Hydrodynamics of large motor yachts - Past and Future Developments

For luxury motor yachts, aesthetics, comfort and operability are of major importance. Designers and shipyards have to meet the challenge of realising the owner’s dream but at the same time they have to work within technical and financial constraints. Aesthetics of the superstructure and the profile of the yacht must go hand-in-hand with technical requirements related to the expected operation of the ship. A yacht owner must be confident that his vessel can sail freely and safely in any part of the world and that it will also be comfortable and be able to realise the required range and speed. The Maritime Research Institute Netherlands (MARIN) has a large track record with respect to the hydrodynamic research on motor yachts. This is partly due to the fact that a large amount of yards dedicated to yacht business is present in the Netherlands, sometimes called the Yacht Valley. Every year a number of motor yacht designs are investigated with respect to their powering, seakeeping and manoeuvring performances. Large luxury motor yachts have dedicated optimisation issues in calm water. Good propulsion characteristics mean low noise, low level of vibrations and attaining the required speed. Range also becomes an issue for these 40 to 100+ m vessels which need to sail long journeys (typically crossAtlantic) to arrive at the desired location of the season. Some owners even require a certain range in combination with a required sustained speed, in order to arrive within a specified time at a specific destination without the need to refuel or restock. Regarding the manoeuvrability of the yacht, several aspects may be distinguished. When the yacht enters congested waterways, the controllability of the yacht determines the safety of the passage. When entering the harbour, the low speed manoeuvring ability of the yacht is of importance. With the current trends of ever increasing sizes of motor yachts and increasing service speeds of the yachts, better performance and improved safety is required for luxury motor yachts. These trends also lead to new technologies being implemented in the design, such as podded propulsion or hybrid propulsion systems. These systems all have their characteristic influence on manoeuvring properties.
Seakeeping aspects are also of major importance as they characterise the motions and related comfort level in transit conditions and at anchor. Comfort in our perspective is here related to motions and seasickness level. Comfort at anchor has become one of the most important features for motor yachts over the last 10 years. Many techniques used to decrease the roll motions at anchor have been developed and will be discussed and compared. In transit conditions two aspects will also be developed, a comfort issue in moderate waves and a safety issue in more extreme conditions. The increasing displacement required for a given yacht length has increased their block coefficient and decreased their intact stability. Such combination may, in some extreme case, yield to relatively high rolling in sternquartering seas and poor course keeping ability, even in moderate conditions. The influence of the steering system on the roll and course stability will be discussed in detail. The dramatic increase in size of motor yachts over the last 20 years has also increased drastically their range of operations and has increased at the same time the probability of being caught in severe storm. In such wave conditions risk of green water, bow flare slamming, course keeping ability and risk of broaching are becoming relatively important. To meet all these requirements, hull form design, weight distribution, choice of appendages and propulsion system must be made carefully. Hydrodynamics, used in the form of calculations and model tests, have proved to be a useful support for the shipyards and designers in the optimisation of this category of vessel. The following presents the numerical and experimental techniques and knowledge gained at MARIN on this type of vessels over the last 20 years.