**1. Introduction**

There are no doubts that since ancient times the seafarers were able to discriminate between "good" and "not so good" vessels albeit this was only possible on a purely qualitative and intuitive level. Such discrimination was necessary and sufficient for a slow but steady evolution of various types of ships. As result, it became possible for ships to reach a certain undisputable level of perfection without any scientific support: medieval sailing ships were capable to perform very long voyages in rough sea waves including round-the-world trips. Of course, development based exclusively on traditions and some empiric rules was extremely slow.

Acceleration of development was not possible without certain scientific contributions unthinkable without quantification of the ship performance and seaworthiness. It can be noticed that the tangible raise of the "quantitative" phase of development of the naval architecture approximately coincided with the transition from sailing ships to mechanically powered ones. Likely, this was not only a coincidence associated with concurrent development of the naval architecture and ship theory but was also facilitated by the fact that any mechanically driven ship is, in many respects, a much simpler technical system than a sailing vessel. In particular, while the speed of a powered ship can be measured rather reliably at some specified standard testing conditions (deep water, calm sea, specified engine rpm) this is practically impossible for a tall ship whose speed depends substantially on the available wind. Further, as the aerodynamic configuration of a sailing ship is extremely variable, it is difficult to quantify its seaworthiness. Finally, it is practically impossible to

**Citation:** Sutulo, S.; Guedes Soares, C. Review on Ship Manoeuvrability Criteria and Standards. *J. Mar. Sci. Eng.* **2021**, *9*, 904. https://doi.org/ 10.3390/jmse9080904

Academic Editor: Michele Viviani

Received: 24 July 2021 Accepted: 17 August 2021 Published: 21 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

establish any reasonable measures of manoeuvring qualities of such ships as, for instance, a sail-driven ship is not able to perform steady turning circles and it is very difficult to reasonably define parameters of any turn executed through rather complex coordination between the rudder deflection and handling of the sails.

However, nowadays the sailing ships occupy a relatively narrow niche of recreational and training vessels and can be treated in some special way. As to the powered ships, all quantitative measures of their performance can be divided into two groups:


All these parameters may be related to the ship hydrodynamics or the strength and reliability of a ship hull and other elements or other operational qualities. Only hydrodynamicsrelated parameters will be considered further. The most common parameter of the first group is the contractual or design speed of the ship. The inability of a newly built ship to reach the specified speed value in the trials is typically treated as incomplete fulfilment of the contract and, unless the problem is fixed, is penalized. Most parameters of the second group are related to seaworthiness and unsinkability although in the several last decades manoeuvring performance of ships also became subject to substantial efforts aimed at the development and implementation of manoeuvrability criteria and standards.

The present article contains an analysis of existing requirements to the manoeuvring performance of civil displacement ships with emphasis given to the well-known manoeuvrability standards of the International Maritime Organization (IMO) [1]. All studies related to the introduction of any standards are characterized by many poorly formalizable issues and they strongly depend on subjective opinions of numerous experts. Final solutions are typically reached after long discussions in form of a more or less acceptable compromise, a retrospective analysis of existing approved standards and criteria and various alternative proposals is rather important.

The authors deliberately abstain from formulating any completed specific proposals complementing or modifying the existing recognized system of manoeuvring criteria but rather focus on discussion of possible approaches and methodology. This discussion constitutes the main content of the present article.

Section 2 of the article is dedicated to the formulation of general requirements to any system of criteria and standards related to maritime safety with a brief review of the existing intact-stability criteria. In addition, relatively recent minimum powering criteria are briefly commented. Section 3 contains a brief review of some existing calm-water manoeuvring criteria and of some earlier published alternative proposals. The influence of accounting for adverse factors and external disturbances on the composition and acceptable values of manoeuvrability criteria is discussed in Section 4, where also some possible novel approaches are outlined.

While the first four sections are mainly of descriptive character, Section 5 is of a more specific nature presenting some analytical developments that can be useful for devising simplified manoeuvring standards accounting for the ship aerodynamic characteristics and wind action. Finally, Section 6 contains a concluding discussion.

#### **2. General Principles of the Criteria and Standards in Naval Architecture**

Most theoretical developments and methods in naval architecture are associated with problems of analysis or design. However, it is reasonable also to talk about the standardizing problem characterised by some specifics which justify placing this problem into a separate class.

Most standards in ship design are associated with various aspects of the ship safety depending on the global and local strength, seaworthiness and manoeuvrability where the latter area was embraced by standardizing activities relatively recently and possesses a shorter historical record. At the same time, studies on the intact stability standards have been actively carried out since the 1930s and they provide rather rich material for comparative analysis of various approaches [2–4]. As a result of such analysis, it was possible to formulate the following reasonable requirements to any consistent set of safety standards independently of the application area.


Apparently, the most straightforward and intuitively transparent criterion of a ship's seaworthiness could be the expected duration of the ship's survival in certain severe weather conditions characterized by parameters of the sea and wind spectra but in practice, some better defined simplified scenarios are needed.

The so-called Second Generation Intact Stability Criteria [2,5–7] envisage 5 different basic failure scenarios including parametric roll and broaching to. However, the existing IMO Code [2] still in effect is based, besides direct requirements to the righting-arm curve, almost exclusively on the so-called weather criterion, i.e., on the estimation of dynamic stability of a dead ship under the action of resonant roll and an unfavourable wind gust. Despite its simplicity, this criterion is quite reasonable and reliable as in fact it resulted from several decades of evolution based on intensive research studies carried out in many countries. The criterion still survives despite considerable efforts aimed at accounting for, say, parametric rolling, broaching or just involving more sophisticated and accurate mathematical models [5,6]. It is also remarkable that while the direct statistical approach introduced by Rahola is considered outdated and no longer acceptable, statistical analysis keeps playing an important role [2,8].

Of course, all safety-aimed criteria and standards have a clear trend to involve more and more sophisticated mathematical models, scenarios and schemes which sometimes seems to be more driven by the interests of developers than by clear practical demands. In particular, it is envisaged that in the mentioned "Second-generation intact-stability criteria", see, e.g., [9], 3 levels or tiers of criteria are to be introduced with the level 1 corresponding to the most simple and conservative criteria while higher-level criteria presume involvement of rather sophisticated schemes and methods. It is supposed that a higher-level criterion is only invoked if the corresponding lower-level criterion is not satisfied. Whether such multi-level system of standards will be definitely beneficial for the overall safety of shipping is not so evident and some specialists reserved a somewhat sceptical attitude [10]. The authors of the present contribution tend to believe that any practical system of criteria and standards should be kept as simple and transparent as possible and, regarding the classification mentioned above, all ideas presented in this paper should be viewed as corresponding to the lowest level.

In addition to the aforementioned standardizing areas, relatively recently a completely new problem emerged—that of minimum installed power requirements [11]. It was triggered by the so-called Energy Efficiency Design Index (EEDI) requirements, see [12] for more details. These requirements encouraged reduction of the installed main engine power as the most effective mean of decreasing the fuel consumption and, concurrently, emissions of all kinds. As such a trend obviously leads to reduction of the capability of ships to withstand adverse conditions thus impairing navigational safety, it gave birth to

rather natural concerns. As result, considerable efforts have already been dedicated to working out certain minimum-power standards limiting the EEDI-driven reduction of the installed power [13].

The minimum-power requirements are connected with the seakeeping and manoeuvring qualities of ships. However, in this article, the authors avoid detailed discussion of the minimum-power problem limiting themselves to the short comments that follow.

Since the appearance of powered ships the required power of the main engine(s) has always been estimated as a compromise between a natural desire to reach the highest possible operation speed and safety and a desire to build a more economical vessel. It is obvious that the latter encourages the designer to reduce the ship's speed and the engine's rating. In some special cases (tugs, fishing vessels, icebreakers) the engine power is governed not by the design speed but by the required bollard pull. While the bollard pull-based power can be determined relatively straightforwardly, a more common task of setting the design speed is more complex and fuzzy. The value of this speed depends on the type of the ship, its size, actual and envisaged fuel prices, traditions and expectations of ship owners and seafarers.

A kind of natural selection during many decades resulted in several typical values of the design speed for various types and classes of ships although those typical values were subject to relatively small variations and corrections especially when new types of ships, engines and propulsors were introduced. However, so far they have never been conditioned by some legal requirements. Thus, in some sense, the introduction of EEDIs is breaking the existing dynamic equilibrium exerting substantial legal pressure that encourages the ship designers to decrease the power of main engines. It was soon understood that in some cases the newly built ships could become underpowered and excessively prone to exogenous factors. As result, the problem of standardizing also some minimum safe power of ships was formulated with the purpose of legal counterbalancing the unfavourable impact of EEDI requirements.

In addition to vulnerability to environmental factors, the underpowering may have one more negative effect related to the stopping distance in crash stop. While the stopping distance or the track reach from the speed at maximum continuous rating in calm water, as specified by the current IMO standards, may remain unchanged as long as poorer reversing capabilities come in certain harmony with the reduced speed, this is not so certain for partial speed and/or in presence of unfavourable wind, sea and current. This issue, closely related to the powering requirements is also dropped here.

As the danger of underpowering was realised, lately a concept of the reserved power to only be used in extreme conditions was discussed [13]. It may happen that this situation will finally lead to the appearance of more adaptable and flexible power plants.

Returning to the general standardization problem, it can be observed that there are three main different standardizing schemes:

1. **Explicit criteria scheme.** It is presumed that the limiting values of certain criteria are explicitly specified and the compliance to them is to be verified by some certified or generally recognized procedures which may include full-scale trials, model-scale tests and Computational Fluid Dynamics (CFD) modelling. For instance, regarding some manoeuvring criterion such as, e.g., the minimum turning radius, its value can be specified explicitly and their fulfilment is then verified by estimating the corresponding measures for the actual ship or design. This approach was realized in IMO standards [1,14] and STANAG standards [15]. The advantage of this scheme is that the standards in form of natural and meaningful manoeuvring qualities are verified directly and the estimation methods can be gradually improved in course of natural scientific progress without affecting the standards themselves. The evident disadvantage is the necessity to select and use some methods not incorporated into the text of the standards: this complicates application of the standards and may introduce additional uncertainty.

2. **Implicit scheme.** Alternatively, the standards can be formulated in form of a set of direct requirements to the rudder effectiveness. The latter must be defined within the standards in some unambiguous way allowing immediate verification at least for the most typical steering devices. This approach was followed in the implicit manoeuvring standards implemented within the Rules of the Russian Maritime Register of Shipping [8], see also [12,16]. In contrast with the IMO standards which were developed and adopted 14 years later, the Russian standards do account for aerodynamic loads albeit in a somewhat hidden way. However, although implementation of these standards was by all means somewhat useful, their straightforward reproduction in the part concerning controllability in wind cannot be recommended: the exploited mathematical models and prediction methods were too simplistic by all reasonable standards and even suffered from some inconsistencies. Even though these inconsistencies were traced by some experts, they practically could not be fixed as all the models and methods are embedded in the standards and their correction, modification and improvement is unthinkable without revision of the corresponding part of the Rules which is impossible without complicated bureaucratic procedures. This is one of the main reasons why the mentioned "hidden" manoeuvring standards remained unchanged since their embedment into the text of the Rules in 1982. Even though these Rules will keep these standards for an indefinite time, it is obvious that any new developments should follow a more "explicit" approach and it makes sense to realize them as some modifications and extensions of the existing IMO standards.

The task of specifying limiting values for criteria involved in any system of standards is also important and non-trivial. The following approaches can be followed:


can be drawn: although any of the meaningful scenarios provides reasonable and useful indicators, no one of those scenarios is guaranteed to be "played" successfully in real world even if the ship meets the finally adjusted standards.

3. **Ergatic approach.** This approach is very common in aeronautics [20] where standardization focuses on ergatic issues, i.e., on the man–machine interaction presuming a definition of acceptable values of parameters of mathematical models of aircraft. Nowadays, this has become a rather routine practice and is performed with the help of interactive flight simulators operated by representative groups of pilots. Probably, for the first time, the idea of applying a similar approach to ship manoeuvring was expressed by Segel [21] and much later, though independently and in a more elaborated form, by Sutulo [22]. In fact, elements of this approach combined with the scenario of steering a ship in a complex bent canal were also used by Nobukawa et al. [23] to establish an acceptable level of directional stability and finally these results contributed to the IMO standards. However, in general, the significance of methods based on interactive simulations in ship manoeuvrability is much inferior to that in the flight dynamics because of substantial differences in dynamic properties, in particular in the values of time lags, between the surface displacement ships and aircraft. In addition, this approach is hardly suitable for standardizing any properties of the craft related to exogenous perturbations.
