this type of tugboats, so they can provide the same pulling force with relatively less horsepower. This coefficient is generally applied as 0.90 if the propellers of the tugboats that are compared are of similar pitch.

If we apply it to our example:

If the propellers of the tugboat, which will carry out the towing, are housed  and have  a similar pitch, the required horsepower will be 2430 BHP (2700 × 0.90).

As a result, the most important thing that shiphandlers should be aware of in the light of this general information is that , bollard pull value is measured when the tugboat is making no speed at all. As mentioned above, a tugboat cannot apply the force stated in the bollard pull certificate to a ship in motion. Since a certain speed must be maintained in order for the tugboat to keep up with the ship, less force can be applied on the assisted ship .

It is an important point to always remember in terms of maneuvering safety that the higher the speed of the ship served, especially in port and escort maneuvers, the lower the force that can be applied by the tugboat.

However, of course, this does not result in the pilots making port maneuvers at the lowest speed. The ideal maneuvering speed is a matter that the pilots will decide on their own based on environmental conditions such as current and wind, and different speeds may be used depending on the situation.

References :

1.Cpt.Cahit İstikbal –What is bollard pull and how to calculate it?

2.UZMAR Gölcük Shipyard Bollard Pull Test Procedures

3. Various Aspects of Bollard Pull Tests and Analysis of Test Results by Dev.Arun Kr.

4. Bollard Pull Test Procedure- Norwegian Maritime Authority

5. IMO Rules / Appendix A - Bollard Pull Testing Procedure

6. New İnternational Standards for Bollard Pull Trials / Bureau Veritas

7. Cpt.Alpertunga Anıker – Master’s Guide to Shiphandling

8. Theoretical and Experimental Measurements of Bollard Pull with Emphasis on Propeller Dimensions - Mohamed Ashraf,Ahmed El Zaalik

9. DNV Towing Reccommendations

10. Technical Insight on Bollard Pull Requırements
https://offshoremarinesurveyors.com/towing-vessel.html

11.Mooring and Towing Manual – Wartsila


Cpt.Alpertunga Anıker

Editor's note:
Opinion pieces reflect the personal opinion of individual authors. They do not allow any conclusions to be drawn about a prevailing opinion in the respective editorial department. Opinion pieces might be deliberately formulated in a pronounced or even explicit tone and may contain biased arguments. They might be intended to polarise and stimulate discussion. In this, they deliberately differ from the factual articles you typically find on this platform, written to present facts and opinions in as balanced a manner as possible.

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TL
Tan Le Vietnam
on 1 April 2024, 14:59 UTC

Please tell me how to get fomula BHP = ( Dx2 / 3 ) × ( v² ÷120 ) or Documents proving the formula
0

Capt. Vishal Prasad Sinha Reliance Industries Limited, India
on 27 February 2023, 14:04 UTC

Excellent explanation
0

ak
abbas khaksar Belgium
on 20 January 2023, 09:38 UTC

Very practical and understandable .
Capt. Abbas Khaksar
1

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