Two speed propeller
Black clouds bearing cold rain showers are racing across the sky. The VHF radio is broadcasting gale warnings. This is not the day to be testing propellers, but nevertheless we are headed out of a marina near Aarhus, in Denmark, on a Bavaria 42 equipped with a Gori three-bladed folding propeller.
The three- and four-bladed Goris are unique in the propeller world. They look much like any other contemporary folding propeller, with blades that are geared at the base and open on hinge pins. What’s different is that the blades can open in two different directions so that either side of the blade can become the leading face when in forward gear.
The “pitch” of a propeller, which is a measure of how aggressively it bites the water, is determined by the shape of the forward faces of its blades. Gori blades are machined with the hinge pins slightly skewed. When opened in the “normal” direction, the blades have one pitch; when opened in the opposite direction, the other side of each blade becomes its forward face, with approximately 20 percent more pitch. Gori calls this the “overdrive” mode.
Putting the propeller into overdrive increases a boat’s speed at any given engine speed, but also increases the load on the engine. What’s the point of doing this?
Engine and Propeller Curves
If you look at the curve representing the power an engine can produce, you see its horsepower or kilowatt output increases with speed (rpm), forming a convex curve. If you now look at a curve representing the amount of energy it takes to spin a propeller, you see the horsepower or kilowatts absorbed also increases with shaft rpm, but the curve is concave. The net result is, the two curves can only be made to cross at one point.
Propellers are almost always sized such that the propeller’s power absorption curve crosses the engine’s power output curve at full engine speed and power. In this way, the two are matched at full speed. Because of the different shape of the two curves, the minute you back off the throttle the curves begin to separate. In effect, the engine is now producing more power than the propeller can absorb. By the time you back off to typical cruising rpms, the curves are widely divergent, meaning the engine has quite a bit of spare capacity.
By increasing the pitch of a propeller in overdrive mode, the Gori propeller increases the power needed to drive the propeller at a given engine speed, which creates a new crossing point between the engine power curve and the propeller curve. The goal is to shift this crossing point until it is at, or close to, cruising rpms, so now the engine and propeller can be matched at cruising speed and at full speed. Overdrive mode also results in a closer match between the engine curve and the propeller curve at all speeds below cruising speed.
The immediate benefit to the overdrive mode is that the engine runs slower for a given boatspeed and creates substantially less noise and vibration. You would assume this is also more efficient—after all, you’ve reduced rpms and the boat is going just as fast—but in fact this is hard to quantify.
Normally, at cruising speed, there is a substantial mismatch between the power the engine is capable of producing and that absorbed by the propeller, but this does not necessarily mean the engine is inefficient. The engine’s governor in fact reduces the amount of fuel being injected into the cylinders to the level required to produce the necessary power.
When a Gori propeller goes into overdrive, the power required to maintain a given boat speed doesn’t change, but because the propeller itself becomes more effective the necessary power is achieved at a slower shaft and engine speed.
The load on the engine, however, at this new lower engine speed is higher than it would normally be, so more fuel must be consumed to sustain it. In theory, if the boat’s speed has not changed the power demanded by the propeller will be about the same, so fuel consumption should be about the same, with no net gain in efficiency. Put another way, consuming more fuel at a slower engine speed in overdrive should equate with consuming less fuel at a higher engine speed in normal mode.
In practice, it doesn’t quite work like this. The lower the load on a diesel engine relative to the power it is capable of producing, the less efficient it is. Increasing the load in overdrive mode should result in some gain in efficiency. Although this will vary according to engine technology. For example, older engines with traditional mechanical fuel injection will see a greater improvement in efficiency than modern electronically-controlled engines with “common-rail” fuel injection. Efficiency increases will also vary according to application and operating characteristics.
Gori, for example, has data derived from tests on a number of different Hallberg Rassy models showing considerable increases in fuel efficiency in overdrive mode. These results are specific to the boats and the engines that drive them, and you can’t assume you’ll see similar benefits in other applications. But, says Gori’s Technical Manager, Sune Ehrenskjold, “typically, fuel consumption tests show about a 20 percent longer cruising range in overdrive.”
Propeller to Propeller Comparisons
There is another side to this efficiency equation factor to consider. Regardless of whether a Gori propeller in overdrive is itself more fuel-efficient, we must also ask how it compares generally to other propellers in terms of converting shaft energy into boat speed.
On most sailboats even a well-matched three-bladed propeller is, at best, only 60 percent efficient. The other 40 percent of shaft energy is dissipated into the water to no useful effect. Some two-bladed folding propellers are as little as 20 percent to 25 percent efficient.
In tank testing conducted by the University of Berlin for a German yachting magazine, the Gori proved to be 53 percent efficient in normal mode, 54 percent in overdrive, and 53 percent in reverse. This compares favorably with other folding and feathering propellers. The efficiency in reverse is particularly high because of the reversing nature of the blades.