While underway, the tug and tow will be subject to both steady and dynamic loads Care must be taken in
planning the tow by selecting the proper gear and deciding on the route and departure date to avoid adverse
weather conditions which may subject the towing systems to loads that exceed the safe working load of any
component within the towing system. A tug must request Optimum Track Ship Routing (OTSR) for each long
Navy design criteria for safe mooring loads are based upon specific conditions of winds, waves and cur- rents.
In contrast to this situation, safe working loads for towing depend much more upon the towing operation and
upon the general category of towing: inshore barges, rescue towing in combat or storm conditions. In the
routine, inshore barge type of tow, safe loads are more easily defined and the design can be based primarily
upon normal operating conditions For ocean towing, the design must consider loads associated with extreme
weather or survival conditions, rather than those characterizing normal operations. Chapter 5 contains detailed
discussions of towline tension, towing resistance and factors of safety for towing system design.
SECTION III. TUGS
TYPES OF TUGS. Most U.S. Navy ships can tow man emergency, but only properly designed and
outfitted tugs make good towing ships. Characteristically, a tug's superstructure is set forward, allowing the
towing point to be close to the ship's pivot point. The towline, secured well forward of the rudder and propellers,
is allowed to sweep the rail without limiting the maneuverability of the tug. In addition to a clear fantail area,
characteristics of a good tug include the following:
a. High horsepower
b. Slow speed
c. Large-diameter propeller(s)
d. Large-area rudder(s)
e. Towing machine
f. Power capstans
g. Towing points
2-3.1.1 Yard or Harbor Tugs. The U.S. Navy operates the YTB (large yard tug), YTM (medium yard tug), and
YTL (small yard tug) These tugs are used primarily in sheltered waters including harbors. On occasion,
however, YTMs and YTBs can be used in open-ocean towing. In addition, the U.S. Army and U.S. Coast
Guard operate a large number of similar tugs and yard craft.
Harbor tugs are classified by shaft horsepower (shp)
YTL-small-400 SHP and under
YTM-medium-400 to 1,000 SHP
YTB-large-1,000 to 2,000 SHP or greater.
The design of harbor tugs and the equipment they employ varies. The typical harbor tug is a single- screw,
deep-draft vessel equipped with a capstan aft, H-bitts forward and aft, towing hawsers and additional lines for
handling ships or barges in restricted waters. The harbor tugs may also be equipped with fenders, very limited
firefighting equipment and deck equipment to support harbor operations.
2-3.1.2 Ocean Tugs. The Navy's ocean tugs are designed to be far more versatile than harbor tugs, not only
in terms of horsepower and range capabilities, but also in terms of the services they can provide to their tows.
Designing purpose-built tugs has not been the primary goal of Navy design practices in the past; most Navy
ocean tugs are multipurpose-built In addition to their power, range and endurance capabilities, the ocean tugs
have the capability to work and survive in heavy weather independently of other auxiliary or support ships.
They also are used in stranding and other salvage operations. Ocean tugs should have