Toroidal Propellers: How Do These “Hollow” Propellers Improve a Boat’s Performance?

In the latest episode of “Ti spiego la barca,” the Barche a Motore podcast in collaboration with Suzuki Italia Marine, the topic discussed was propellers. You can find the full episode here.

Some readers have asked us for more information about a type of propeller that’s often talked about but not often seen: toroidal propellers. What are they? What are they used for? What are their advantages? We’ll explain below.

Toroidal Propellers

The name is complex, as is the geometry, but the efficiency is extremely high: they’re called toroidal propellers, and in the world of recreational boating, they’re synonymous with the name Sharrow Marine. The Michigan-based company acquired the patent from MIT (the Massachusetts Institute of Technology) and then optimized it to such an extent that, compared to a conventional propeller, the toroidal propeller is far superior in terms of quietness, fuel efficiency, speed (and even price). But, in a nutshell, what is the operating principle?

By utilizing a closed-loop design—hence the name—and eliminating the free ends of the propellers, turbulent vortices are dissipated: the water is accelerated more uniformly, producing constant propulsive thrust with minimal induced drag. So is everything perfect? The main limitation to their widespread adoption—beyond specialized or military applications—is, as we mentioned earlier, the price. For a high-powered outboard motor, the average cost is around 5,000 euros.

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What Is a Toroidal Shape?

What is a toroidal shape? It is a donut-shaped geometric surface generated by the rotation of a circle around an external coplanar axis. Did you understand any of that? Don’t worry—let’s try to clarify things now. Let’s start with the classic example of the “screw,” which is used to explain how a propeller works (and if you’re still unsure, there’s always this video) to explain that, in fact, aclassic propeller“screws” through the water, which allows the boat to move. In this process, some energy is lost due to the shape of the propeller.

This is the well-known slip—that is, the difference between the theoretical speed calculated by multiplying the pitch by the engine RPM and the boat’s actual speed. The toroidal propeller has a ring-shaped geometry designed to better contain the flow, thereby reducing these losses. In technical terms, the advantage lies in the smaller difference between the propeller’s theoretical pitch and the boat’s actual forward speed: in other words, it reduces the loss and increases propulsion efficiency.

In toroidal propellers, in fact, the ring-shaped geometry helps to better contain the flow, limiting lateral losses and vortices that reduce efficiency. For this reason, both in theory and in test trials, the boat can plane sooner, maintain cruising speed at lower RPMs, and consume less fuel—especially in conditions where the propeller is designed to perform at its best.

The Practical Advantages of the Toroidal Propeller

In a nutshell? The propeller loses less energy, and the boat is “propelled” more efficiently. This can result in faster planing, cruising at a lower RPM, and, consequently, lower fuel consumption. There’s also another interesting aspect: more efficient propulsion tends to generate less vibration and less noise, with an immediate benefit to onboard comfort. This is a detail that matters a great deal on medium-sized motorboats, especially when sailing longer distances.

Toroidal propellers are not a magic wand. They do not guarantee the same results on every hull, with every engine, or under every operating condition. Just as better tires allow for a smoother ride on the road, propellers of this shape can undoubtedly offer significant advantages.

The advantage isn’t just in the numbers. More efficient propulsion can also result in less vibration and quieter operation—two factors that greatly affect comfort, especially on medium-sized motorboats used for longer trips and cruises.
In other words, the toroidal propeller doesn’t change the laws of physics—but it does seek to make better use of them, reducing the gap between theoretical thrust and the thrust actually achieved in the water.

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But they represent a very interesting technical solution because they address a key aspect of boating: making better use of available energy by reducing waste. In short, the idea is this: don’t change the engine’s power, but use the water more efficiently. And this is precisely where toroidal propellers aim to make a difference.