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.

Introduction

Monohulls and catamarans, often equipped with foils, trim tabs, and/or interceptors that control the trim angle and minimize wave-induced motions, are nowadays the most established concepts for high-speed vessels. Interceptors are relatively new concepts and are illustrated in Figure 7.5. The vessels have transom sterns, that is, there is a flat part of the aft ship below the mean free surface that is perpendicular to the centerplane. Catamaran designs include the wave-piercing and semi-SWATH (small waterplane area twin hull)-style hulls. The length of high-speed catamarans used for passenger transportation in coastal water is typically 30 to 40 m. Both monohulls and catamarans longer than 100 m have been built. Trimarans and pentamarans are new types of multihull vessels that are considered. They consist of a long center hull with smaller outrigger hulls. The outrigger hulls are important for static heeling stability. The larger vessels are typically ro-pax ferries, which means they carry passengers and allow roll-on/roll-off payloads, most often cars.

Calm water resistance of semi-displacement vessels is dealt with in Chapters 2 and 4. This chapter concentrates on linear wave-induced motions and loads; however, added resistance in waves is also handled. Common statistical procedures for calculating short- and long-term responses based on linear results in regular waves are also shown. One important load aspect is slamming, which is relevant for all high-speed vessels. This is dealt with in detail in Chapter 8. Maneuvering is considered in Chapter 10.

Introduction

Figure 5.1 shows an example of a surface effect ship (SES). Figure 1.8 gives a fish-eye view. The vessel is supported by an air cushion that is bounded by flexible seal systems at the bow and stern and by two side hulls. The aft seal is usually a flexible bag consisting of a loop of flexible material open against the side hulls, with one or two internal webs restraining the aft face of the loop into a two- or three-loop configuration. In equilibrium position, there is a very small gap between the bottom of the bag and the water surface. An example is a gap height of 3 cm. The bow seal (skirt) is usually a finger seal consisting of a row of vertical loops of flexible material. Details are shown in Figures 5.2 and 5.3. The seal material is rubber.

There is an air fan system (Figure 5.4) that provides the excess pressure in the air cushion and lifts the SES up, thereby reducing the water resistance. The excess pressure in the air cushion causes a water level inside the cushion lower than the level outside. Typically, the air cushion carries 80% of the weight of the vessel. The buoyancy of the side hulls carries the rest of the weight at zero speed. When the vessel speed increases, the vertical side hull forces due to the water are caused by both hydrostatic (buoyancy) and hydrodynamic pressures.

Introduction

Before we can describe wave resistance in detail, we need to introduce wave theory. This theory is also needed in the description of wave-induced motions and loads on a high-speed vessel. We first present linear wave theory in regular harmonic waves in deep and finite water depths. This includes analysis of wave refraction. An irregular sea state can be represented as a sum of regular waves of different frequencies and wave propagation directions. Recommended wave spectra that describe the frequency content in irregular waves are then given.

Linear wave theory assumes that the wave slope is asymptotically small. Not all waves occurring in reality can be described by linear wave theory; an extreme example is breaking waves. Figure 3.1 illustrates plunging, breaking waves generated in a wave flume. We see breaking waves on a beach, but they can also occur in the open sea in deep water. A strong current in the opposite direction of the wave propagation steepens the waves. This is a phenomenon known in connection with the Agulhas current off the east coast of Africa. A typical feature of a nonlinear wave is that the vertical distance between the wave crest and the mean water level is larger than the distance between the mean water level and the wave trough.

Scatter diagrams of significant wave heights and mean wave periods are needed in operational and design studies. These diagrams describe the probability of occurrence of different sea states for a given operational area.

Introduction

Planing vessels are used as patrol boats, sportfishing vessels, service craft, ambulance craft, recreational craft, and for sport competitions. The Italian vessel Distriero, designed by Donald L. Blount and Associates, won in 1992 the Blue Riband Award for the fastest transatlantic passage without refueling. The average speed was 53.1 knots. The vessel is a 67 m–long planing monohull equipped with gas turbines and three waterjets with a combined horsepower of 60,000. Use of hydrodynamic test facilities was an important part of the design. However, the amount of research on planing vessels is relatively small, considering the large amount of different recreational craft that exist. Our focus is on monohull vessels, but catamaran types also exist. Most of the planing vessels have lengths smaller than 30 m. Recreational craft are typically smaller than that.

A vessel is planing when the length Froude number Fn > 1.2 (Savitsky 1992). However, Fn = 1.0 is also used as a lower limit for planing; that is, we cannot set a clear line of demarcation between planing and nonplaning conditions just by referring to the Froude number. During planing, the weight of the vessel is mainly supported by hydrodynamic pressure loads, with buoyancy having less importance. The hydrodynamic pressure both lifts the vessel and affects the trim angle.

Figure 9.1 shows a typical high-speed planing hull.

In this chapter, we summarize and expand somewhat on those results from quantum mechanics and spectroscopy most germane to our study of statistical thermodynamics. We then prepare for revisiting intensive properties in Chapter 9 by considering the nature of thermodynamic calculations before the advent of quantum mechanics. From a pedagogical point of view, the previous three chapters have focused on the properties of a single atom or molecule. For our purposes, the most important such properties are the allowed energy levels and degeneracies corresponding to the translational, rotational, vibrational, and electronic energy modes of an independent particle. Exploiting this knowledge, we proceed to a macroscopic assembly of atoms or molecules, with a focus on calculations of thermodynamic properties for any pure ideal gas. Assemblies composed of different particle types subsequently permit the evaluation of properties for both nonreacting and reacting gaseous mixtures, including equilibrium constants for various chemical reactions. Finally, re-applying spectroscopy to such mixtures, we examine the utility of statistical thermodynamics for experimentally determining temperature or concentrations in realistic gaseous mixtures at high temperatures and pressures.

Energy and Degeneracy

Our foray into quantum mechanics and spectroscopy has led to relations giving the energy and degeneracy for all four energy modes – translation, rotation, vibration, and electronic. If we insist on mode independence, any consideration of diatomic molecules also mandates the simplex model, which presumes a combined rigid rotor and harmonic oscillator.