Optimized Hull Form design at Initial Design Stage considering Manoeuvrability, Propulsive performance and Sea-keeping performance
Tsuyoshi ISHIGURO (IHI Marine United INC., Japan)
Takuya OHMORI (Ishikawajima-Harima Heavy Industries Co. Ltd., Japan)
Abstract: This paper first describes the relationship between principal particulars and three performances, propulsive performance, maneuverability and sea-keeping performance. Based on the tank test data, design impact by the selection of principal particulars on each hydrodynamic property is discussed for the latest full ship whose fullness is increased by great amount by her limited dimensions. Secondly, comparisons between CFD calculation and tank test results for each hydrodynamic property are shown to validate its estimation accuracy as a practical design tool. Finally, procedure of hull form design including the affirmation of IMO standard is discussed.
According to the increased demands for fuel oil saving in a field of merchant ship, shipyards are always requested to design hull form with highest propulsive performance and maximum dead weight within limited hull dimensions. However, manoeuvrability and sea-keeping performance act as constraints in pursuit of the above situation, especially for a full ship like VLCC
After the establishment of IMO manoeuvrability standard, shipyards are clearly faced with the design criteria for manoeuvrability and thus, procedure and tool for optimum hull form design should be established to maximize propulsive performance within a restriction of manoeuvrability and sea-keeping performance.
This paper first describes a design restriction for maneuverability to satisfy excellent propulsive performance and sea-keeping performance. Correlations of principal particulars and aft hull form to hydrodynamic characteristics for three performances are discussed based on tank test data. Secondly, results of CFD estimation for three performances are compared with tank test results and its accuracies are discussed. Finally, design procedure to realize the optimum hull form by compromising three performances is shown and the impact of present IMO standards on this procedure is discussed.
2. PRINCIPAL PARTICULARS AND THREE PERFORMANCES
In this chapter, relationship between ship's fullness, especially fore and aft fullness individually, and maneuverability, propulsive performance and sea-keeping performance is discussed based on the tank test data. Further, impact on hydrodynamic design raised by the continuous demand to increase the dead weight within a same dimension is described.
Concerning sea-keeping ability, resistance increase in waves, which is related to the sea margin under the actual voyage, is discussed as a main design parameter.
2-1 Selection of ship's fullness
It is a first step to determine fore and aft fullness when the principle dimensions such as Length, Breadth and draught are given at the initial design stage. In general, fore and aft fullness is described by the following expressions.
L : Length between perpendiculars
B : Ship's Breadth
Cpf : Fore prismatic coefficient
Cpa : Aft prismatic coefficient
Fig.1 shows the relationship between these coefficients and hydrodynamic properties, which consist of manoeuvrability, propulsive performance and sea-keeping performance. In general, fore fullness affects the wave making resistance in still water and resistance increase in waves which are categorized i n propulsive performance and sea-keeping performance respectively. On the other hand, aft fullness is closely related to (a) viscous resistance and self-propulsion factors and (b) directional stability, which are categorized in propulsive performance and maneuverability respectively. It is well known that hydrodynamic characteristics are strongly affected by the aft frame line shape, what is so-called U-type and V-type frame line shape.
Classification of three performances related to the hull form design
As is shown in Fig.2, block coefficient of the recent full ship is increased by great amount according to the requirement to maximize her dead weight within same principal particulars, which is already limited by physical restrictions in harbor.
Fig. 2 Necessity of compromised design for recent full ship
To increase the block coefficient, fore and aft prismatic coefficients are increased consequently. However, excessive increase of fore fullness leads the growth of wave making resistance and resistance increase in waves which results in an inferior propulsive performance both in calm sea and under the actual voyage.
Since design of an energy saving hull form is an essential aspect for a merchant ship, maximum value of fore fullness must be strictly limited regardless to the increase of block coefficient.
The above situation naturally generates the increase of aft fullness, which also leads to the inferior propulsive performance by the growth of viscous resistance. To minimize this tendency, a ship designer normally aims to apply the V-type aft frame shape.
On the other hand, since maneuverability, especially course keeping ability of a full ship, naturally tends to be inferior according to the increased ship's fullness, a ship builder should recover the lack of this ability by applying the adverse frame-type, U-type aft frame shape.
The above situation requires the ship builder to implement compromised design on aft body shape considering the conflicting two performances, maneuverability and propulsive performance. Discussion is continued further in detail based on the tank test data.
2-2 Propulsive performance and aft fullness/frame line shape
Fig.3 shows form factor on a basis of run coefficient, which are derived from tank test with various kinds of aft fullness and two different frame line shapes. In this figure, circle mark corresponds to the ship with U-type aft frame line and painted marks correspond to the V-type aft frame line whose typical figures are shown in Fig.4.
Fig.3 Comparison of Form factor for different run coefficients
Fig.4 Configurations of aft frame line shape