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

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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)

CD (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)

CB (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 (Fn), which represents the state of the art.

CB (Watson, 1998)

CB (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.

CB (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.