Page 244 - Ship Construction.DJ Eyres 6Ed

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Ch20-H8070.fm Page 233 Wednesday, October 18, 2006 6:59 AM
Fore End Structure 233
best chain lead to ensure the anchor could be raised and lowered smoothly
and housed properly. Today this can be achieved with the CAD hull model.
Tubular hawse pipes are generally fabricated, and castings are welded
at the shell and deck to prevent chafing (see Figure 20.5). Additional stiff-
ening in way of the hawse pipes is required at the side shell. On higher
speed vessels a recess is often provided in the shell for anchor stowage;
this helps to reduce any drag caused by the stowed anchor and prevents
--- ใช้เพื่อการศึกษาเท่านั้น---
serious damage in the event of a collision.

งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Bow Steering Arrangements
Double-ended ferries are provided with a rudder at either end which is
locked in position when it is at the fore end of the vessel under way.

Bow Thrust Units

For manoeuvring in confined waters at low speeds, lateral bow thrust
units are particularly useful. These are often found in research vessels, or
drilling platform vessels where very accurate positioning must be main-
ึ
tained. They are also to be found in large ships and cross-channel vessels
where they are provided as an aid to docking. The thrust unit consists as a
rule of controllable pitch or reversible impeller fitted in an athwartships
watertight tunnel. Control of the unit is from the bridge, but the driving
motor is in way of the impeller. Thrust provided by the impellers are low;
16 tonnes is perhaps the largest fitted, but the unit size does not need to
be large as small thrusts are very effective. It is true however that the
greatest thrust is provided at zero speed and as the vessel gets under way
the unit becomes much less effective.
From a construction point of view the most important feature is the
provision of fairing sections at the ends of the athwartship tunnel in way of
the shell. It has been shown that an appreciable increase in hull resistance
and hence power may result if this detail is neglected. The best way of
avoiding this is to close the tunnels at either end when they are not in use.
This is possible, flush mounted, butterfly action, hydraulically operated
doors being available for this purpose.