Motor yacht drives: Shaft, sterndrive or pod?

Whoever wants to buy a motor yacht is often faced with the choice of the right drive. In addition to the size of the yacht, the type of use and performance, the buyer should also consider the various types of drive available on the market before making the purchase in order to avoid disappointment later on. What are the advantages of shaft systems, so-called sterndrives and a pod, specifically: the IPS system from Volvo Penta? SeaHelp gives an overview.

Anyone who wants to move on the water under motor, can not avoid a few thoughts about the type of drive of the particular motor yacht (or the built-in machine for sailing yachts). We present three of the most important types of propulsion and describe advantages and disadvantages.

The classic: the shaft – little power loss, but reversing and wheel effect

Many yachts are still equipped with shaft systems today. The advantages are obvious: for the most part, the system is relatively manageable, and the direct transmission of turning power ensures that very little power loss occurs. Only about one percent of the transmitted power is lost in the form of bearing heat, it is said; with single gear units, the losses are still very low at two to three percent.

The term shaft system has only come into use since other types of propulsion, such as sterndrives, jet drives and outboard motors (the latter two will not be discussed in detail here), have been used in yachts and it has become necessary to differentiate between the various propulsion systems. The so-called propeller nacelles for large vessels (pods) also play no role here, with the exception of Volvo Penta’s smaller IPS system for recreational boating.

The shaft system is the classic propulsion system; for recreational vessels, there are three designs

The marine shaft device may initially consist of one or more parallel drive trains. In recreational marine applications, the shaft device is divided into three different designs: L-drive – the inboard engine transmits its power directly aft via the drive shaft to the propeller, possibly via a gearbox, V-drive – the inboard engine transmits its power with a first drive shaft forward to a reversing gearbox and from there aft to the propeller.

A special feature is the saildrive – a compact system used on motor and sailing yachts with low propulsive power: from the engine, which is installed in the hull of the yacht, a permanently flanged shaft with a drive shaft (propeller shaft) leads vertically downward and exits at an opening in the hull. Rubber sleeves are fitted between the shaft and the hull to seal the hull. At the end of the shaft, the drive shaft is deflected at right angles and emerges horizontally from the shaft. Mounted to this is the propeller.

Volvo Penta IPS System: Saildrive
© Volvo Penta

 

Gearboxes are often part of shaft systems – important for reducing speeds

In shaft drives, the actual shaft is held by bearings. The rear bearing, called the stern tube bearing, is sealed from seawater by a stern tube seal, such as a stuffing box. Normally, a shaft train consists of a straight line from the engine flywheel to the propeller. In limited space conditions, a gearbox may also serve to compensate for angular and/or vertical misalignment.

Gearboxes are often part of the shaft system. These serve to reduce the engine speed to achieve specific speeds for favorable drive efficiency. In addition, the gearboxes often have other outputs to drive generators or pumps; for example, powerful hydraulic pumps are required to operate the stabilizers; bow and stern thrusters must also be supplied.

Advantages of the shaft system: the systems are simple in design, easy to maintain and inexpensive

Advantages: the systems are mostly simple in design, well laid out, easy to maintain, and relatively inexpensive compared to other propulsion systems. Another advantage: a balance in the ship, since with shaft drives the position of the motors can be variably determined (for example, by adjusting the shaft length accordingly). Finally, shaft systems also have relatively low service and maintenance costs.

The disadvantages of shaft systems include the fact that they (usually) do not contain any maneuvering elements, such as a rudder, and sometimes have a very pronounced wheel effect. In addition, for this type of drive to change the direction of rotation, known as reversing, either the motor itself or the gearbox must be reversible, or the propeller must be a controllable pitch propeller. Reversing the motors usually involves loads on the motor and a time delay; controllable pitch propellers and gearboxes for reversing are expensive.

Pro and contra wheel effect: besides negative characteristics, it can also help maneuvering

The wheel effect can be remedied, for example, by installing several shaft systems or those with two propellers: here, the direction of rotation of the two propellers is adjusted (usually in opposite directions) so that almost no more wheel effect occurs, and maneuverability can thus be significantly improved. However, the wheel effect can also be a valuable aid to maneuvering if you know it and know how to use it properly.

Flink, maneuverable and quiet: the sterndrive combines the advantages of inboard and outboard motors

The sterndrive is a gearbox (so called because of its design) with clutch for powerboats. This drive is mostly attached to the transom plate and is thus located outside the boat, while the engine is located inside the hull. The shaft exiting the engine horizontally is directed in a Z-shape to the propeller shaft located lower down, hence the name.

On fast recreational boats under ten meters (33 feet) in length, this type of propulsion is common. The reason is simple: in a sense, sterndrives combine the advantages of inboard engines with those of outboard engines. Compared to pure inboard engines and classic shaft systems, for example, boats with sterndrives are much more maneuverable.

 

Volvo Penta IPS System:
© Volvo Penta

 

Sterndrive inboards are mostly quieter than comparable internal combustion outboards

Compared to outboard engines, boats with inboard engines and sterndrives are mostly much quieter. In addition, the range of economical and powerful diesel engines is very small for outboards, while in the inboard sector, including alternative propulsion concepts, the choice of engines is greater.

For a long time, another advantage of the sterndrive in combination with an inboard was its relatively low price: a comparable outboard was more expensive than an inboard with a sterndrive. However, the situation has changed fundamentally in recent years: Outboards are increasingly displacing sterndrives from the market, and it is becoming more and more common nowadays to find 40-foot boats, for example, with two, three or four powerful outboard engines hanging from the stern, which are often less expensive in terms of overall performance than a comparable inboard engine with the same power output.

Outboard engine technology is mature; production is in high volumes

In addition, the engine technology of outboards is now very mature, often the same engines are also used in motorcycles, quads, skidoos and jet skies, and are therefore also produced in larger numbers, which in turn affects the price.

Regardless, the sterndrive (and outboard) has the unbeatable advantage over a shaft system in that the propeller shaft can be trimmed so that one hundred percent of the thrust goes aft (and not, as with shaft drive, always a little down as well).

Disadvantage of stern drive: the articulated joint between the drive and the hull is subject to wear

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The disadvantage is that the articulated connection between the sterndrive and the hull, as well as its seals and all moving parts, are subject to constant wear, resulting in high service and maintenance costs.

Another disadvantage often seen is that the motors in a sterndrive must always be mounted directly in front of the transom plate, and thus can adversely affect the balance of the boat. The level of power has therefore been set by experts at a maximum of 350 hp for sterndrives.

IPS: more comfortable with joystick, and: faster, farther, more maneuverable and quieter to the destination

In 2004, Volvo first presented the IPS (Inboard Performance System) propulsion system, which has since defined what is feasible in recreational boating at the highest technical level. The technology is based on pod propulsion, comparable to the propulsion systems of passenger ships, the propeller gondolas.

IPS propulsion was developed as an alternative to conventional shaft drives. Mercury also developed a similar system specifically for its engines called Zeus, a pod drive for large cruisers and pleasure boats.

 

Volvo Penta IPS System:
© Volvo Penta

 

The IPS (pod) system is superior to shaft systems in terms of maneuverability and performance

The distinguishing feature of Volvo Penta’s IPS propulsion system is the forward-facing, counter-rotating traction propellers that generate horizontal thrust. The drives are located in the aft section of the stern and are integrated directly into the hull. The Swedish propulsion system now consists of ten models and is superior to conventional shaft systems in terms of maneuverability, on-board comfort and performance.

Specifically, the advantages of an IPS (pod) system over classic shaft systems or stern drives are better maneuverability; according to the manufacturer, up to 20 percent greater speed and acceleration and up to 40 percent greater range can be achieved.

 

Volvo Penta IPS System:
© Volvo Penta

 

CO2 emissions balance and fuel consumption are significantly better with the IPS (pod) system

Furthermore, CO2 emissions and fuel consumption are said to be 30 percent lower each, it is much quieter on board (minus 50 percent subjectively perceived noise minimization), and finally, valuable space on board is also saved due to the compact design and the way it is (externally) mounted. Note from Volvo: the data given here are approximate values for full gliders at cruising speed – compared to corresponding shaft systems.

The IPS systems are available as double, triple and even quadruple units; three drive series are optimally matched to the respective engines. The IPS1050 and IPS1200 models are the latest additions to the system, with the D13 yacht diesel engine serving as the powerplant.

 

 

In 2006, Volvo Penta launched the joystick for IPS – for even more comfort on board

Those who decide to install an IPS propulsion system on their yacht can also take advantage of the joystick introduced by Volvo in 2006: with the joystick in the helm, the boat can be turned with just one touch on the platter (Mercury offers something comparable for its Zeus system).

If you push the joystick to the side, the boat moves sideways. Port maneuvers become a breeze, and the Electronic Vessel Control (EVC) system, which connects and manages internal communications between the engine and drivetrain, joystick and screen, ensures that the operator can control everything with just a single joystick.

Volvo Penta IPS System:
© Volvo Penta

 

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Dynamic Positioning System allows the skipper to maintain his position over ground

Another advantage of IPS & Joystick: with the Dynamic Positioning System (DPS), the skipper can easily maintain the course and position of the boat automatically even in rough conditions, for example in the waiting area in front of a lock when there is no place to moor, when waiting in front of the gas station or a marina place that will soon become available.

 

Volvo Penta IPS System: Dynamic Positioning System (DPS)
© Volvo Penta

 

Volvo Penta recently unveiled its new Assisted Docking System (ADS), which combines proprietary software with the GPS-based Dynamic Positioning System and its own Inboard Performance System (IPS) into a complete package. The goal: to further enhance the boating experience, even when really rough docking conditions such as strong crosswinds and/or currents make maneuvering difficult.

Volvo Penta IPS System:
© Volvo Penta

 

Disadvantage IPS- (pod-): the system is more expensive than comparable drives and limits the usable space in the stern of the yacht

One of the disadvantages of the IPS (or Zeus) pod system: it is not cheap to buy. And: the orientation of the propeller shaft – similar to a shaft drive – is never directly aft; trimming in this regard is not possible. Measurements have shown that the same boat equipped with a diesel inboard sterndrive consumes about 30 percent less fuel and is also five to ten knots faster.

Another disadvantage is that the engine compartment must always be located in the rear third of the yacht with podded propulsion, which can restrict the use of the stern area of the hull under certain circumstances. Aft of the propulsion system always remains a poorly usable space – exception: tender garage.

If the IPS system fails, there is often no emergency operation; technicians must be adept

In addition, the entire propulsion system can only be controlled with an electronic control system, which in a sense means dependency, because the system cannot be easily bypassed (emergency operation may not be possible). A line in the propeller can thus under certain circumstances already mean the end of the vacation (total system failure). Technicians must be well trained and properly equipped to help.

However, once you have a pod system on board, you won’t want to do without it. The ease of use and excellent maneuverability (in combination with the other advantages mentioned above) – when the system is working properly – are very convincing arguments for many.

For more information about Volvo Penta, see here.

Further articles from the category “Test & Technology” can be found here.

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