Hull Form Exploration in the Early Stage of Design
Authors: Igor Mizine, Charles Rogers, Bruce D. Wintersteen
The objective of the ship design synthesis process is to derive a ship's physical and performance characteristics based on mission requirements and selected technology and configuration options. To accomplish this objective an effective compromise must be achieved between the many competing requirements and constraints that form the available design engineering disciplines that are addressed during the design synthesis process include; mission systems and cargo requirements, hull form geometry, hull subdivision, deckhouse geometry and subdivision, structures, appendages, resistance, propulsors, machinery arrangements, weight estimates, required arrangeable area and volume, intact stability and seakeeping.
The hull form is a critical component of the design synthesis process. The hull is subdivided with decks and bulkheads to establish the compartment configuration (to the watertight compartment level) within the hull and to determine if the required mission capabilities and systems can be accommodated. The hull form is the principal boundary for the structural design. Required appendages must be integrated with the hull form. The propulsor design (propellers, waterjets, etc.) depends on resistance and the water flow around the hull form. The hull form significantly drives the propulsion power required and significantly impacts the location of the principle machinery equipment within the hull. While the weight estimates draw directly from the structural design and machinery equipment and other known data (mission systems), many of the other weight groups are estimated by algorithms. These algorithms are very dependent on hull volume and the distribution of that volume within the hull. Hull hydrostatics, stability and seakeeping are all very dependent on the hull form.
The investigation of hull form variations during early stage design has long been limited by the capabilities present in the available design tools and their supporting framework. While some excellent hulls have been designed in parallel or preceding the overall ship design process, the limitations in design tools and their integration have often left the design process with a significant unknown as to whether the selected hull form is truly the best configuration for the ship and its mission. The full form has a significant influence on almost every subsystem and discipline involved in ship design, not just hydrodynamics.
The routine Navy practice during early stage design has been to perform analysis based on a single baseline hull form point design, which is usually derived from dimensional scaling of existing designs or prototypes. This practice limits analysis of the hull form related characteristics and performance in concert with other tradeoffs and analysis of the disciplines that are very much influenced by the hull form. In some cases, this approach has perpetuated the undesirable characteristics of the selected starting hull form. In many, if not most recent designs, the limitations of our design process capabilities have produced less than optimal hull form configurations, especially in view of the operational profile, which determines the life cycle cost. In addition, late design improvements in hull form such as stern flaps or bulb changes result in the ship exceeding the design requirements that drive cost into the ship, i.e. larger engines installed then required to meet the ship's KPP for speed.
The paper explains how it is possible to overcome this limitation and how to restructure the ship design processes to facilitate effective investigation of hull form variations as part of the design synthesis process. The development of the hull form along with the overall development of the ship design configuration can be effectively integrated during the early stages of design when sufficient flexibility remains to enable the most effective design across all disciplines.
This paper addresses the process, tools, and methodologies the authors have been developing and applying for several ship design projects to enable the effective development of the hull form and the investigation of hull form variations and their impact on the overall ship effectiveness. The approach used to facilitate the effective integration of the range of design and analysis tools necessary to support the process is described. The methodologies and theories used to investigate the potential range of hull form alternatives and assess their relative performance are presented. Examples of analyses done for actual design projects are provided, along with lessons-learned and recommendations for further refinements and improvements to the processes presented.