The blade number may also have been chosen so as to avoid global or local natural frequencies of the ship’s structure and probably also the maximum propeller diameter that can be accommodated. This will normally be based on a propeller type agreed during the specification stage, because it is very likely that some preliminary propeller design studies will have been conducted at that time. Following the creation of the design specification the synthesis of the design can commence. The resulting specification should be a jointly agreed document into which the owner, shipbuilder, engine builder and propeller designer have contributed: to do otherwise can lead to a grossly inadequate or unreasonable specification being developed. ![]() The definition of the problem is principally the specification of the propeller design point, or points in the case of controllable pitch propellers or ships with significant changes in operating condition, together with the constraints which are applicable to that particular design or to the vessel to which it is to be fitted. In the case of a propeller design, the conceptual and abstract design approach shown in Figure 22.1 can be considered in the following way. Therefore, in addition to the synthesis phase of Figure 22.1, which might be conducted using a mean radial wake distribution, there is the analysis and optimization phase to study the effects of the propeller operating, for example, in the full wake field variations or at off-design conditions. Indeed, in some instances the sea trial condition is an off-design point. ![]() Although there is a unique design point, in general the propeller operates within a variable circumferential wake field and may also be required to work at off-design power absorption conditions. Since in general a propeller can only be designed to satisfy a single design point, this involves a unique specification of the power absorbed, shaft rotational speed, ship speed and a mean radial wake field: the controllable pitch propeller being the partial exception to this rule when it would be normal to consider two or more design points. These general design ideas, although abstract, are nevertheless useful and are directly applicable to the propeller design process. Nevertheless, when a design does not perform as expected, it is normal, as in the generalized design process, to return to an earlier phase of the process to explore the reasons for failure and propose remedial action. Hence, the evaluation stage is frequently the sea trial phase within the ship-building program. However, in propeller design the luxury of a prototype is rare, since the propeller is normally a unit volume production item. This phase is the final proof of the design concept from which its success is determined, since it usually involves the testing of a prototype in the wider engineering context. When the design loop of synthesis, analysis and optimization is complete, the process then passes on to the evaluation phase. The analysis process may also include model testing in either a towing tank, cavitation tunnel or other facility. ![]() Consequently, we may synthesize several individual components of the propeller design, then analyze and optimize them and finally return to see what effect this has on the remaining parts of the system. In some cases it is also likely that this appeal process may lead to the identification of areas for longer-term research to enhance future design solutions.ĭesign is an interactive process in which several steps are negotiated and where results are evaluated, after which it may be necessary to return to an earlier phase of the design procedure. However, the design loop must be flexible enough should an unresolvable conflict arise with the original definition of the design problem, to allow for an appeal to be made to change the definition of the design problem. This iterative approach is needed so as to refine the design to that required: that is, a design that complies with the original specification and also has an optimal property about it. To provide a notionally optimal solution, the synthesis phase cannot exist in isolation and has to be conducted with the analysis and optimization phases in an interactive loop. ![]() Phases of engineering design.įollowing the design definition phase, the process moves to the synthesis phase where the basic propeller design is formulated using the various capabilities that are at the designer’s disposal.
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