Overview

Our constructions

Yachts 

For many years, the design, construction and sailing of yachts has been a fascinating art about which whole books are regularly published. This is because the science is too complex for precise solution – and indeed, few yachtsmen would wish it otherwise. Some tenable theories have, in fact, been evolved to help in explaining certain of the performance characteristics of sailing boats.A yacht, of course, obeys the fundamental theory described generally in this book for all surface ships. In addition, a yacht is subject to air forces acting on the sails and to water forces due to its peculiar underwater shape – forces which are negligible for ordinary surface ships. Sailing before the wind, a yacht is propelled by: (a) the vector change of momentum of the wind, defl ected by the sails; (b) lift generated by the sails acting as aerofoils; because an aerofoil requires an angle of incidence, yachtsmen prefer sailing with wind on the quarter rather than dead astern, particularly when fl ying a spinnaker, to give them more thrust. When sailing into the wind, the yacht is propelled only by a component of the lift due to the sails, acting as aerofoils. Lift and associated drag depend upon the set of the sails, their sizes, shapes, stretch and material, the angle of heel of the boat, the relative wind velocity and the presentation of the sails to the wind and to each other. Because the yacht does not quite point in the direction in which it is going and is also heeled, the hull too acts as an aerofoil, experiencing hydrodynamic lift and drag which exactly balance out the air forces when the boat is in steady motion. The transverse couple produced by air and water forces is reacted to by the hydrostatic righting moment to keep the boat in stable equilibrium. Large angle stability and dynamical stability are clearly of great importance. Longitudinally, the relative position of hydrodynamic lift, the centre of lateral resistance and the lateral component of air lift determine whether the rudder carries weather helm,  or lee helm. Lee helm is dangerous because, if the tiller is dropped or the rudder goes free by accident, the boat will not come up into the wind but veer away and increase heel. Ideally, to minimize rudder drag, a yacht should carry slight weather helm in all attitudes and it is towards this ‘ balance ’ that yacht designers aim. The resistance of a yacht calculated or measured in the manner conventional for surface ships, is not a very helpful guide to the yacht designer. Minimum resistance is required at small angles of yaw and small angles of heel, and these are different from the conventional fi gures. Resistance in waves is also of considerable importance and varies, of course, with the response of the boat to a particular sea – because of augment of resistance due to pitching, a yacht may well sail faster in Force 4 conditions than it does in Force 5, or better in the Portland Reaches than off Rhode Island. A yacht designer must therefore achieve minimum resistance yawed, heeled and in waves, good longitudinal balance in all conditions and satisfactory stability. The rig must not upset the longitudinal balance and must give maximum performance for sail area permitted, in all conditions and direction of wind, particularly close to the wind. While some theory helps, the process overall remains much an art. 

Fugure ( A )

Tugs

Tugs perform a variety of tasks and their design varies accordingly. They move dumb barges, help large ships manoeuvre in confined waters, tow vessels on ocean voyages and are used in salvage and fi refi ghting operations. Tugs can be categorized broadly as inland, coastal or ocean going. Put simply, a tug is a means of applying an external force to any vessel it is assisting. That force may be applied in the direct or the indirect mode. In the former the major component of the pull is provided by the tug’s propulsion system. In the latter most of the pull is provided by the lift generated by the fl ow of water around the tug’s hull, the tug’s own thrusters being mainly employed in maintaining its attitude in the water. The main features of a tug (A Figure  ) are an effi cient design for free running and a high thrust at zero speed (the bollard pull ), an ability to get close alongside other vessels, good manoeuvrability and stability. Another way of classifying tugs is by the type and position of the propulsor units: (1) Conventional tugs have a normal hull, propulsion being by shafts and propellers, which may be open or nozzled, and of fi xed or controllable pitch, or by steerable nozzles or vertical axis propellers. They usually tow from the stern and push with the bow. (2) Stern drive tugs have the stern cut away to accommodate twin azimuthing propellers. These propellers, of fi xed or controllable pitch, are in nozzles and can be turned independently through 360° for good manoeuvrability. Because the drive is through two right angle drive gears these vessels are sometimes called Z-drive tugs. They usually have their main winch forward and tow over the bow or push with the bow. (3) Tractor tugs are of unconventional hull form, with propulsors sited under the hull about one-third of the length from the bow, and a stabilizing skeg aft. Propulsion is by azimuthing units or vertical axis propellers. They usually tow over the stern or push with the stern. 

Icebreakers and ice strengthened ships 

The main function of an icebreaker is to clear a passage through ice at sea, in rivers or in ports so that other ships can use the areas which would otherwise be denied to them. Icebreakers are vital to the economy of nations such as Russia with ports that are ice bound for long periods of the year and which wish to develop the natural resources within the Arctic. Icebreakers need: ● To be specially strengthened with steels which remain tough at low temperature. ● Extra structure in the bow and along the waterline. ● High power propulsion and manoeuvring devices which are not susceptible to ice damage. The shape of the stern is important here. ● A hull form that enables them to ride up over the ice. This is one way of forcing a way through ice; the ship rides over the ice edge and uses its weight to break the ice. The ship may be ‘ rocked ’ by transferring ballast water longitudinally. The hull is well rounded and may roll heavily as protruding stabilizers are unacceptable. ● Good hull sub-division. ● Special hull paints. Icebreakers are expensive to acquire and operate. Other ships which need to operate in the vicinity of ice are strengthened to a degree depending upon the perceived risk. Usually they can cope with continuous 1 year old ice of 50–100 cm thickness. Typically they are provided with a double hull, thicker plating forward and in the vicinity of the waterline, with extra framing. They have a fl at hull shape and a rounded bow form. Rudders and propellers are protected from ice contact by the hull shape. Inlets for engine cooling water must not be allowed to become blocked. 

Fishing vessels