Feature 4 | MEGA YACHTS Optimisation of Superyacht Hulls
James Roy, yacht design director at BMT Nigel Gee and naval architect Rob Sime, draw on their combined superyacht design and commercial vessel experience to explore the practical limitations of optimising superyacht hulls.
O
perating costs and fuel costs in particular are the biggest driver in the commercial vessel market.
Te desire to reduce these costs has always provided a focus for the optimisation of the vessel’s hull to develop efficient ships with minimal powering requirements; even a significant investment in hull form optimisation at the design stage can quickly achieve a payback. Large yachts, however, spend far less
time at sea than commercial vessels and consequently, for an average yacht, fuel costs represent a far smaller proportion of annual operating costs. Additionally for those who own and operate these yachts the adage that: “If you have to worry about filling her up then you probably can’t afford it” holds true and cost is in general not an issue. However, with rapidly increasing fuel prices, ever more stringent environmental legislation and the desire to be seen to be green, there is an increasing focus in the superyacht industry to improve efficiency. Large yachts tend to have relatively high
hotel loads and, given the relatively low hours at sea, much of today’s focus within the industry is on reducing generator fuel burn through energy efficient technologies. Hull efficiency still remains a very significant part of the overall through life efficiency, and whilst optimisation of a yacht hull is oſten undertaken, it is typically compromised by other factors relating to the functional aspects of the design. It is these factors that present different challenges to the optimisation of a commercial vessel. BMT Nigel Gee undertakes optimisation
across both the yacht and commercial vessel industries. Critically the work is not limited to the pure naval architecture of the optimisation process but also to complete vessel design and engineering (from concept through to delivery) allowing valuable insight into how the processes differ between yachts and commercial vessels. This makes use of optimisation techniques predominantly based on
The Naval Architect July/August 2010
An optimised 85m yacht about to undergo physical testing.
experience, married with an appropriate level of computational analysis. With modern computer techniques it is very easy to launch into the generation of 3D lines too early in the process without sufficient focus on a formal optimisation methodology. Additionally the use of computational fluid dynamics (CFD), whilst of significant potential value, has its limitations. Where utilised, such techniques must in our experience be tempered with optimisation variable limitations specific to large yachts gained from practical experience, some of which will be cited as specific examples in this article. The hull design process begins with
three critical parameters; displacement, length and speed. Te latter two variables define the Froude number which gives an indication of the basic hull type, where to set our goals and how challenging they will be to achieve. With a commercial vessel there will be a well defined operating profile with target speeds oſten derived from analysis of
required transit times or freight rates against optimum voyage speed analysis. Tis is not so for yachts where the cruising speed and maximum speed are oſten decided upon by the captain or the owner. Additionally there is no defined, or even typical, operating profile to apply so deciding the optimisation point in an analytical manner can be very difficult. It is also oſten the case that the maximum speed desired may be subject to one-upmanship resulting in a large difference between the cruising speed and the maximum speed, further complicating the optimisation process. 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. Te third variable in our initial assessment
is the displacement in relation to length and the next step is to undertake an assessment of the yachts overall design concept. Is the vessel a slender, low volume design or a
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