Maritime Journal

For more than a century, marine propulsion has changed surprisingly little. Engines have become cleaner, quieter and more efficient, but the basic principle remains the same: diesel is burned, pistons drive a crankshaft, and power passes through shafts and gearboxes before finally reaching the propeller.

Belgian inventor Gerd Van Driessche believes he has rewritten almost the entire process.

After spending four decades developing an idea that he says simply wasn’t possible until advances in computing, materials and manufacturing caught up, his company, Hydro Puls Systems, has developed Hydro Puls Direct-Drive (HPDD) – a compact hydraulic propulsion system that dispenses with the conventional marine engine altogether.

Designed initially for workboats, harbour tugs, ferries and inland vessels, the modular system runs on ammonia and, according to Van Driessche, combines high efficiency with zero exhaust emissions, minimal maintenance and a dramatically simplified drivetrain.

The technology is currently approaching its first full-scale prototype, meaning many of its performance claims still await independent validation. But if the concept proves successful, it could represent one of the more radical departures from conventional marine propulsion seen in decades.

Rethinking the engine

Rather than using combustion to rotate a crankshaft, the HPDD system converts combustion pressure directly into hydraulic power. Two opposed pistons move just 1.2mm inside a digitally controlled linear cylinder. Their movement pressurises hydraulic fluid to more than 600 bar before sending it to an accumulator, from which the stored energy can drive propulsion units or hydraulic equipment around the vessel.

The result, says Van Driessche, is a propulsion system with far fewer moving parts than a conventional engine.

Gone are the crankshaft, connecting rods, bearings and gearbox that inevitably absorb energy through friction and mechanical losses.

“We don’t need gearboxes, we don’t have a shaft, we don’t have, you know, 2,000 things that are moving, only two pistons… And we don’t have bearings, it’s lubricated with water.”

Instead of lubricating the pistons with oil, the system uses water, while hydraulic oil circulates continuously in its own sealed circuit to transmit power.

According to the company, the simplified mechanical design contributes to a net efficiency of 63% – much higher than that achieved by conventional marine diesel engines.

“We are burning at 63% efficiency,” says Van Driessche. “A diesel engine is about 25%. Don’t forget, the combustion itself is really bad. Then you have your shaft, you have your gearboxes and so on. You will save money. I’m sure of it.”

Powered by ammonia

While hydrogen has become one of shipping’s most talked-about future fuels, Hydro Puls Systems believes ammonia offers a more practical route for many vessels because it is easier to store.

Rather than carrying hydrogen on board, HPDD cracks a proportion of the ammonia into hydrogen immediately before combustion to improve ignition.

“We crack it a little bit to hydrogen,” Van Driessche explains. “Twenty per cent mass goes to hydrogen to burn it more easily.”

And unlike conventional engines, which require exhaust systems and emissions treatment, HPDD is designed as a sealed combustion system. Hydro Puls Systems says this eliminates carbon dioxide, particulate emissions, nitrogen oxides and ammonia slip, leaving only distilled water and nitrogen as by-products.

“What your readers will be interested in is there is no exhaust, there’s nothing going out,” says Van Driessche. “Everybody is speaking about slip, and so on. It’s not possible in our system. There is nothing coming out, only water and nitrogen.. That’s really revolutionary.”

Those claims, like the efficiency figures, remain subject to independent testing and certification as the prototype programme progresses.

More than propulsion

And with HPDD, propulsion is only part of the story.

Because the system generates hydraulic power directly, the same energy source can operate cranes, thrusters, winches and other hydraulic equipment without additional drive systems.

The combustion process also produces distilled water, which Hydro Puls Systems believes could reduce the need for freshwater storage or onboard desalination.

“That is a very important thing in little ships, because they don’t need a tank, they don’t need to fill up. Every hour you have 126 litres of water being produced, which is more than sufficient, and it’s clean water, with nothing in it.”

The company also says the nitrogen produced by the system can potentially be used for onboard cooling.

Perhaps HPDD’s greatest attraction for workboat operators is its modularity.

Each module produces around 300kW (approximately 400hp), with additional units simply added to increase output. Rather than designing different engines for different vessels, Hydro Puls Systems envisages using identical building blocks across multiple sectors.

“When you want 800 horsepower, we connect two of them. When you want 1,200, we connect three of them, because it’s hydraulics,” Van Driessche says. “There are no shafts. That’s the beauty of it.”

That flexibility could also make retrofitting existing vessels easier.

Because power is transmitted hydraulically instead of mechanically, the engine does not have to be positioned directly over the propeller shaft, giving naval architects greater freedom over machinery layout.

“The advantage is we don’t need to place this engine right on the shaft,” says Van Driessche. “This I can put in the front, you can put it high. Just with a tube of oil, we’re going to feed our propeller.”

Operators could also introduce the technology incrementally.

“You don’t need to buy directly a big thing,” he says. “You can, for example, use it for the generator. Then you can go step by step. You don’t need to change everything directly. That’s the beauty. It’s modular.”

The road ahead

For all the ambition behind HPDD, Hydro Puls Systems remains at the start of its commercial journey.

The first industrial prototype is being assembled in India, after which the company faces the far greater challenge of proving the concept in real-world operation, obtaining class approval and convincing a traditionally conservative maritime industry that a century-old approach to propulsion can be improved upon.

Investment will also be essential if production is to move beyond the prototype stage.

But shipping is no stranger to revolutionary ideas: batteries, methanol, hydrogen, fuel cells and ammonia engines are all competing to re-fuel the sector. The difference with Hydro Puls Systems is that Van Driessche believes changing the fuel alone is not enough: the engine itself also needs rethinking.

And there’s another major benefit, he says: there is no need for shorepower: HPDD modulates automatically, he says. 

“A ship with an installed power of 900hp (three modules) does not need that during manoeuvring and while docked,” he says. “HPDD will modulate to one module that keeps the hydraulic accumulator pressurised: it starts and stops to maintain the 600 BAR pressurehp

“The captain will not notice any of this; he can simply manoeuvre as always. His throttle operates the hydraulic cylinder and not the HPDD, which runs fully autonomously – the same principle as the range extender in a car that only charges the battery.”

Whether HPDD ultimately delivers on that ambition will only become clear once the technology has been independently tested and proven in service. But if Van Driessche’s vision is realised, the next revolution in workboat propulsion may come not from a new fuel, but from abandoning the conventional marine engine.