In contrast to boats, where outboard engines are common, motor yachts are equipped exclusively with stationary propulsion systems. «With the development of technology» even large enough boats (length 10-12 m) and houseboats could be equipped with outboards. But they can not be called motorboats yet, they are more like caterpillar boats, transitional link in the chain of evolution of powerboat to yacht.
We will talk about outboard engines in detail in the article devoted to powerboats. Here we will talk about different types of stationary (placed inside the hull) power plants. They differ in two main parameters: type of fuel and construction of drive unit which transmits energy from the motor to the propeller.
Gasoline, diesel, electricity
Most boats are fueled by gasoline, or are propelled by an electric motor powered by a battery. Diesel engines on the smallest boats are rather an exception.
When the length of the hull crosses the limit of 10.6 meters (35 feet), the engine options in terms of fuel usage increase dramatically.
Moreover, it is in the 10.6-13.7-meter (35-45-foot) segment that boat owners get the richest choice of engines.
You want a diesel shaft. You want a gasoline-powered turntable.
Then the variety decreases again. Diesel engines start to dominate on yachts from 14 metres onwards. As the hull length increases so do the engine power requirements, and petrol engines show themselves to be less efficient.
Pros and cons of gasoline boat motors
The main advantage of petrol internal combustion engine is compactness and low weight, compared to diesel engines of the same power. In addition, petrol engines are quieter and produce less vibration.
However, gasoline is flammable, explosive and more expensive than diesel. In addition, a gasoline engine consumes an average of 40% more fuel.
Benefits of diesel engines for yachts
Diesel internal combustion engines are more fuel efficient and less fire hazardous. Diesel itself is less expensive than gasoline, and exhaust emissions have a lower percentage of carbon monoxide, making them less harmful to the health.
With proper maintenance, a diesel engine is capable of living twice as long as a gasoline engine.
But the weight of a diesel engine is on average twice that of a similar petrol engine. Just because of its large dimensions, they are seldom used at small boats. Also, diesel engines cost essentially more than their gasoline counterparts.
In this article, we have already reviewed in detail how to make a choice between petrol and diesel engine.
Electric motors: for lovers of nature and silence
As the name implies, in the case of an electric motor, the energy for propeller rotation is obtained not from the thermal expansion of gas, as in the internal combustion engine, but from electricity accumulated in the battery.
At anchorage, onboard batteries can be recharged from the shore network. During the voyage, they are usually replenished by solar panels. Hydrogenerators are rarer: while the yacht is in motion, the water flowing under the bottom turns the propeller connected to the dynamo-machine which generates electricity. Somewhat exotic options include wind turbines and hydrogen fuel cell engines.
The main advantages of electric motors are absence of harmful emissions into water and atmosphere and noiselessness. With an electric engine on board, the crew can safely enter even protected waters. Engine operation will not harm local flora and fauna. There will not be a trail of petroleum products on the surface of the water.
However they have only a limited power, so they can't accelerate a large boat very well.
Yachts running solely on electricity are rare. Brands Greenline(Slovenia) and SilentYachts (Austria) offer pure electric versions of their models but hybrid propulsion systems are much more popular. We will talk about them below.
Hybrids: the advantages of both electric and diesel engines
Hybrid yachts have both traditional combustion engine and electric motor.«Most of the time they cruise on diesel engine or in» mixed-mode, but if necessary they can also switch to pure electric power.
Usually, the electric mode is used during standing times. Batteries supply all the life support systems on board so that one can be at anchor in peace and comfort. So, Greenline owners can forget about the generator for up to three days of mooring.
Drive types: «shaft», «column», «under»
Stationary propulsion drives offer a wide range of designs, each of which has advantages and disadvantages.
A shaft-driven engine is the simplest type of a marine stationary engine. The motor is installed at midships (direct drive) or in the stern of the boat (V-drive). It is not only a question of the layout, but also of the shaft design. «In both cases a shaft rotating around its axis» (the shaft itself) moves away from the boat transom. At the end of the shaft, which comes out of the boat hull, the propeller is fixed.
The rudder blade is not connected to the propeller and is located behind it, closer to the transom.
The shafts are relatively cheap and reliable. They are used even with very powerful engines. In addition, they are easier to service when parked than the swing-out propeller shafts (discussed below) and they last longer.
The midship-mounted propeller shaft has a simpler design and uses less fuel. However, the location of the engine room at midships can be inconvenient. This deprives the owner of a master cabin in the most comfortable position on the lower deck.
In the V-drive type aft shafts are divided into two elements, positioned at an angle to each other, as the < symbol. Its position frees up space at midships, but on large yachts this can make planing difficult. In addition, because of the complicated design, such shafts are less economical.
The propeller shaft is a great option for large yachts that are always in the water and over time can develop polyps. Apart from the propeller itself, there is almost no place for them to settle.
Since the shaft does not participate in the maneuvering of the boat, but only pushes it forward, it has no movable rubber bellows, unlike the tilt-up column, which must be replaced periodically. The shaft is equipped with a gland seal, which can eventually begin to leak, but its maintenance is inexpensive.
The disadvantages of the shaft include the fact that the propeller and rudder blade go deep into the water. With a shaft, the boat is unlikely to be able to sail in shallow water and come close to shore. Although from the point of view of safety when rescuing a drowning person, this position of the propeller is an advantage, not a disadvantage.
To mitigate this disadvantage, the shafts of some modern models «are recessed» into the hull and placed in semi-tunnels.
For example, Swedish shipyard Nimbus does it that way.
One more disadvantage of shafts is that on planingyachts, the engine with such a drive is less economical in fuel consumption compared to a tilt-up column. The maximum available speed in this case will also be lower.
But it is easier for the shaft to maintain speed when accelerated. And stability is ensured not by engine trimmers, but by shifting the center of gravity to the midship.
In addition, the shafts are required to use the so-called electrochemical «cathodic protection» to prevent corrosion. For this purpose a metal anode protector is installed on the shaft, which degrades instead of the shaft. The protector must be replaced annually. But the same is required for the tilt-and-turn column.
The shafts are very demanding on the quality of the hull and the location of the boat's center of gravity. Tilt-up columns are more tolerant of design flaws.
But if done properly, a boat with shafts is more maneuverable. Two shafts can be run in opposite directions, so that one propeller pushes the hull forward and the other backward. Thus, the boat will rotate around its axis practically on the spot. With tilt-up propellers, of course, you can do a similar trick. But the efficiency of such maneuver will be lower.
Keep in mind, that operating any boat with shafts requires a certain amount of skill.
But even with good handling skills, a boat with one shaft is a lot more difficult to handle than a boat with one tilt-shaft. But with two shafts (which, fortunately, are more common), the difference is noticeably smoothed out.
The main manufacturers of shaft-driven marine engines are MAN, Caterpillar, MTU (Rolls-Royce), Volvo Penta, Cummins.
Tilt&Turn column (Sterndrive)
The tilt-turn column is a hybrid of a stationary and an outboard engine, taking the best of both worlds. This type of drive first appeared in the early 1960s and quickly became very popular. Since then, they have been the main competitor to shafts.
Compared to shafts, swing-out speakers are cheaper, quieter and more fuel-efficient.
A motor with such a drive is always mounted inside the boat as close to the transom as possible and is attached to an external propulsion system - essentially the lower half of the outboard engine. The motor rotates a small drive shaft, on the end of which a propeller is attached.
The positioning of the propeller and its compactness (compared to the shaft drive) allows the engine room to be smaller and free up space for the living quarters.
As with a conventional outboard, the outer part of the rig can be tilted and rotated to change the direction in which the engine pushes the hull.
Because of this, a boat with a tilt-and-turn unit does not need an additional separate rudder pen like a boat with a shaft.
Thanks to the position of the engine, the bow-tilt hull has better hydrodynamic properties, because the propeller on the transom doesn't create additional water resistance under the bottom and doesn't brake the boat. As a result a boat with such engines can reach higher speeds than one with shafts of the same power.
The position of the propeller and its mobility also allows to sail in shallow water and come close to the shore without the fear of striking the bottom with the propeller. In addition, like an outboard, the column can be raised (although the range of movement of columns is less than that of outboards and the draft is greater). During planing, the trimmer increases the stability of the boat.
However, due to the additional moving parts and difficult accessibility, maintenance of the trimmer (especially in marine yachts) is much more expensive than maintenance of the shaft or outboard engine.
It is worth bearing in mind that such an engine would have to be completely removed during repairs.
Depending on the model, in order to get full access to the engine at this time, it may be necessary to free the cockpit from furniture and even dismantle the aft deck.
Also, if the turntable is installed on a boat that is permanently in the water rather than being taken ashore for storage, the outside of the engine will be an excellent springboard for polyp attacks. In this case there is a need for regular cleaning and coating of the speaker with an antifouling agent.
Fouling is particularly undesirable due to the bellows. These parts themselves tend to crack over time, and sharp barnacles can pierce them even faster. The result is a leak and, in the worst case, flooding of the boat.
Another vulnerable place is the water intakes, which are part of the engine cooling system. If spawned polyps clog the intakes, the engine will overheat.
For wakeboarding a boat with a kickstand is a good choice. On the other hand, for water-skiing it's better to go for flatter waves, which leaves the boat with a shaft.
The most well-known manufacturers of turnpike engines are Konrad, Mercury Marine and Volvo Penta. Used engines also include many Outboard Marine Corporation models that are no longer in production.
Pods are a relatively new phenomenon in the marine engine market. They have only been available since 2005-2006. Because of their design, they are also called «bottom drives».
The pods are self-contained units for each propeller, mounted directly on the bottom near the stern. And these propellers are always at least two.
The pod configuration reduces the amount of space that needs to be allocated to the engine room. On planing, there is no risk of air entrainment and consequently, cavitation causing erosion.
But the ability to enter shallow water will have to be sacrificed for that.
is sacrificed. And it's not very convenient to service such system, because mechanics have to have access to the bottom.
While shafts can be found on every hull, pods are suitable only for planing or at least semidisplacement yachts.
Compared to shafts, pods are 10-30% more fuel efficient.
Cummins Zeus pods are usually equipped with integrated interceptors. But their propellers face towards the stern and push the boat, like other drives.
The Volvo Penta IPS (Inboard Performance System) pods do not have interceptors. «But since 2015 they have» (Forward Drive) - the original twin propellers pointing towards the bow. Due to it, unlike other drives, this one pulls the boat instead of pushing it. But such system is not without disadvantages.
The maximum speed of a boat with pads is 15-20% higher than with shafts. Volvo Penta also promises a 40% increase in range.
Pods are available only in diesel and always come with a joystick control system, which eliminates the need for manual gear shifting.
The ease of operation and increased maneuverability makes these boats much easier for beginners to moor.
The pods are more expensive to maintain than the shafts. They don't need oil seals, but regular replacement of expensive synthetic machine oil and anode treads more than make up for it. In addition, pods are very sensitive to water ingress, and fewer mechanics know how to fix them. In the event of a breakdown during a long trip, this could be a problem.
It's also worth considering that simply replacing shafts with pods on an already built boat probably won't work. They require a certain type of bottom.
When pods were first introduced, shipyards had to modify some existing models to allow customers to use their engines with pods.
In addition to Cummins and Volvo Penta, Reintjes (Fortjes, which have two propellers facing in different directions) are also producing pods.
There are special pods for electric motors. An azipod (or «azimuthal pod») is a system where the electric motor is located in a 360⁰ rotating nacelle outside the hull around the vertical axis. Therefore, a separate rudder is not necessary for a boat with an azipod.
These pods are not fitted with a gearbox, which is normally responsible for reducing engine speed and increasing torque on the output shaft. Azipods don't need it, because the propeller is mounted directly on the motor shaft and the torque is transmitted directly, without any losses.
Such pods were developed by ABB, Swiss corporation. The first installation in the world was received by Seilin tugboat in 1990.
Note that azipods are very popular in Russian shipbuilding. Such engines are used e.g. on icebreakers «Alexander Sannikov» and«Andrey Vilkitsky».
In total, there are more than 80 icebreakers and ice-class ships in the world with azipods.
ABB has also announced the installation of its pods on at least two Oceanco superyachts. They will equip the 115m Tuhura, whose design was unveiled in 2018, and an unnamed project as part of the Oceanco shipyard's NXT environmental programme, announced in autumn 2020.
Bringing the engine into the nacelle naturally frees up a lot of usable space on board. In addition, no vibration from the propeller is transmitted to the hull. Asipoda boat passengers will also appreciate the low noise level.
Another important advantage is economy.
According to ABB, azipods consume 20% less fuel compared to shafts.
In addition, azipod boats are maneuverability and good handling.
Disadvantages of azipods are generally the same as those of other pods: special requirements to the hull design, high price and complexity of installation and maintenance, large draft due to hanging gondola.
Arneson's Surface drive
One of the most economical drives are considered Arneson @ Twin Disc Surface Drives. According to Arneson Industries, the drive design reduces water resistance by 50% compared to more traditional submersible drives.
In the case of the Arneson drive, only the propeller blades and the skeg - the protective aft keel ledge - touch the water.
Designer Howard Arneson developed these propellers mainly for catamarans and planing hulls like «Deep V».
This drive can be found on Pershing and Mangusta sports yachts.
«Depending on the model, Arneson propellers can either steer the» boat by themselves, or be used in combination with a separate rudder blade.
By reducing water resistance, boat speed is increased by 15-30% and fuel consumption is reduced by the same amount.
The Arneson drive is superior to a tilt steering column in fuel efficiency. And in terms of vibration levels, so are shafts.
Minimal contact with water also means minimal corrosion. The screws are also not prone to cavitation. This saves significant maintenance costs.
At the same time, however, due to the large number of moving parts, the drive requires more attention.
The propeller positioning high on the transom allows the boat with an Arneson drive to be as close to shore as possible. The propeller depth can be adjusted while running.
The downside of this design is problems with stern mooring. The propellers protrude strongly to the rear and as if lengthen the hull.
One of the advantages of the Arneson drive is that, unlike the pods, it does not require a special hull design.
Such a drive is also very easy to maintain.
Apart from Arneson Industries directly, drives of similar type can be found in ZF (SeaRex) and France Helices (SDS). Close to the Arneson drive are the Mercury M8 and Mercury Bravo One XR Sport Master swashbucklers.
Water jet engine (water jet, pump-jet, hydrojet)
The main difference between water jets and other propulsion systems is that the propeller is not in the open water, but inside the propulsion system. This design, which is called «impeller», allows to reduce water overflows at the propeller blade ends and at the same time to reduce power losses. The impeller forces the water out of the«nozzle» of the water-jet and pushes the boat forward. It is controlled by changing the direction «of the nozzle».
For the first time, it was invented to use a directional jet of water for propulsion in the middle of the 17th century. In modern water vehicles, water-jet propulsion has long been the prerogative of jet skis and water toys. Today they are installed even on mid-size yachts (including 30-meter ones like Overmarine Mangusta 108). In the future, perhaps, the development of technology will allow the construction of reasonably priced water jets for the largest boats as well.
Since the propeller of the waterjet is enclosed, a boat with such a drive can safely approach people and animals in the water.
The waterjet also does not produce unnecessary noise underwater and does not transmit vibration to the hull.
The waterline location of the water pump on the transom also allows you to get close to shore and sail in shallow water. The location of the water pump is more compact than a tilt-up propeller and allows more room in the stern.
As the number of moving parts is small, this drive is sufficiently reliable. The main part of the engine is hidden inside the hull and is not in contact with water, so it is not exposed to corrosion and cannot be damaged by an accidental collision. In addition, the boxed propeller is less susceptible to cavitation and corrosion due to it.
Another obvious advantage of the waterjet is its speed characteristics.
Even in rough weather, boats with such propulsion are capable of speeds up to 40 knots. Moreover, they can turn on 360⁰ while remaining in place.
The main drawback of such a drive is low maneuverability at low speeds due to insufficient pressure of the water flow coming out of the water-jet. Besides, water cannons are rather noisy and uneconomical (though manufacturers are improving these characteristics with time).
The initial price of waterjets, compared to other drives, «bites». Their maintenance is also specific and will require an additional investment from the boat owner.
The main manufacturers of water jets are Castoldy, HamiltonJet, Alamarin-Jet Oy, Scott Water Jet, Marine Jet Power (MJP).