Feature 4 | MEGA YACHTS
Flow visualisation tests.
beamy, high displacement motor yacht? Tese factors have a critical influence on the direction that we follow, as displacement in relation to length is the most influential parameter on resistance. With a commercial vessel the naval
architect will be in control of the design intent behind the concept. Tis is rarely the case with large yachts where the stylist (aka the yacht designer) will have defined the design intent and many of the defining features of the yacht. Tese will have already been laid down and “sold” to the owner. It then becomes the job of the naval architect to work with the stylist to try and maintain as much of that design intent whilst quantifying the compromises that may have to be made in order to improve the hydrodynamic efficiency of the overall design. Additionally at this stage the naval
architect will undertake a preliminary stability assessment in order to confirm that the beam, oſten already decided by the stylist, will be acceptable. In conjunction with this, preliminary powering estimates will also be made to verify the space reserved for the main machinery space. Following the initial assessment the
designer sets the target values for the optimisation of key form coefficients and parameters including block, prismatic and maximum sectional area coefficients, longitudinal centre of buoyancy and floatation, wetted surface area and immersed transom area. If a bulbous bow is to be used the basic parameters are set down; bulb type, centreline profile shape, sectional area and immersion. Additionally the effects of appendage and propulsion system integration are considered including propeller diameter, tip clearance and rudder configuration amongst others. It is normally at this point that many of the aforementioned features of the stylist’s
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design intent start to present barriers to effective optimisation. Two examples spring to mind. Te first is a 100m motor yacht where the arrangement of the design had been centred around a central vertical ‘feature’ staircase running from the tank deck right to the sky lounge of the yacht. Te longitudinal location within the yacht was fixed by the stylist and considered immovable. The end result of this was that the engine room had to be pushed relatively far aſt in the design, resulting in a less than optimum longitudinal centre of buoyancy (LCB), form coefficients and steep buttock angles, all contributing to an increase in resistance. Te second example was an 85m motor yacht where the design featured a large swimming pool in the aſt lazarette (commonly called the ‘beach club’ on large yachts). In order to accommodate
“With a well
executed project, part of the job of the naval architect is to try and influence the owner’s
decisions to bring the target speeds and optimisation point to sensible parameters.”
this, and integrate the shaſtline, the hull had to be deepened aſt resulting in increased transom area and non optimal LCB: Te consequence of which was a significant increase in vessel resistance and reduced propulsion coefficients. The next step in the process involves
optimisation of the sectional area curve. Tis is an exercise which is oſten forgotten, but achieving the correct area profile is critical in achieving a set of hull lines which combine the desired characteristics with efficient hydrodynamic performance. It is generally
The Naval Architect July/August 2010
started with the sectional area curve for a basis yacht such that the designer has a known and quantified baseline. Once the sectional area curve has been satisfactorily developed the next step is to modify the parent lines to obtain the optimum midship section shape and area distribution. At this stage, BMT takes into consideration
for further practical constraints which limit our ability to achieve the desired sectional area shape. Many of these are common to both commercial vessels and yachts alike, such as integration of the selected propulsion system. However some are in general limited to yachts only, for example roll fins on yachts are designed for operation at zero speed. In the absence of hydrodynamic liſt they generate the required righting moments by being over sized (typically 30-40% greater area than conventional roll fins) and operating in a paddle fashion. Tese large fins are oſten constrained in their longitudinal position by internal arrangements and frequently present a serious challenge to integrate within the beam-keel envelope whilst maintaining an optimised sectional area curve and undistorted waterlines. Bow thrusters are another common
challenge on yachts. Typically, in an attempt to maximise internal volume for accommodation, they are pushed very far forward in the general arrangement. Tis can lead to fuller forebody waterlines and sections with the resulting half angle of entrance being increased significantly to achieve the minimum required tunnel length. Having developed a set of lines
optimised to the principal parameters, as far as possible, within any accepted compromises, the next step in the process involves more refined consideration of section shapes, waterline shapes and buttocks. Generally by this stage there will
A properly optimised bow can offer significant savings.
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