Section 12 Decks, Bulkheads, Superstructures and Appendages Introduction, interior Decks and Bulkheads Weather Deck Superstructures Hull Appendages and Sailing Yacht Keels

Introduction The hull of a ship accounts for its largest surfaces, and often the most complex and demanding surfaces in terms of shape requirements. Decks and bulkheads are simple surfaces (planar in most cases), but need complex outlines in order to meet the hull accurately. Superstructures are often complex assemblages of many large and small surface elements, with important aesthetic and functional requirements. Hull appendages — for example, stern tube bossings, bow bulbs, sonar domes, sailing yacht keels, bilge keels, and rudders — must be shaped to perform critical hydrodynamic functions.

Interior Decks and Bulkheads Definition: typically horizontal or vertical planes trimmed by intersection with the hull. The longitudinal subdivision by watertight bulkheads has to meet hydrostatic requirements for damaged flotation and stability. The bulkheads and/or interior decks also form the principal compartmentation of the ship’s interior, so their locations interact with requirements for locating machinery and cargo.

Weather deck Weather deck surface: any deck exposed to the outside and the windward side deck. occasionally planes, horizontal or with some fore-and aft inclination — but are much more commonly given camber (transverse shape) in order to encourage shedding of water to gain structural stiffness to gain interior volume without increase of freeboard.

Weather deck In small crafts, it is common for the deck camber to be specified as a circular arc having a constant crown to breadth ratio from 6 to 8%. The relationship between radius R, crown h, and chord c (i.e. breadth for a deck) for a circular arc is 2Rh = h^2 + c^2/4 This shows that for constant h/c, R is directly proportional to c: R/c = (c/h)/8 + (h/c)/2

Weather deck If the weather deck is required to be developable, this imposes substantial constraints on the design. A general cylinder swept by translation of a camber profile along a longitudinal straight line is the simplest solution; however, this tends to make a very flat deck forward, where it becomes narrow. A shallow cone made from the deck perimeter curves, with its apex inside the superstructure, is often an advantageous construction. Large commercial ships usually have planar deck surfaces, the outboard portions slanted a few degrees, combined with some width of flat deck near the centreline.

Superstructures In merchant and military ships, superstructures consist of flat surfaces for relatively easy geometric constructions from trimmed planes and flat quadrilateral or triangular patches. Where the superstructure meets the deck, some plane intersections with deck surfaces, or projections onto the deck, will be required. An interesting recent trend in military ship design is the “stealth” concept for reducing detect-ability by radar. The basic concept of radar is to scan a region of interest with a focused beam of pulsed high-frequency radio waves (wavelength of a few mm or cm) and listen for reflected pulses (echoes) at the same or nearby wavelengths.

Hull Appendages The most common hull appendages for ships are bow bulbs, stern tube bossings , sonar domes, bilge keels and rudders. Though in each case there is a possibility of integrating the appendage with the hull surface (and admitting there are going to be borderline cases where it is difficult to decide whether to add on or to integrate). It is often far more convenient to leave the main hull surface alone and retrofit it by attaching the appendage as a separate surface.

Hull Appendages For example, Fig. 40(a) shows a B-spline surface for the fore body of a destroyer, using a 5 x 5 net of control points.

Hull Appendages Fig. 40(b), five rows and five columns of additional control points have been inserted in order to provide enough control points in the forefoot area to form an integrated sonar dome; the dome is shown in Fig.40(c).

Hull Appendages However, there are now some 30 superfluous control points in the bottom and stem regions, and it will be very difficult to position them all in such a way as to obtain anything like the fairness of the original simple surface in these areas.

Hull Appendages Figure 41 shows the alternative of treating the sonar dome as the appendage that it is. Outside a well-defined line on the hull (a snake), the hull surface is unaffected by the presence of the dome.

Hull Appendages The dome designer is then free to focus on the shape of the dome, and does not have to worry about side effects on the remainder of the surface.

Sailing Yacht Keels A sailing yacht needs to generate hydrodynamic lift forces to resist the component of sail force that tends to push it sideways. It also needs roll stability to resist the heeling moments arising from sail forces. In monohull yachts, a keel appendage is the most common answer to both these needs. shaped so as to carry this weight as low as possible, while providing an effective lifting shape of sufficient lateral area, adequate streamlining, and low wetted surface.

Sailing Yacht Keels A sailing yacht can be viewed (to a degree) as an airplane flying on its side, with its two wings — keel on one side and sails on the other — having quite different shapes and proportions primarily because of the large difference in density (a factor of about 830) between the two fluids they operate in.