Why Aren't Diesel-Electric Motors More Widely Adopted in Semi-Displacement Yachts?

The Intriguing Question:

Given that diesel-electric motors act as a continuously variable automatic transmission, it is intriguing to wonder why they are not more widely adopted by builders of semi-displacement yachts. One possible reason could be the expense and weight of such motors, which can be prohibitive for smaller boats, despite their benefits in terms of fuel efficiency. Perhaps a marine architect or yacht builder could provide a more detailed explanation.

Cost and Weight Considerations:

For large cruise ships, the cost and weight of diesel-electric motors are less of a concern. However, for smaller boats, these factors can significantly impact the feasibility of such systems. The expense and weight associated with implementing diesel-electric motors might make them less attractive compared to other propulsion solutions.

Variable Pitch Props and Fuel Efficiency

Another factor to consider is the availability and effectiveness of variable pitch props. These have been in use for decades and can offer significant benefits in terms of optimizing mid and top-end efficiency. If a diesel-electric motor is considered, using variable pitch props can further enhance the performance and efficiency of the yacht.

The Role of Prop Design

Think of the prop as a variable ratio transmission, depending on the blade geometry and screw angle. This concept is crucial in understanding how prop design impacts the overall efficiency of the yacht. One key consideration in boat design is hull speed, which must not be ignored. Efficiency of travel is extremely dependent on the speed relative to hull speed. While it is complex, boat designers often model these elements to get a first pass at a design. Most boats that are used for fixed positions like docks do not require extensive engineering on optimizing travel efficiency, as these boats are mostly stationary.

The Role of Propulsion in Boats

Slapping water with a blade is inherently inefficient, no matter what transmission is used. This fact is a significant drawback in boat design, making the search for more efficient solutions crucial. Additionally, diesels have a flat power curve, which makes the transmission less critical for them compared to other types of engines. While diesel engines are good for sustained lugging (continuous low-speed operation), they do have their drawbacks, including pollution.

Conclusion

While diesel-electric motors offer many advantages in terms of fuel efficiency and operational flexibility, their adoption in semi-displacement yachts is hindered by factors such as cost, weight, and the ease with which variable pitch props can be utilized. Balancing these factors with the inherent inefficiencies of propulsion in boats is a continuous challenge in the field of marine engineering.

Keywords: Diesel-electric motors, semi-displacement yachts, variable pitch props, hull speed, fuel efficiency