A brief introduction to gas-turbine engines was presented in Chapter 1. A review of the different engines, included in this chapter, reveals that most of these engine components are composed of “lifting” bodies, termed airfoil cascades, some of which are rotating and others stationary. These are all, by necessity, bound by the hub surface and the engine casing (or housing), as shown in Figures. 2.1 through 2.5. As a result, the problem becomes one of the internal-aerodynamics type, as opposed to such traditional external-aerodynamics topics as “wing theory” and others. Referring, in particular, to the turbofan engines in Chapter 1 (e.g., Fig. 1.3), these components may come in the form of ducted fans. These, as well as compressors and turbines, can be categorically lumped under the term “turbomachines.” Being unbound, however, the propeller of a turboprop engine (Fig. 1.2) does not belong to the turbomachinery category.

The turbomachines just mentioned, however, are no more than a subfamily of a more inclusive category. These only constitute the turbomachines that commonly utilize a compressible working medium, which is totally, or predominantly, air. In fact, a complete list of this compressible-flow subfamily should also include such devices as steam turbines, which may utilize either a dry (superheated) or wet (liquid/vapor) steam mixture with high quality (or dryness factor). However, there exists a separate incompressible-flow turbomachinery classification, where the working medium may be water or, for instance, liquid forms of oxygen or hydrogen, as is the case in the Space Shuttle Main Engine turbopumps. This subcategory also includes powerproducing turbomachines, such as water turbines.

Definition

A gas turbine engine is a device that is designed to convert the thermal energy of a fuel into some form of useful power, such as mechanical (or shaft) power or a highspeed thrust of a jet. The engine consists, basically, of a gas generator and a power-conversion section, as shown in Figures 1.1 and 1.2.

As is clear in these figures, the gas generator consists of the compressor, combustor, and turbine sections. In this assembly, the turbine extracts shaft power to at least drive the compressor, which is the case of a turbojet. Typically, in most other applications, the turbine will extract more shaft work by comparison. The excess amount in this case will be transmitted to a ducted fan (turbofan engine) or a propeller (turboprop engine), as seen in Figure 1.2. However, the shaft work may also be utilized in supplying direct shaft work, or producing electricity in the case of a power plant or an auxiliary power unit (Fig. 1.1). The fact, in light of Figures 1.1 through 1.5, is that different types of gas-turbine engines clearly result from adding various inlet and exit components, to the gas generator. An always interesting component in this context is the thrust-augmentation devices known as afterburners in a special class of advanced propulsion systems (Fig. 1.4).

Gas-turbine engines are exclusively used to power airplanes because of their high power-to-weight ratio. They have also been used for electric-power generation in pipeline-compressor drives, as well as to propel trucks and tanks. In fact, it would be unwise to say that all possible turbomachinery applications have already been explored.

Writing a book on the hydrodynamics of high-speed marine vehicles was challenging because I have had to cover all areas of traditional marine hydrodynamics, resistance, propulsion, seakeeping, and maneuvering. However, there is a need to combine all aspects of hydrodynamics in the design of which high-speed vessels are very different from conventional ships, depending on whether they are hull supported, air cushion supported, foil supported, or hybrids.

High-speed vessels are a fascinating topic, and I have been deeply involved in research on high-speed vessels since a national research program under the leadership of Kjell Holden started in Norway in 1989. We also started the International Conference on Fast Sea Transportation (FAST), which has a much broader scope than marine hydrodynamics. I have also benefited from being the chairman of the Committee of High-Speed Marine Vehicles of the International Towing Tank Conference (ITTC) from 1990 to 1993. Further, this book would not have been possible without the work done by the many doctoral students who I have supervised. Their theses are referenced in the book. Parts of the book have been taught to the fourth year, master of science students and doctoral students at the Department of Marine Technology, Norwegian University of Science and Technology (NTNU).

My philosophy in writing the book has been to start from basic fluid dynamics and to link this to practical issues for high-speed vessels. Mathematics is a necessity, but I have tried to avoid this when physical explanations can be given.

Introduction

The power of the installed propulsion machinery is an indirect measure of the maximum resistance of a vessel. However, the actual amount of this power that can be transformed into thrust to counteract the resistance depends on the efficiency of the propulsion device. For an ACV and an SES, power is also needed to lift the vessel. For an SES, this is about 10% to 20% of the power needed for propulsion. Casanova and Latorre (1992) have collected data on installed horsepower in different types of high-speed marine vehicles (HSMV).

Our focus in this chapter is on resistance and propulsion in calm water. When we consider a ship with constant speed on a straight course in calm water conditions, the balance of forces is simple: the ship resistance must be equal to the thrust delivered by the propulsion unit.

It is most common in model tests and in numerical calculations to consider the ship without an integrated propulsion system. The resistance is therefore evaluated without the presence of the propulsion unit. We will follow this approach. This means the ship resistance RT is defined as the force that is needed to tow the ship in calm water with a constant velocity U on a straight track (of course, the towing unit must not affect the flow around the ship).

The fundamental units of measurement in mechanics are mass, length, and time. The basic units in the SI system are kilograms (kg), meters (m), and seconds (s) for mass, length, and time, respectively. Special names are given to derived units in the SI system. The unit force is one newton (N), which is equal to one kilogram-meter per second squared (kgms−2). One pascal (Pa) is the unit of pressure and stress. This is the same as one newton per meter-squared (Nm−2). The unit of work or energy is one joule (J), that is, one newton-meter (Nm). The unit power is one watt (W), equal to one joule per second (Js−1). Prefixes denote decade factors, such as kilo- (k) for 103, mega- (M) for 106, giga- (G) for 109, tera- (T) for 1012, centi- (c) for 10−2, milli- (m) for 10−3, and micro- (μ) for 10−6.

Table A.1 presents the relationship between the SI system and some other commonly used measurement systems. Table A.2 lists values of density and viscosity of water and air, whereas Table A.3 shows how the vapor pressure varies with the temperature. The standard acceleration of gravity (g) is equal to 9.80665 ms−2. The standard atmospheric pressure at sea level is 1.01325 × 105 Nm− 2. The surface tension of the interface between air and water varies between 0.076 Nm− 1 and 0.071 Nm−1 for the temperature range 0° to 30°C.

Introduction

Slamming (water impact) loads are important in the structural design of all high-speed vessels. Further, the occurrence of slamming is an important reason for a shipmaster to reduce the ship's speed. It is also an important effect in calculating operational limits (see section 1.1). The probability of slamming is found by defining a threshold relative impact velocity for slamming to occur (see eq. (8.142)). An often-used criterion is that a typical shipmaster reduces the speed if slams occur more frequently than three out of 100 waves passing the ship. It may be misleading to talk about a threshold velocity. There is no threshold for slamming as a physical process. Further, the conventional way of defining a threshold velocity does not reflect the effect of the structural shape. For instance, for a high-speed vessel with slender lines in the bow, the procedure may say that slamming occurs on the bow part of the hull, although, in reality, it is not a problem. In order to come up with better criteria, it is necessary to study theoretical models for or perform experiments on water impact against wetdecks and hull structures typical for high-speed vessels. This is also necessary in order to develop rational criteria for operational limits due to slamming. The criteria should be related to slamming loads used in the structural design or, ideally, to structural response due to slamming.

Wetdeck slamming (Figure 8.1) is important for multihull vessels.

Introduction

In this chapter, we concentrate on wave resistance and ship-generated waves (wash) of high-speed vessels in calm water conditions. The focus is on semi-displacement vessels and air cushion–supported vehicles. Wave resistance of hydrofoils is discussed in section 6.8.

Wave resistance

Figure 4.1 shows the numerically calculated relative importance of resistance components of a 70 m–long catamaran in deep water. The main particulars are presented in Table 4.1. The ship speed U is 40 knots in calm water. The effect of head sea waves with different significant wave heights H⅓ is also shown. The added resistance due to the incident waves and wind are accounted for by selecting a representative wind velocity and mean wave period for each H⅓. Corresponding data for a 70 m–long surface effect ship (SES) are shown in Figure 4.2, with the main particulars given in Table 4.2. The ship power is kept constant in the calculations, which means increasing speed loss with increasing H⅓. Voluntary speed reduction, for instance, as a result of excessive vertical accelerations, is not accounted for. The speed loss is most pronounced for the SES, as shown in Figure 4.2. The SES speed has dropped from 50 knots in calm water to about 35 knots in H⅓ = 5 m. The speed loss for the catamaran is not shown. However, because viscous frictional resistance is mainly proportional to UH2, the curve for frictional resistance as a function of H⅓ shows that the speed loss is not large.

Introduction

High standards for maneuverability are required for high-speed vessels to operate, particularly in congested areas, where emergency maneuvers may be necessary to avoid collisions. An important aspect is training of personnel who operate the vessels. A maneuvering simulator is then a useful tool. This requires mathematical models that reflect the very different physical features of the various categories of high-speed vessels. The maneuvering characteristics of a vessel are documented in terms of turning circle maneuver, zigzag (Z) maneuver and crash astern test. The IMO (International Maritime Organization) maneuvering criteria from 2002 for ships longer than 100 m are described in Table 10.1.

Figures 10.1 and 10.2 define a turning circle maneuver and a zigzag maneuver, respectively. The course changing ability of the ship is expressed by the turning circle maneuver. The ability to bring the ship to a straight course is determined by the zigzag maneuver. The crash astern test is illustrated in Figure 10.20. Later, in section 10.5, we give examples of a turning circle maneuver, zigzag maneuver, and crash astern test.

The hydrodynamics clearly differ between high-speed maneuvering on one hand and low-speed maneuvering and dynamic positioning on the other hand. Let us assume calm water conditions and divide the hydrodynamic loads on the hull into potential and viscous flow effects, as we did for ship resistance (see Chapter 2). However, an interaction exists in reality between these two effects.

Introduction

Hydrofoil vessels in foilborne conditions generally have good seakeeping characteristics, create small wash, and have small speed loss due to incident waves. This is particularly true for fully submerged foil systems. Foils are normally designed for subcavitating conditions. However, the possibility of cavitation is then an important issue. Our discussion assumes subcavitating foils.

Johnston (1985) pointed out that important aspects when selecting foil and strut configurations of fully submerged hydrofoils are:

• Maintenance of directional and roll stability

• Stable recovery when a foil comes out of the water (broaches)

• Graceful deterioration of performance in severe seas

• Safety

The designer tries to maximize the foil's lift-to-drag ratio and the speed for cavitation inception. Further, the weight of the strut-foil system must be minimized with due consideration of structural strength.

Abramson (1974) discussed relevant structural loads for monohull hydrofoil vessels. In this context, important aspects are slamming, hull-bending moments in foilborne conditions, and bending of the forward foil and strut during recovery from a forward foil broach. Slamming on the side hulls of a foil catamaran is not considered a problem. The reason is large deadrise angles. Because a monohull hydrofoil vessel typically uses a planing hull with relatively small deadrise angles, slamming loads matter. If a foil catamaran is hullborne in bad weather, wetdeck slamming must be considered. The possibility of grounding and hitting of objects like logs against the strut-foil system must also be considered.

Flutter of foils and struts could cause catastrophic failure, but this has never occurred.