in spite of a number of well publicised problems they have gained acceptance in that market. Podded propulsion has been slower
to gain acceptance in the yacht market, but this is changing as the benefits are becoming more widely appreciated by designers and owners. The main advantage is that podded
propulsors provide fully vectorable thrust and do not require a conventional shaſt line, shaſt brackets, rudders, steering gear and stern side thrusters with their associated drive and control gear. A possible disadvantage may be that
the space required for the unit’s steering module may compromise available space in a beach club area at the stern of a yacht. Podded propulsors are expensive but,
when set against the saving from not fitting shaſts and rudder etc, the difference is not so significant. Although the reliability of pods has
improved, they still have the disadvantage that, if problems do occur, the yacht has to be drydocked to carry out repairs to the pods.
Stabilisation Te design of stabilisers installed on yachts differs from those fitted to commercial ships in that they are generally:
• not retractable • of larger area
• offset on the stabiliser shaſt These stabilisers are designed by
manufacturers such as Quantum, Naiad, Ship Dynamics, etc. to operate with a range of GM values for the yacht at zero speed. Te disadvantage is that they have a large
area and as such have a significant effect on speed. A recent innovation that has been fitted
to a number of BCTQ newbuilds is the ‘Magnus Effect Stabilizer’ from Quantum. These retractable rotating cylinders generate liſt by rotating the cylinder in the flow. In order to generate flow whilst the yacht is at anchor, the cylinder is oscillated fore and aſt.
Hull form refinement Over the last few years there has been a change from the conventional yacht form incorporating an extreme raked and flared
The Naval Architect July/August 2009 Motor Yacht Kogo - diesel electric with podded propulsion.
bow through the vertical bow to the reverse rake or ram bow type of hullform. Naval architects will argue the merits of
the different types in terms of seakeeping, ship motions and available waterline length in relation to overall length, although there is probably an element of fashion involved as well. As yacht speeds have increased efforts
have been made to reduce resistance and hence installed power. Te naval architect will optimise the Cp of the hull for the design speed and probably incorporate a bulbous bow. Te optimum arrangement of bulbous bow will require it to protrude above the waterline at rest. Tis is normally not acceptable from an aesthetic point of view and therefore the bulb will be arranged completely below the waterline with some compromise in performance. At higher speeds the hull may well trim by the stern giving increased drag. In this instance a stern wedge can be considered to reduce the trim and consequential drag. It should also be remembered that
the use of a podded propulsion system allows the elimination of the normal hull appendages including shaſt lines, bossings, shaſt brackets and rudders etc to give a significant overall reduction in resistance.
Environmental trends Owners are now looking for green technologies on their new yachts and
exhaust emissions have long been an area of concern. The modern diesels with common rail technology have reduced emissions and the focus is now on exhaust treatment. For smaller engines, catalytic converters have proved very effective. However, these become a problem for the larger diesels. The size of the catalytic converters for diesels over 3000kW becomes impractical and manufacturers are investigating alternatives to address the situation. Diesel electric propulsion would also
help emissions as the diesels are generally smaller than propulsion engines, making exhaust treatment possible. Tey can also be operated at a more efficient point on the power curve. Sewage treatment plants have
now been developed to meet stricter requirements, particularly in the USA, and this has led to the use of filters in the discharge from the plants. Although maintenance levels have been increased, the pollutants in the discharge are now extremely low and can easily comply with the latest regulations. In short, it is clear that the large
custom yacht market continue to provide interesting and varied challenges to all aspects of naval architecture, but specifically, stability, powering, structural design, and regulatory compliance. NA
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