Yacht power choice and design
by Roger Lean-Vercoe
The vast majority of todays 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, its 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 yachts 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 yachts 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 todays 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.