For owners interested in beginning a new design/build project or a refit of an existing yacht, there will likely come a point in the process that requires an important choice to be made regarding power and propulsion, and there are several variables that should be considered in order to achieve desirable results.
There are more choices and combinations to consider today than ever before, and sometimes the choice can be made simpler by ruling out certain options based on general characteristics of different types of propulsion systems.
The process of selecting an appropriate propulsion system is vital to the success of a design. While all types of systems mentioned here can offer advantages in the correct application, choosing the wrong system for a particular vessel and operational profile can result in design requirements not being achieved.
Submerged propellers
Historically, submerged propellers on inclined shafts have been the most popular propulsion option for large and small yachts. A submerged propeller is installed on a shaft that is supported by an underwater strut, and is coupled to a reduction gear connected to an engine, either directly or remotely via an intermediate shaft. Direct-mount gears are more common, but V-drive gears can also be used to help minimise the internal volume required for the propulsion system.
Submerged propellers typically rely on rudders to provide turning force, though steering can also be achieved with asymmetric thrust on vessels with more than one propeller, provided they can be independently controlled. A propeller-strut-rudder system offers high reliability, low complexity, and minimal maintenance combined with a relatively low capital cost.
Over time, modifications to the standard system have evolved, including tunnels in the hull bottom that allow the shaft angle and operational draught to be reduced, and adjustable-pitch propellers that allow for some increased flexibility in the performance/speed/weight relationship. However, it is important to realise that on higher-performance vessels (approaching and above 50 knots) efficiency of adjustable-pitch propellers decreases due to higher appendage drag and propeller cavitation. Further, adjustable-pitch propellers are more likely to cause excessive noise and vibrations at high speed.
Surface drives
A surface drive, like a traditional submerged prop on an inclined shaft, consists of a fixed-pitch propeller at the end of a shaft whose forward end is attached to a reduction gear. However, whereas traditional inclined shafts penetrate the bottom of the hull, surface drives penetrate the transom.
Surface drive systems are installed almost exclusively on high-speed vessels, as they offer a lightweight option that is relatively easy to install and is very efficient at high speeds. High efficiency results from reduced appendage drag (no struts or rudders), a lower thrust angle (closer to horizontal), and surface-piercing propellers (while running at speed, only part of the propeller is underwater). Surface drives provide for low operational draught, but compared to traditional submerged propellers, they exhibit lower efficiency at low and medium speeds, reduced low-speed manoeuvrability, and inferior performance in backing-down operations.
Surface drive systems typically produce what is called a ‘rooster tail’, a wake pattern that can be undesirable, throwing a considerable amount of water into the air behind the vessel as the propeller blades break the surface of the water.
Waterjets
Another propulsion option typically associated with high-speed applications is the waterjet. A waterjet system consists principally of an inlet duct, an impeller, and a nozzle mounted on the vessel’s transom. The impeller spins inside the duct, pulling water from beneath the hull and forcing it through the nozzle to provide thrust. Nozzles can be steerable or non-steerable, and can be fitted with reversing buckets that allow the vessel to back down.
There have been significant advancements in waterjet technology related to impeller efficiency and inlet duct shape. Waterjet systems also offer advantages such as low operational draught, good high- and low-speed manoeuvrability and relatively low noise and vibration, and they can be adapted to large variations in vessel displacement and arrangement allowances.
Like surface drives, though, waterjets lose efficiency at low speeds and they are typically more expensive to purchase and maintain than alternate propulsion systems. Plus the fact that the inlet ducts are located inside the vessel typically increases the interior volume required for the machinery.
Pod propulsion
Pod propulsion systems have evolved from being applied almost exclusively to large commercial vessels, as they have been for a long while, to more recently being installed on smaller recreational vessels.
Pods can offer a long list of operational benefits including low-speed joystick-operated manoeuvrability, dynamic positioning, relatively simple installation and compact machinery space requirements. Pod drives typically offer high efficiency, resulting in good performance and better fuel burn at moderate to high speeds as compared to traditional submerged propellers, but typically, efficiency gains are realised only in planing or semi-planing operation modes.
Currently, pod propulsion is readily available for yachts up to approximately 30m, though some larger and lower-speed yachts (over 60m) are using pods similar to those used on commercial ships.