Metacentric
Height, Stability
Typical metacentric height for a stern drive
tug is approximately 1.5 m. Tugs with less initial
stability need to have their stability checked carefully
and may still end up with stability problems.
The
stability for the specified bollard pull needs to
be checked at an early stage, especially if the
design is rather narrow. The stability requirement
is not only for a static situation, as - in normal
towing conditions - both the tug and the assisted
vessel are moving, the assisted vessel dragging
the tug along. Especially important this is if the
tug is assisting vessels at higher speed or used
for escort towing.
Weight
The weight of the azimuth propulsors may come as
a surprise and often it will be difficult to get
a proper balance, if not accounted for in the first
basic design. The units are way back in an area
with little or no hull volume. Hence, the centre
of gravity for the rest of the tug needs to be kept
more forward than on other tug types.
Trim
and Draft
At least the trim of a tug should be possible to
be altered, to improve the performance and prevent
ventilation of the propellers in braking and astern
running. For ice going tugs the possibility to trim
the vessel fast is important, as is the possibility
to increase the draft for ice navigation.
