Ship Design

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ELEMETS OF SHIP DESIGN, R. Munro
-
Smith (1975)
-
1995


SHIP KNOWLEDGE, A Modern
Ansiklopedia, K. Von Dokkum, 2003


PRACTICAL SHIP DESIGN, D.G.M.
Watson, 1998


Ship Design and Performance for

Masters
and Mates
, Dr C.B. Barrass, 2004


Ship Design for Efficiency and Economy
,
H. Schneekluth and V. Bertram, 1998






Dimensional Relationships (Watson, 1998)


The fact that there are six dimensional
relationships linking the four main ship
dimensions of
L, B, D

and
T
and that it is
necessary to use three of these to solve
either the weight or volume equations has
already been noted (Watson, 1998). The
relationships are:



B =f(L) D

=f(L)



D =f(B) T=f(L)



T
=
f(D
)
T =f(B)


Estimations of the length for a new design,


Barrass, 2004


1. Ship
length
is controlled normally by the space available at
the quayside.

2. Ship
breadth
is controlled by stability or canal width.

3. Ship
depth
is controlled by a combination of draft and
freeboard.

4. Ship
draft
is controlled by the depth of water at the Ports
where the ship will be visiting. Exceptions to this are the
ULCCs and the Supertankers.

They off
-
load their cargo at single point moorings located at
the approaches to Ports.


Constraint for length

CD=DW/
Δ

(Munro
-
Smith, 1995)

C
D
(Barrass, 2004)


CD will depend on the ship type being
considered. Table 1.1 shows typical values
for Merchant ships when fully loaded up to
their Summer Loaded Waterline (SLWL)
(Draft Mld).



L/B (
Von Dokkum, 2003
)

The ratio of length and beam can differ quite darmaticly
depending on the type of vessel. Common values:

Passenger ships

: 6
-
8

Freighters


: 5
-
7

Tug boats


: 3
-
5

A larger L/B value is favourable for speed but unfavaurable
for manoeuvrability.

(Barrass, 2004)


From a study of a large number of Merchant
ships, it has been shown that in modern ship
design practice, the parameters L and B can
be linked as follows:


L/B (Watson, 1998)

By 1975, when Fig. 3.7 was originally presented,
ships of more than about 130 m in length were
almost invariably being built with an
L/B
ratio of
6.5; ships of up to 30 m in length, such as fishing
boats, usually had an
L/B
ratio of 4; whilst vessels

whose length lay in the range between 30 and 130
m followed a linear interpolation pattern between
L/B
values of
4

and
6.5
.


L/B (Watson, 1975)

L/B (Watson, 1991)

L/B (Watson, 1998)


A low
L/B
ratio that is undesirable
as such
but the fact that a short length and therefore
a high Froude number for a given
displacement are an unavoidable results of
having a large beam.


B/D (Von Dokkum, 2003)

B/D ratia varies between 1.3 and 2. If this value
becomes larger, it will have an unfavourable
effect on the stability (because the deck will be
flooded when the vessel has an inclination) and
on the strength.

B/D (Watson, 1998)

This relationship is closely related to stability since KG is a
function of depth and KM is largely a function of beam.


Figure 3.9, which was originally presented in the 1975 paper,
reverts to lines of constant
B/D
and shows a plot of depth
against beam for a number of ship types. It was found that there
were two distinct groups. The first group consisted of

deadweight carriers comprising coasters, tankers and bulk
carriers had an
B/D
ratio of about 1.9
. The second group
consisted of volume carriers comprising fishing vessels and
cargo ships whose depth was limited by stability considerations
and which had a
B/D
ratio of about 1.65.

B/D (Watson, 1975)

B/D (Watson, 1991)

The 1991 plot included in Fig 3.8 largely confirms these
groupings with tankers and bulk carriers again averaging at a
B/D
of 1.9.


The second group brought container ships and refrigerated
cargo ships together at the slightly
increased
B/D
value of
1.7
. The higher
B/D
value (1.7 vs 1.65) for volume carriers in
1991 may be a consequence of the need to limit the depth of

these ships because of the stability inferences of making
provision for the carriage of containers on deck.


B/D (Watson, 1991)

B/D (Watson, 1998)


Factors which in general may require an increased
B/D
value include: higher standards of stability for
whatever reason these may be needed; the carriage
of deck cargo; reductions in machinery weight
raising the lightship KG; and the finer lines needed
for high speeds giving reduced KM for a given
beam.


Factors which may permit a reduction in
B/D
include the provision of a large ballast capacity in
the double bottom; absence of deck cargo; relatively
light superstructure and cargo handling gear;
absence of sheer and/or camber; and lines designed
to provide a particularly high KM value.


T/D (Watson, 1975)


T/D (Watson, 1998)



The high freeboard that this low ratio indicates
shows the concern for seaworthiness that is so


necessary a feature of the design of these ships.


L/D (Von Dokkum, 2003)


L/D varies between 10 and 15. This
relationship plays a role in the determination
of freeboard and longitudinal strength



L/D (Watson, 1998)


In deadweight carriers, stability is generally in
excess of rule requirements and depth and
breadth are therefore independent variables. For
these ships, control of the value of
D
is
exercised more by the ratio
L/D
which is
significant in relation to the structural strength
of the ship and in particular to the deflection of
the hull girder under the bending moment
imposed by waves and cargo distribution.

L/D (Watson, 1998)


The largest
L/D
ratios were formerly used on
tankers whose “A’ type freeboard needed a
comparatively small depth for the required draft
and whose favourable structural arrangements
with longitudinal framing on bottom, deck, ship
sides and longitudinal bulkheads together with
the fact that this type of ship has minimum
hatch openings meant that the steel
-
weight
penalty for an unfavourable
L/D
value was
minimised.


L/D (Watson, 1975)

L/D (Watson, 1991)

L/T (Watson, 1998)


This is essentially a secondary relationship
resulting from either of the following
combinations of relationships:



T =f(D)

or

T=f(D)

and

D =f(L)

and

D =f(B)
and
B
=
f(L)


B/T (Von Dokkum, 2003)


The B/T

ratio
, varies between 2.3 and 4.5.
A larger beam in relation to the draft (a
larger B/T value) gives a greater initial
stability.


B/T (Watson, 1998)


This is again a secondary relationship,
resulting in this case from either of the
following combinations of relationships:



T =f(D)

or

T=f(D)

and
D =f(B)

and

D =f(L)
and
B =f(L)


Optimisation of Main Dimensions (Watson, 1998)

C
B
(Watson, 1998)


The last factor required to complete the equation linking
dimensions and displacement is the block coefficient.
A
first principles approach to the determination of the
optimum block coefficient for a ship would involve a
trade
-
off calculation in which the increment in building
cost resulting from the increased dimensions required for a
fine block coefficient is compared with the saving in
operational cost obtained as a result of the reduction in
power which fining the lines achieves. This is a major
exercise but fortunately it is rarely necessary to adopt such
an approach, the more general procedure being the use of
an empirical relationship between block coefficient and the
Froude number
(F
n
),
which represents the state of the art.



C
B
(Watson, 1998)

C
B

(Barrass, 2004)


The slope ‘m’ varies with each ship type, as
shown in Figure 1.1. However, only parts of
the shown straight sloping lines are of use
to the Naval Architect. This is because each
ship type will have, in practice, a typical
design service speed.



C
B

(Barrass, 2004)

(Barrass, 2004)

Optimisation of the Main Dimensions and CB

Early in the design stages, the Naval Architect may have to
slightly increase the displacement. To achieve this, the
question then arises, ‘which parameter to increase, LBP,
Breadth Mld, depth, draft or CB?’

Increase of L

This is the most expensive way to increase the displacement.
It increases the first cost mainly because of longitudinal
strength considerations. However, and this has been proven
with ‘ship surgery’, there will be a reduction in the power
required within the engine room. An option to this would be
that for a similar input of power, there would be an acceptable
increase in speed.

(Barrass, 2004)

Increase in B

Increases cost, but less proportionately than L. Facilitates an
increase in depth by improving the transverse stability, i.e. the GMT
value. Increases power and cost within the machinery spaces.

Increases in Depth Mld and Draft Mld

These are the cheapest dimensions to increase. Strengthens ship to
resist hogging and sagging motions. Reduces power required in the
Engine Room.

(Barrass, 2004)

Increase in CB

This is the cheapest way to simultaneously increase the
displacement and the deadweight. Increases the power required in
the machinery spaces, especially for ships with high service speeds.
Obviously, the fuller the hullform

the greater will be the running costs. The Naval Architect must
design the Main Dimensions for a new ship to

correspond with the specified dwt. Mistakes have occurred. In most
ship contracts there is a severe financial penalty clause for any
deficiency in the final dwt value.