J-1 2.2 Bollard Pull. Bollard pull is the zero speed pulling capability of the tug. It is a measure of the usefulness of the
ship in a stranding scenario or in holding a large tanker or aircraft carrier off a lee shore. However, the bollard pull figure
must be under- stood
Ideally, bollard pull is tested when a tug is built and certified by one of the classification societies. Bollard pull tests
sometimes are performed after major engine overhauls Tug owners whose tugs have been tested usually provide a copy
of the certificate attesting to the bollard pull figure
Bollard pull, like horsepower, is a selling point for tugs and is sometimes overstated. For instance, there are rules of
thumb for converting propeller power (SHP) to bollard pull, such as one ton pull per 100 horsepower for a conventional
propeller, or 1.2 to 1.5 tons pull per 100 horsepower for a propeller fitted with a nozzle. The owner may save the cost of
a bollard pull test and simply apply one of the factors to convert propeller power to bollard pull without ever knowing what
the real figure is It is unlikely that this owner will ever select a conservative conversion factor
European owners generally report bollard pull in their literature and reputable salvage tug owners generally are able to
produce a certificate to document the test. American owners, and the worldwide offshore oil support industry, rarely
report bollard pull. When they do, the figure may not have been validated by a test. Horsepower is probably a more
reliable measure among ships of these types.
Bollard pull is not the only useful measure of the puling capability of a tug. Except in the case of a stranding, the
objective of the tug is to move its tow. In this case, some of the tug's power is expended on overcoming the hull
resistance of the tug itself, and some on the hydrodynamic resistance of the towing hawser Bollard pull can be
maximized by propeller and nozzle design, but at the expense of towline pull at towing speeds This adversely impacts
free- running speed and fuel usage. Most tug designs, however, are optimized for towing.
Tugs generally are expected to operate in the 4- to 8-knot speed range. Modern tugs usually use propeller nozzles so
that bollard pull still is quite high, but with a significant disadvantage in tug speed and fuel consumption A tug optimized
for rescue towing probably would not employ nozzles, being most concerned with high speed running to the casualty, and
accepting some loss in efficiency of the tow itself. Appendix L provides additional information on the tradeoffs involved in
tug design. Figure M-1 displays towline pull vs. speed for typical tug designs using controllable-pitch propellers turning
inside nozzles. The figure is adapted from Blight and Dai, Resistance of Offshore Barges and Required Tug Horse-
power, OTC 3320, 10th Offshore Technology Conference Proceedings, 1978 (Ref. 26).
The foregoing aspects of tug design and owners' claims demonstrate that a tug should be considered as a balanced
design, with some being more suitable for some tasks, others for other tasks. The balance extends to the task as well
Chartering a 20,000 IHP salvage tug to tow a 200-foot barge would be just as inappropriate as sending a 5,000 HP
platform supply ship, with no tow hawser or winch, on a rescue tow mission.
J-2. OCEAN-GOING TUGS FOR HIRE.
This section provides sample specifications for typical ocean-going tugs and statistics on the number of tugs available
J-2.1 OCEAN-GOING TUG EXAMPLES.
Figures J-2 through J-4 are drawings of typical salvage tugs, point-to-point towing tugs and anchor- handling/supply tugs.
Table J-1 provides data on these and other tugs
J-2.2 DECLINE IN SALVAGE TUG AVAILABILITY . Traditionally, salvage and towing companies maintained their best
ships "on station" waiting for a casualty to occur. The "station" could be a semi- permanent strategic location such as
Jamaica, Gibraltar, Aden or Singapore, with backup by a shore