Bollard pull, Tow gear size etc

Ref : Oilfield seamanship volum 8, page 173.

Bollard pull

1 kw = 1.341 horsepower

Maximum instantaneous bollard pull is the value achieved during a test pull when the vessels momentum contributes to the load on the test line. It’s usefulness for practical calculations is of little value.

Maximum bollard pull is the value achieved when the engines are running at full load and can only be sustained for very short periods without risk of damage to the engines.

Maximum continuous bollard pull is the sustainable static pull with the boats engines running at, and load to, makers recommended continuous rating. It is the most useful measure and the one most often quoted in vessel specification.


Calculating maximum continuous bollard pull

Bollard pull is a function of engine horsepower, the most often use measure of engine horsepower is referred to as break horsepower. This is the power at the vessels propeller shaft, ( BHP ). Power may also be quoted in kilowatt(kW).

1 kw = 1.341 horsepower
Bollard pull is influenced by propeller design and the fitting of kort nozzles or shroud.

The following methods are equally suitable.

Method 1 : After Noble Denton and Associates

Bollard pull (tones) = 1.1√BHP ( add 10% if nozzles are fitted )

Method 2 : After US Navy

Bollard pull ( in tones)
multiply shaft horsepower x 25 to 30, for open propellers – divided by 2240
Bollard pull (tones) = (BHP x 30 ) / 2240
multiply shaft horsepower x 30 to 35, for enclosed propellers(kort nozzeles) – divided by 2240
Bollard pull (tones) = (BHP x 35) / 2240

Ref : Oilfield seamanship volum 8, page 173.

Main tow wires the usual method of deciding on the size and length of the main towline is to express it as a ratio of the bollard pull. The wire size may be reduced for geographical area where gales are rare.

Other tow system components Shackles, tow pennant and towing springs are sized according to the size of the main tow wire expressed as tow wire break load multiplied by a factor.


Ref : Oilfield seamanship volum 8, page 174.

Three separate tables are shown :


Table 1 – tow gear size :

Note : BP = Bollard Pull, BL = Break Load

Tow line 2 x BP of towing unit
Tow pennant 1 x BL of tow wire
Chain spring 1.3 x BL of tow wire
Chafe chain 1.3 x BL of tow wire
Nylon tow spring 2.3 x BL of tow wire
Weak link 0.9 x BL of tow wire
Shackles SWL = BP of towing unit

Table 2 – tow gear size :

Note : BP = Bollard Pull, BL = Break Load
Bollard pull(tones)
Part 1 Tow line break load = 3 x BP < 40
Tow line break load = (3.8 - BP) x BP 40 ~ 80
50
Tow line break load = 2 x BP > 90

Part 2 a.Towing springs short in field BL x 1.5
Inter field/ocean BL x 2
b.Shackles/connections BL x 1.5
c.Tow pennants Same size as main wire
d.Weak links Main towline diameter less 10%

Table 3 – Bollard Pull Ref : Oilfield seamanship volum 8, page 175.

Main tow wire break load 2.5 x BP
Synthetic tow spring 5 x BP
Weak link 2 x BP
Shackles/connections SWL 1.2 to 1.5 BP

Table 4 – Length of main tow wire Ref : Oilfield seamanship volum 8, page 175.

In general the length of the main tow wire is related to the size of the vessel bollard pull and the area of operation.
Frequently used ratios for minimum tow wire length are:

Fine weather areas Minimum length bollard pull x 1200 mtrs
Break load

All other area Minimum length bollard pull x 1800 mtrs
Break load

Most modern vessel with horsepower above 6000 BHP will have tow wires at least 800 to 1000 mtrs and vessel over 10000 HP will have tow wires up to 1800 mtrs in length.