Chapter 3 is a general, rather short and partly descriptive introduction to general wave theory, without application of any differential equation. The emphasis is on mechanical waves, e.g., acoustic waves.

Referring to a simple illustration, a verbal explanation is given by the essential, but perhaps paradoxical, statement that to absorb wave energy from a wave by means of an oscillating system, it is required that the system radiates a wave which interferes destructively with the incident wave. Then various mathematical relations are derived concerning the conditions for an oscillating body to remove energy from an incident wave. The mathematical conditions for wave-power absorption may be illustrated as a paraboloid-shaped 'island' on an infinite complex-plane 'ocean' surface. The top of this 'island' corresponds to maximum absorbed power. An additional matter is the optimum control of a wave-energy converter (WEC) body. Thus far, the WEC body's shape and oscillation mode have been taken into account, but not its physical size. The latter is an important parameter related to the cost of the WEC, when the Budal upper bound is explained and discussed. Another important phenomenon, related to the Keulegan–Carpenter number, is discussed, in relation to an example of a WEC body. In a final section of the chapter, a WEC body, oscillating in several modes of motion, is discussed.

Chapter 8 concerns a group of WEC units that may be realised in a more distant future, namely groups or arrays of individual WEC units and two-dimensional WEC units, which needs to be rather big structures. Firstly, a group of WEC bodies is analysed. Next a group consisting of WEC bodies as well as OWCs is analysed. Then the previous real radiation resistance needs to be replaced by a complex radiation damping matrix which is complex, but Hermitian, which means that its eigenvalues are real.

Chapter 4 introduces basic differential equations and boundary conditions for gravity waves propagating along a water surface. Assuming low wave amplitudes, equations are linearised. Then a quantitative discussion is given for harmonical (sinusoidal) waves propagating either on deep water, or otherwise on water of constant depth. Phase and group velocities are introduced, and then formulas are derived for the potential energy and the kinetic energy associated with a water wave. A closely related result is an important formula for the wave-power level, which equals the wave’s group velocity multiplied by the wave’s stored – kinetic + potential – energy per unit of sea surface. An additional subject is the wave’s momentum density. A section concerns real sea waves. Further, circular waves are mathematically described. Two sections of the chapter concern mathematical tools to be applied in Chapters 5–8 of the book. A final section considers water waves analysed in the time domain.

The first part of Chapter 7 deals with oscillating water columns (OWCs). The concepts of radiation conductance and susceptance are introduced. The former is related to the radiated power, whereas the latter represents the reactive power. Expressions for the power absorbed by the OWC are derived, which are analogous to those of the oscillating body WEC. The potential energy of the OWC is also discussed. The last part of Chapter 7 deals with wave energy converters that move in modes other than the six conventional rigid-body modes. The theory of generalised modes are described, and some examples are given to illustrate the utility of the theory.