Not only does an owner's choice of power affect speed, build cost and fuel burn, but it also has an impact on accommodations. How does an owner's choice of either diesel or diesel-electric affect guest area space? What are the advantages and disadvantages for interior design purposes of each? How do installations affect on board comfort?
The case for more space
Yacht owners of today demand more from their vessels than their predecessors did three decades ago, and one of their main demands is for more space. Specialist areas such as beach clubs, gymnasiums, dive rooms, health spas, cinemas practically unheard of in the past are now regularly demanded by owners of even the smallest superyacht. The problem is fitting them in.
A prospective owner's choice of conventional diesel propulsion or a variant, such as diesel-electric, can have a direct bearing on the space available for the amenities he requires.
One might think that the solution to accommodating more amenities is simple: just build a yacht with slightly more volume by adding a deck or increasing the beam. But things are not that straightforward.
In the same time frame, international maritime laws have clamped their icy jaws around the internal volume of a yacht, and have selected its gross tonnage (GT) as the yardstick for the marker points when regulations governing crewing and safety requirements are imposed.
All yachts above 23m LOA are subject to such regulations, whose impact increases at regular intervals: 500GT, 1,200GT, and 3,000GT. Most prospective owners aim to stay just inside their chosen size limit.
Naval architects, designers and builders are under pressure to reduce volume lost in voids or allocated to areas that don't excite an owner
The catch is that gross tonnage is not a measure of weight, but of enclosed volume, so an owner inclined to add a dive room or some other volume-hungry facility has to be wary of breaching the next upward limit, which will certainly take him into areas of greater cost and regulatory complexity. So a yacht's internal volume has become more valuable in recent years. As a result, naval architects, designers and builders are under pressure to reduce volume lost in voids or allocated to areas that don't excite an owner.
Crew areas used to be under pressure in this way, but over the years the realisation that a happy crew usually means a well-run yacht with good service a distinct owner benefit has removed the pressure from crew spaces, as have the upcoming maritime labour regulations which, among many other features, will specify the minimum size of crew cabins.
Inevitably, the next target for slimming down becomes the machinery spaces, which, although vital to the running of the yacht, can often yield a degree of their volume without adversely affecting reliability. In pursuit of allocating even more space to guest areas, some builders of large yachts have removed engine control rooms while, in smaller yachts, the tender garage often intrudes on the high level space above the engines. But perhaps even more volume can be turned over to the owner's use by choosing an entirely different propulsion system.
The vast majority of today's yachts are propelled by the same system: diesel engines powering propellers located beneath the stern of the yacht. The two are joined by rigid propeller shafts which determines the location of the engine room, as they must be connected directly to the engines through a gearbox.
There are very good reasons why this system is so popular. Primarily, it's cost effective, practical and simple. Originally, engine rooms powering this configuration were almost universally positioned low down at the centre of the vessel, where their weight would increase stability, and more important, reduce pitching (the vertical motion of the bow and stern) and hence enhance the yacht's seagoing comfort.
While such ideal attributes might put a smile of satisfaction on the face of the naval architect, they made designers of interiors less happy. A centrally positioned engine room meant that lower-deck guest accommodation would usually have to be positioned both fore and aft of the machinery, so all cabins would suffer equally from engine noise and vibration, while two stairways, eating into available volume, were required between the main deck and lower decks, the aftermost often being difficult to position.
The usual configuration of two cabins forward and three aft also made cabin servicing more difficult, and while giving the two groups of guests a degree of individual privacy, it also split them into two distinct parties, with the 'preferred' cabins being in the forward area where propeller noise is not an issue.
There is another propulsion solution, found in commercial passenger ships, that might serve to further increase guest volume
Designers saw the possibility of improving this layout by positioning the engine room farther aft, between a stern lazarette and the centrally positioned guest cabins. This new machinery position solved many issues. It increased available volume; it moved some cabins away from the main source of noise; it opened the possibility of a direct lower-deck route between the central block of guest cabins and the yacht's laundry, which made cabin servicing easier; and it also ensured that guests felt equally treated. Of course, it made the yacht more susceptible to pitching and gave the naval architects an added problem in that the new engine room position had displaced fuel and water tanks to a position further forward. As these commodities were consumed, the trim of the yacht had to be adjusted by moving remaining stocks forward or using ballast tanks. It was, nevertheless, such a significant improvement for the guest areas, as well as an important space-saving measure, that almost all of today's yachts are laid out in this space- and service-efficient manner.
But there is another propulsion solution, found in commercial passenger ships, that might serve to further increase guest volume at the expense of machinery space, without falling afoul of the reduction of efficiency and difficulty of servicing that results from simply making the engine room smaller: diesel-electric propulsion.
Diesel-electric propulsion is the mainstay of cruise ships, but is only employed in a very small number of larger yachts.
Modern cruise ships are propelled by two or more Azipods electrically-powered pods fixed externally to the hull that provide both propulsion and steering. Set on a 360° swivelling mount, the units incorporate fixed-pitch propellers directly driven by an electric motor.
Large cruise ships might have up to six units, distributed along each side of their hull. Cruise ships favour this mode of propulsion for several reasons, the most important being that because they have a huge demand for 'hotel' load electricity to run needs such as lighting, cooking, air-conditioning, there is a need for a very large electrical supply. Many of these demands are at their lowest when the ship is underway (typically at night) so it makes sense to utilise a single power source that will satisfy both propulsion and house needs at the same time.
Diesel-electric power has other useful economies, too: the propellers are generally forward facing, so the water that flows over the props is undisturbed by shafts or shaft brackets, and hence, they provide five or six per cent more efficiency. They also produce less noise and vibration than standard shaft-driven propellers and a stern thruster is not needed. And dynamic positioning systems (DPS) can easily be built into the system.
The main volumetric savings of diesel-electric propulsion come from the fact that the azimuthing thrusters are driven by multiple generator sets (either diesel or, more exotically, gas turbines) that are physically smaller than the propulsion system of a conventional yacht, whose larger propulsion-dedicated diesels (that operate alongside the gensets) often necessitate a two-storey engine room located in a predetermined, near-central portion of the vessel that is usually of high value to the owner.
The major disadvantage of diesel-electric propulsion, and perhaps the reason why it has not found general acceptance, is its cost
A diesel-electric yacht can, on the other hand, have a smaller, single-storey engine room, or perhaps even multiple engine rooms, to feed its Azipods, and these can be located in parts of the yacht that have less interest for guest facilities. Suddenly, such interesting prospects as direct and level access from guest cabins to a beach club in the stern can become reality without the second story of a conventional engine room blocking the path. There is a modicum of further space saving in that the gensets drive both the household and propulsion loads, thus delivering an efficiency that reduces the total power requirement, and hence the physical volume of the engines.
The major disadvantage of diesel-electric propulsion, and perhaps the reason why it has not found general acceptance, is its cost, which can be up to double that of a conventional power train. Seen just as a volume-saving device, diesel-electric propulsion is hard to justify, but the additional opportunities it provides for increased propulsion efficiency, lower levels of noise and vibration, an easy interface with DPS, and the possibility that it provides for a more guest-friendly layout, make diesel-electric an interesting proposition.
Will it catch on more generally? This will depend on the availability of lower-powered Azipods than the ones currently available, which are suited only to larger yachts, combined with a general lowering in cost of the units. We can only wait and see.