Assessing Modern Marine Propulsion

By Christopher Swanhart

It used to be there was only one choice for marine propulsion—wind. Steam power was next, then it was the internal combustion engine. Typically, this power has been delivered to the water via submerged propellers on inclined shafts. And even with the multitude of propulsion types available today, and the significant advances in systems efficiency, inclined shafts remain a constant on mid- to large-sized sportfishing vessels. Let’s take a look at why.

Submerged Propellers

Whether going slow or fast, submerged propellers on inclined shafts provide predictable performance, are inherently reliable and can be serviced almost anywhere. For sportfishing boats, this arrangement provides additional benefits. All appendages are under the hull, allowing for clear access to fishing lines and the fish, which is not the case with options like outboard motors, stern drives and waterjets.

And while these other systems do provide better propulsive efficiency at high speeds, submerged props offer relatively high efficiency at low speeds. This in turn means good fuel burn while trolling, which can be most of the hours spent on the vessel. Overall propulsive coefficient—defined as the ratio between the effective horsepower and the shaft horsepower—for submerged propellers can be over 20 percent higher as compared with waterjets at low speeds. The waterjets don’t become significantly more efficient until about 40 knots and higher.

In a practical sense, 50 knots at WOT for most craft with submerged propellers on inclined shafts can be thought of as an upper ceiling. Yes, there are some boats exceeding this speed, but propeller propulsive coefficients, thus overall efficiency, are reaching their limits around this speed with this type of arrangement.

Even though inclined shafts have been around for a long time, new advancements are still being made in attempts to further improve performance. Hull features such as propeller tunnels offer reduced draft, lower shaft angles and some level of lift when backing down. With props in tunnels, shaft angles can be reduced from approximately 12 degrees to 8 or 9 degrees. This results in higher propulsive coefficients and less propensity to cavitate.

The Magnus Effect

Shaft tubes are always a good topic for debate and are used to mitigate the Magnus effect, which occurs when a cylinder is rotating in a fluid. On a sportfish, this is the shaft spinning in the water. The rotation causes a force which adds drag to our arrangement. It’s how a curve ball curves, or a heavy topspin forehand turns downward. To remove the rotating cylinder, a tube is fitted around the shaft. The tube is sized to match the strut barrel diameter to promote clean flow.

It doesn’t rotate, but is quite larger in diameter, so we have more cross section in the flow. This somewhat offsets the benefit of removing the Magnus effect. The shaft tube does introduce some headaches regarding service, and the shaft bearing still needs water to be cooled. But the performance gains
outweigh these slight annoyances.

Wake-Adjusted Appendages

Another relatively recent development in hydrodynamics is the use of wake-adjusted appendages. Geometry of struts and rudders are finely tuned using computational fluid dynamics. This results in somewhat odd shapes that reduce drag by being more flow friendly. The design effort of these customized appendages is time consuming and costly as the water flow across the shafts and through the propellers and rudders is quite complicated. Yet improved performance and fuel burn are not the only benefits. Due to the cleaner flow, noise and vibration can be greatly reduced as well.

Across the board, power to weight ratios have increased greatly for gasoline and diesel engines. In our sportfishing yachts, this means access to much more horsepower for not much more weight. With combined improvements to hull form and appendage design, we are seeing large boats hitting
speeds previously not considered realistic. Massive outboard engines have forced boat builders to alter their thinking. This has left the recreational boat owner wondering when the arm’s race will end. However, inclined shafts remain the standard choice in the world of sportfishing. As improvements are made across all marine propulsion types we see an increase in both efficiency and usability resulting in tough but exciting choices for owners.

Originally from Annapolis, Chris moved from aerospace to the marine industry early in his career.  For over twenty years, he has taken pride in being able to build lasting relationships with many builders of world class yachts. He has managed countless new designs and builds, including patrol and rescue craft, production boats, and high end custom sportfishing yachts, with a design focus on high performance hydrodynamics, hull forms, and structures.