The design spiral is a methodology for developing ship designs. As ships are complex systems with highly interdependent variables, it quickly becomes impossible to calculate factors simultaneously. Instead, the design spiral describes a process of iterative refinement to 'zero in' on an efficient design. Each successive iteration is referred to as a 'spin' of the spiral. Phases or cycles are considered once a given level of technical refinement has been achieved. It is important to note that is not a concrete, defined checklist: particularly in the early stages, the order of evaluation may be rather hap-hazard.
While the number of spins is highly dependent upon the project and time constraints, there is a general progression of iterations, with some recognized loose milestones. These milestones are often important decision-points, particularly in relation to the projects funding.
The owners requirements phase determines the client/owners desires and needs in a vessel. For larger vessels, this may include market evaluations or traffic studies. For fleet expansions, this may include elements of interoperability or similarity with existing vessels. Owner requirements can change over the course of a project as feasibility of design aspects become apparent, supporting documents (such as environmental studies) are developed, or as the owners own requirements change over time.
The preliminary design circumscribes only the most basic of dimensions, layouts, equipment and capabilities. For the most part, it is a rough feasibility check. A number of variations will be evaluated, whose varied parameters should be guided by sensitivity analysis, such as by a state of the art report. The concept phase should lead to decisions on major parameters, such as hull type and number of vessels required (i.e., one large vessel? two smaller vessels?). Of these concept designs, one or two will be accepted for continued evaluation and will serve as the base parameters of the design project.
The chosen concept from the concept design is put through more rigorous analysis. Clashes in major components will be identified in this cycle. Capacities are worked out, and major parameters have values determined (length, breadth, etc.). During the preliminary phase, expected calculations based on typical equipment will be performed. Shafting systems, general structural scantlings (see midship section), initial lines plan, general arrangement, ship specific cargo handling, storage, and working deck systems will be conceptualized. These will be to a suitable level of detail to allow sub-contractors to quote equipment, packages, or systems.
At this point, all major components have been settled upon, the hull lines are finalized, and the package is suitable for bid tendering to ship yards. All major vendor equipment shown will be project-specific, a preliminary stability book will be created using weight estimate information, and all classed drawings will have been reviewed and approved. Once this point is reached, additional changes are likely to be subject to ECO's.
The detail design phase is the final phase of development. During this phase, assembly drawings are created, steel sheets are nested, pipe spools drawings created, etc. The detailed design phase leads to final production drawings which will be used to physically build the vessel. In contrast to earlier stages, the detailed design stage will use lofted geometry.
The amount of time for a project can vary greatly with ship size, complexity, experience of the designers and available reference project and/or materials. One estimate for a moderate size vessel, shown here to demonstrate the relative increase in effort of later stages (in person work-days):
This is a sample evolution of the spiral. Varying sources will use different orders, combining or seperating various points out.
The mission profile is a document outlining the requirements of the owner, route, operational profiles, and the constraints that will be imposed on the vessel. While it may seem initially trivial, this document is important to maintaining the focus of a given project. The final design product's merit will be its ability to achieve the goals laid out here. For larger projects, the mission profile is important for keeping all members of the design team working towards a cohesive solution. Depending on the situation (i.e., a series-vessel being ordered by a client with a qualified owners representative), the mission profile may aided or substituted by a complete specification.
The hull form may be approached in a number of ways, such as the parent hull method. The parametric analysis can also be used to develop a desired set of length, breadth, draught, and depth as well as coefficients of form from similar ships to provide an adequate start point. For ships with specialized bows and sterns (such as icebreakers), a midship section can be developed to suit the cargo, to which a suitable ends can be worked in. On further spins of the spiral, hullform is refined to minimize wasted volume and ensure adequate displacement. Any deficiencies of stability, resistance, or arrangement will also be corrected.
The structural arrangement of the vessel (see structural design) is important to the determination of weights & centres, cargo area intrusions, and buildability of the project. Initial design will focus around the placement of large stress-concentrating features (such as hatch openings), frame system, frame spacings, the placement of bulkheads to meet subdivision requirements, and to adequately transfer ship loads to the shell plate. Once a structural system has been roughed out, scantlings can be determined by or checked against classification requirements. Further spins will focus on increasing the level of detail shown and the structural efficiency of the design (as well as any updates arising during project development).
The general arrangement is the placement and sizing of shipboard areas such as to suit the task of the vessel. The placement of heavily loaded items, such as moorage or lifting devices will ideally be supported by efficient structure. Regulatory requirements, such as passageway sizes and fire codes, need to be met. Naval architects will strive for an efficient workflow and habitable spaces. Tankage will be calculated to determine range, maintainable trims, and for stability analysis. On subsequent spins, the general arrangement tends to undergo only minor rearrangements, however the complexity of the drawing increases as the ships equipment lists become more complete. For large and complex vessels, additional arrangement drawings will be created for spaces, i.e. a vessel with a large number of staterooms an accommodation arrangements drawing would be desirable.
Weights & centres will be tallied to estimate centre of gravity locations for lightship, outfit, and load conditions. The distribution of weight can affect hull deformation, determines displacement, and can be used to estimate build costs. An accurate weight estimate is also imperative to producing meaningful stability data.
The hydrostatic properties of the ship will be analyized to ensure adequate stability and seakeeping ability; including various conditions of load and damaged states. These will be used to produce a preliminary stability book, with a final version being issued after the ship is launched and inclined.
Of particular importance to diesel-electric or all-electric ships, though pertinant to all ships.
Expanding on the dictums of the general arrangement, choices of furniture, coverings, wall and ceiling sheathings, etc.
Most commercial vessels primary role, goal, and purpose for existance is to make money for the owner. For privately owned vessels, the overall cost will generally be constrained. When designing naval vessels or other public vessels, operational (lifecycle) and initial costs are both major considerations. As such, when passing this stage of the design spiral, the mission profile will be compared against the operational profile for compliance. Build cost will be estimated (i.e., by the Carreyette method); with consumables, occupancies, outfit, capacities, etc. used or estimated to determine operating costs. Cash flow analysis, break even analysis, and net present value analysis are all ways of demonstrating the economic feasability of the vessel.
Yachts are one class of vessel which tend to follow a truncated version of the design spiral. As a luxury vessel, where presence is highly valued, the vessels hullform, exterior, and interior may all be designed by different firms. Where this is the case, aspects of the design may be frozen relatively early in the design process (for example, the vessels lines), with subsequent turns of the design spiral working to refine the design within these constraints.
Naval vessels will have additional 'stops' on the cycle for radar cross section, acoustical & vibration analysis, thermal and electromagnetic emissions, damage control & subdivision, radiological/biological/chemical resistance and fleet compliance (interoperability).
While many naval architects would agree that this is, in general, the path design work takes, the design spiral is not without its detractors. Some have chided the reductionist focus, i.e., attempting to break down a design into manageable components, which may exclude greater life-cycle savings (for example, the inclusion of integrated systems as in the all electric ship). There is also the charge that some factors may be missing from the spiral, for example environmental evaluation. It is therefore important for those in charge of a given project to also be able to see the 'greater picture', as well as to ensure a good flow of communications between resources; applying the spiral as a framework rather than a checklist.
The initial spiral analogy was published in 1959 by J. Evans. Dr. Ian L. Buxton introduced elements of engineering economics in a 1972 RINA publication. In 1981, D. Andrew considered the spiral as a helix, demonstrating increasing constraint as the design continues.