maximum design power and optimum efficiency at a designated towing speed, which may be anything from zero speed
(bollard) to the free route speed. Maximum engine power and design speed were established early in the design evolution
In all cases the greatest thrust (bollard pull) is produced at zero speed, with the towline pull diminishing as the towing
speed increases. When the tug reaches its maximum free route speed, all its horsepower is used in propelling it. At this
point the available towline pull is zero.
5-8.2 ESTIMATED BOLLARD PULL. A rule of thumb for estimating bollard pull for a normal, well- designed tug and
propulsion system is: open screw, 25 to 30 pounds per SHP; and Kort nozzle, 30 to 35 pounds per SHP. It is quickly
apparent that the nozzle vessel produces better thrust under bollard conditions. This advantage normally is lost as the
towing speed increases. Figure 6-1 provides available towline pull vs. speed through the water for Navy tow ships To use
Figure 6-1, the ship's speed THROUGH THE WATER must be known. Note the comparison between T-ATF and ATS
The low-speed pulling advantage of the T-ATF is reduced as towing speed increases, and finally disappears at higher
speeds. This is the result of the propeller nozzles fitted on the T-ATF.
Figure 6-1 can be used directly during towing opera- tions In planning tows, the planner must be aware that a region on
the upper left part of the graph may not be achieved by the tug. The reason is that avail- able shaft torque or engine
power may be exceeded
Appendices K and L provide additional information on evaluating the capabilities of tugs.
The towing system is the main battery of a towing ship and the towing machine or winch is a major element of this
system. These devices must perform several functions. Different machines have different features with which to
perform their required functions. The following paragraphs present summary discussions of these functions and features.
5-8.3.1 Functions of Towing Machines and Winches . The principal function of towing machines is to aid in handling
and controlling the towline Other functions may be to.
Act as a hard point or attachment point for attaching the towline to the tug
Serve as a means of paying out and heaving in during the towin operation
Serve to transport or stow the towline as it is heaved in
Act as a quick-release device for disconnecting the towline if necessary during an emergency
e. Act as an automatic tension control device to limit or relieve the peak dynamic loads in the towline system and
thereby enhance the life and utility of that equipment
Monitor and read out tow hawser conditions such as tension and scope
5-8.3.2 Features of Towing Machines and Winches . All towing machines and winches in Navy use have features
with which to perform the attach- ment, payout and heave-in functions. All of the tow- ing machines and some of the
traction winches can act as quick-release mechanisms for disconnecting the towline. However, quick release, automatic
ten- sion control and tension/scope measuring and read- out systems will vary
For instance, the diesel-driven SMATCO towing winch on the T-ATFs has no automatic tensioning device and often is
operated "on the dog". If an emergency occurs while the winch is on the dog, the dog cannot be released to allow the
drum to free-spool and let the towline run out. The diesel engine must be started,