Motorboat Expert

Independent Yachting Magazine

Shaft drive, outboard motors, sterndrive and IPS pod. Everything you need to know

Shaft drive, outboard motors, sterndrive and IPS. Everything you need to know

Updated on May 31st, 2024

It is clear that the performance and efficiency of a motorboat depends on the drive no less than on the engine. Today you can see four dominant systems besides the jet: direct shaft drive, outboard motor, sterndrive and pod drive. Naturally, proponents of different systems make excellent arguments in favor of using one or another drive.

But in most cases it is not an either/or question, because you cannot install an outboard motor on a superyacht or a pod on a tender.

Once upon a time there was only one option – shaft drive. Then came outboard motors, which at that time were only suitable for very small boats. Later, a stern drive appeared, which made it possible to diversify the design of motor boats, increase their speed and maneuverability, but which also could not compete with direct drive on yachts longer than 10 meters.

And already at the beginning of the 21st century, Volvo introduced a completely new IPS drive system, which brought a number of undeniable advantages, and, in addition, can be installed on fairly large boats, including superyachts.

Let’s take a look at what each of these systems can offer us, so that when choosing a motor yacht you can know exactly their advantages and disadvantages.

Shaft drive

As old as time, this drive system is almost as reliable as a blacksmith’s hammer. That is why he is still with us and has no plans to leave.

Shaft drive

The simplicity of the design means that all boats that do frequently experience rough sea conditions have no other alternatives. In the event of any breakdown, you will not only be able to quickly determine its cause, but also, quite possibly, fix it yourself. Show a fisherman the IPS system and he will just laugh. No, traditional shafts and rudders will be here for a very long time. Perhaps forever.

But what are the disadvantages of shaft drive?
Well, among the most obvious is the non-adjustable angle of attack of the propeller, which is not pointed straight back, causing its efficiency to decrease. For slow displacement yachts, such as trawlers, this is not extremely critical, but for fast boats it greatly reduces efficiency and negatively affects their economy.

But in fairness, it should be noted that, unlike other systems, direct drive very efficiently transfers torque from the engine to the propeller. The main energy losses go to heating the bearing and amount to less than 1%.

The second most important disadvantage is often called the need to install the engine amidships, which affects the interior layout and increases engine noise in the saloon and cabins while underway.

However, placing the heaviest item amidships is not such a bad idea if the yacht’s seaworthiness is a priority over speed. In addition, if desired, you can move the engines closer to the stern and use an intermediate gearbox. This will result in a V-shaped version of the shaft drive.

However, we still prefer to have a spacious engine room amidships for easy engine maintenance, leaving the aft underdeck space for the tender. Modern engine construction technologies, as well as the development of hybrid and all-electric motors, make the need for additional sound insulation less relevant.

Another disadvantage is the poor maneuverability of boats with one central shaft drive. But for most skippers it’s just a matter of practice. We must take into account that bow thrusters are found on almost every yacht today. And Volvo Penta began to offer its joysticks for almost all types of propulsion systems.

Outboard motors

Today, outboard motors are not just for small inflatable tenders. With the increase in their power, they increasingly began to be used for large boats. And there are a number of good reasons for this.

At one time, an inboard motor of comparable power for a sterndrive was less expensive than an outboard. However, recently everything has become exactly the opposite. With the exception of exclusive high-tech models, mounting multiple outboards on a transom is less expensive than two inboards, but they are lighter, more convenient and cheaper to maintain. And probably safer.

An often overlooked benefit of outboards and sterndrives is their ability to flip up when hitting an underwater obstacle. However, this only works successfully at low speeds.

Another great advantage that cannot be overstated is the complete absence of technological holes in the hull below the waterline. Indeed, the drive shaft of the onboard engine, the water intake for the cooling system and other holes are what we all always strive to, if not avoid completely, then at least minimize.

Of course, they have shortcomings that you have to put up with. For example, they are definitely noisier and have a shorter lifespan (although we must not forget that we are moving into the era of electricity and before our eyes there is an active displacement of gasoline engines from their status quo). Plus, they take up the stern of the boat, so you’ll have to give up the swim platform. However, there are boat designs that try to solve this problem.

Outboard motors

Outboard motors sometimes raise the center of gravity high above the waterline and do not contribute to good boat stability. And of course they have limitations on the size of boats they can be installed on.

But in general, we really love outboard motors because they greatly simplify the life of the boat owner in terms of maintenance, and also free up space inside the cockpit.

Stern drives

The sterndrive is the result of a desire to take the best aspects of outboard and inboard motors for planing boats. Its greatest strengths are the low noise level from the motor located inside the stern, high speed, efficiency and maneuverability.

Stern drives

Additionally, like outboard motors, a sterndrive can provide an optimal angle of attack for the propeller and lift when approaching shallow water. And, unlike the shaft drive, it also does not require a rudder.

But for many boat buyers, the most important advantage is the free stern space and the presence of a swimming platform.

Disadvantages? First of all, this is the cost of repairs, which will be inevitable due to the large number of expensive parts located outside the boat in an aggressive environment.

Another disadvantage is that the sterndrive is not well suited for boats longer than 12 meters. This is due to the fact that the motors must be located directly at the transom plate, so the boat’s hull is difficult to balance. Therefore, the power of inboard sterndrive motors is usually limited to 350 hp.

Pod drives

Volvo Penta with its IPS is the most famous system of this type of drive. Besides the fact that they were the first to come to this territory, they also installed multi-directional propellers in front of the pods, which even today still looks unusual.

Pod drives Volvo Penta IPS

A little later, similar drives came to market, but with a traditional propeller arrangement, from Cummins Zeus for larger vessels. And after them, ZF Marine and Caterpillar started producing their own pod drives.

Right off the bat, we can say that Pod drives are unmatched in maneuverability. The IPS system with a joystick allows you to almost not worry about the direction of the wind and current during mooring, and if necessary, the on-board computer, guided by GPS signals, will accurately keep the yacht at a given point, regardless of weather conditions.

Joystick Volvo Penta IPS

And while sterndrives are likely to be more efficient on smaller boats, on larger yachts you can gain significant speed and economy advantages over a shaft drive. These benefits can be up to 30%.

Unlike sailors who are accustomed to a huge ballast keel under their sailboat, for motor yachtsmen having pod with forward-looking propellers under the bottom can be a psychological inconvenience that takes some getting used to.

As for the feeling of safety, of course, the greatest peace of mind for the skipper is the single straight propeller shaft covered by a deep keel – a classic design of a displacement trawler. Pods, from this point of view, are at the opposite end of the scale and rightly cause a slight sense of anxiety even with a working echo sounder.

However, according to Volvo Penta, when running aground or colliding with an underwater obstacle, IPS are designed to break away from the hull of the yacht without damaging it.

Pod drives are constantly being improved, and the range of boat sizes that can be equipped with them is expanding. Superyachts over 50 meters in length have been successfully propelled by IPS, which can have two, three or four pods. Using more engines, but less power, reduces vibration, increases their service life and increases fault tolerance.

Obviously, you guessed that the main disadvantage of such drives is the price. And not only the initial price, but also the cost of maintenance. And while you may indeed save on fuel compared to a shaft drive, it is unknown whether this will pay for the cost of sudden repairs.

In addition, overcomplicating a system, as all experienced yachtsmen know, is rarely beneficial, but often only increases the likelihood of failure. The IPS system is becoming more and more complex and packed with an abundance of electronics. There is a possibility that at some point the very electronics that are designed to help you will simply not allow you to start the engine at the slightest malfunction.

But of course our Luddite syndrome ignores the fact that electronics still perform many useful operations and exercise unprecedented control over important engine components. Electronics allow our yachts to be much safer and provide an incredible level of comfort for the skipper.

Systems such as Volvo Penta’s IPS can now be used very effectively for all types of engines, including electric motors. Over the years, these systems have proven their reliability, so they are now used on coastal service vessels.

Editor’s choice

We are certainly proponents of smart progress and believe that IPS is great in every way, so we welcome it on all fast motor yachts.

For small tenders and RIBs, outboard motors are an ideal option due to their lightness and practicality.

Works of art such as the wooden classic Riva boats and the like are best equipped with stern drives to hide the engine out of sight.

And when it comes to long expeditions, difficult conditions, real trawler yachts and simply reliable boats, we will always remain on the side of the good old shaft drive.