Tugs are designed to facilitate the safe handling of lines and
provide maximum stability when towing. These requirements generally
dictate a relatively large deck area with the minimum superstructure
adequate to allow support of operations. This provides the largest
possible unobstructed arc around the vessel for line handling, and
clearance when operating under overhanging areas of the hull of
an escorted vessel. The result is the "tapered in" appearance
of the deckhouse and pilothouse on most tugs that have to do side
towing, astern towing or ship docking. The unobstructed arc considerations
are of course less important on notch tugs or ATB (Articulated Tug
Barge) sets, where there is much less need to handle lines.
The low freeboard provides maximum stability when towing by allowing
lines to lead in as low as possible on the hull, this is desirable
to provide the greatest margin of stability in the event of an unexpected
mishap while handling the tow. On a modern ocean going tug like
the Crowley Invader class, what you see above the surface
may look low and sleek:
This is in fact a very large and powerful tug capable
of extended operation in open ocean under the most adverse sea conditions.
The machinery on the Invader class is two twenty cylinder
EMD 645 engines rated at 7000 horsepower total. In order to allow
for extended operation, the 136 foot long tug is capable of carrying
155,000 gallons of #2 diesel fuel. The loaded draft is 20 feet,
quite deep for a tug, and there is a tremendous hull area below
the water surface that is not evident when viewing the vessel:
Tugs are "displacement" hull vessels, the
hull is designed so water flows around it, there is no consideration
for having the vessel "plane". Because of this the hull
form is limited to a maximum speed when running "free"
that is about 1.5 times the square root of the waterline length.
As the tug approaches this speed when running "free" it
is perched between the bow wave and the stern wave. Since the hull
cannot plane, application of additional power when approaching maximum
hull speed only results in a larger bow wave, with the tug "squatting"
further into the trough.
Below: A Crowley Invader class tug underway
in choppy seas demonstrates how the tug settles in or "squats"
between the bow wave and stern wave when running free at maximum