Shaft vs IPS: which is better?

Now that IPS pods have been on the market for over 15 years, enough experience has been accumulated to consider the question from all sides 

Construction 

The shaft drive, which is the simplest and most common in shipbuilding, is an engine located inside the hull, connected via a gearbox to a shaft with a rear-facing pusher propeller at the end. 

When a boat is fitted with Volvo IPS pods, the engine is located inside the hull, too, but there is also an external turning part in the water, including a gearbox and a forward-facing propeller, which is of a puller type. 

One should not confuse Volvo IPS pods with a similar propulsion system manufactured by Cummins Mercruiser. The product is called Zeus, and although it is a pod drive, too, it's got pusher propellers. 

We are not going to compare IPS and Zeus pods in this article, as both manufacturers have their good reason to state that either a pusher or puller propeller is more efficient. What we are going to look at here is IPS pods and shafts. Why? Because it’s the data for the shaft drives that Volvo Penta used for its first reports on Volvo IPS impressive results: the top speed is 15-20% higher, and the fuel consumption is 30% lower than that of yachts of similar length and power with shaft drives. 

To put it simply, it is the difference you get if you equip two absolutely identical hulls with engines of the same power, but one with a shaft drive, and the other one with IPS pods. In addition, IPS can boast minimal levels of noise and vibration, amazing maneuverability and a compact size, so the engine room can be reduced to save more usable space inside the hull. Isn’t it wonderful? However, we can still see a lot of yachts equipped with shaft drives in the market. Why so? 

Which yachts is IPS good for?

Initially, Volvo IPS pods were meant for small boats under 50 ft (15 m). But with time, the power of pods has increased considerably up to 1000 hp per unit (Volvo IPS 1350), so it’s been a long time since they started mounting them on larger, 25-30-metre yachts, often combining not just two, but three or four pods. “Sets” like this are frequently used by such world-famous shipyards as Dutch Van der Valk or Italian Azimut.  

Unfortunately, it is impossible to convert any shaft drive yacht into an IPS one. IPS pods can be mounted on particular bottom configurations only, so production yachts are initially designed to be fitted with IPS. A lot of Italian and British manufacturers do so, but they have shaft drive models, too. Occasionally, one can find a model that is available both with IPS pods and a shaft drive, but their underwater hulls are different. 

The fact that a lot of shipbuilders still have shaft drive models makes one wonder: why hasn’t everyone switched to IPS pods yet?  

This is probably partly due to people’s conservatism, and partly to economy, as shafts are cheaper. Moreover, there are types of yachts that are incompatible with IPS pods. If you sail in high latitudes or in cold weather, when there can be broken ice in the water, shafts are a lot more reliable. For this reason, you will not find IPS on expedition vessels that might navigate some northern waters. It doesn’t make any sense mounting IPS pods on low-speed trawler-type yachts with engines of about 150 hp either. 

Operation 

Yachts equipped with IPS pods are often positioned as simplest to handle even for inexperienced drivers. And the opportunity to moor and maneuver a yacht at a low speed in a cramped space with the help of a joystick – even sideways – does seem very attractive. 

Another obvious benefit is reduced fuel consumption. In case a yacht is equipped with three or four IPS pods, there are some extra advantages, too. Quite a large vessel may develop a high speed, while it is very quiet on board, and the engine room is super compact.  

However, there are some operational nuances, too. Quite often the captain or the owner turn on two out of three or four pods, as it seems to them that they can save engine life this way. Or, for instance, when one of the IPS pods fails, instead of repairing it, people go on using the remaining ones. All of this overloads the engines and increases their wear.

Maintenance

Most owners’ conservatism regarding IPS pods has to do mainly with the fear that they are easier to damage or even to lose altogether. The IPS propellers are forward-facing, which means they are the first to meet all the plastic and debris floating in the water. 

However, there is no statistical confirmation to this assumption. Service companies do not sell forward-facing propellers more often, and all kinds of propellers are damaged equally often. 

In case of a serious collision with an underwater obstacle, the lower part of the IPS pod simply gets “cut off” and goes to the bottom. Is it a shame? Yes. But in fact, this helps to protect the yacht’s hull from damage. Firstly, she will not sink, and secondly, to eliminate the consequences of the accident you will not have to spend money on fixing the hull – you will only have to replace the lower part of the IPS pod.

As for a yacht with a shaft drive, the consequences of running aground or colliding with an obstacle are rather unpredictable. Sometimes the damage is minimal, but sometimes the shafts can bend, propellers can rip the hull open, so you are lucky if the boat doesn’t sink.

Although it’s not cheap to replace the lower part of the IPS pod, it is just a spare part that can be bought and mounted quite fast. You can’t use your boat for several weeks at most, and the financial issue is solved with the help of insurance.

If you happen to bend the shaft or the props go into the hull, then there is a risk of sinking. And the repairs definitely won’t be cheap. In addition, shafts and propellers are often not stock spare parts. The repairs will take at least a few months, even if no work on the hull is required.

On the other hand, if shafts can be fixed almost anywhere, Volvo IPS pods require an authorized service company. One should also remember that the benefit of shafts is not so much the ease of maintenance, as its minimization. 

If you plan to rent a yacht out or operate it very intensively, a shaft drive is definitely a better choice. Lower maintenance and repair costs are guaranteed, too. Even if there are some problems with the shafts, like wear or vibration, the boat will still run and nothing will fall off. While IPS pods need to be looked after a lot more carefully.