Real-World Comparison Reveals Big Savings

Real-World Comparison Reveals Big Savings

Conventional Vs. Hybrid Propulsion Systems in Work Vessels


Owners and operators of tugboats, dynamic positioning vessels, ferries, fishing boats and research vessels are seeking a propulsion solution that not only meets their maximum performance needs but also optimizes their fuel, maintenance and overall lifecycle costs.

Conventional diesel-mechanic propulsion systems operate most efficiently in towing, ship-assist and bollard-pull operations, but their use is much less efficient in standby, position-keeping and transit modes. Unfortunately, the latter is where these vessels spend the vast majority of their time.

To address this challenge, Caterpillar has developed a hybrid propulsion system that switches between diesel-mechanic and diesel-electric operation. It’s designed to help vessels meet their contractual requirements for performance and speed while minimizing operational costs. This paper outlines the differences between the two systems and the results of a recent customer case study.


A conventional diesel-mechanic system features larger main engines and fewer generator sets, while a hybrid propulsion system consists of smaller, lower-rated main engines, additional generator set capacity and added booster motors and drives.


Conventional Diesel-Mechanic Propulsion System
Hybrid (Diesel-Mechanic + Diesel-Electric) Propulsion System


Regardless of operating mode, a conventional diesel-mechanic system requires the main engines and one or two generator sets to run at all times to power the Z-drives and electrical loads, respectively. A hybrid propulsion system, on the other hand, uses only the amount of engine power required for the task at hand.  Here’s an example of how it works in various tugboat applications.

Standby & Low-Speed Transit Mode

In this operating mode, the hybrid system operates in full diesel-electric mode. Use of smaller generator sets instead of the main engines reduces fuel and maintenance consumption, and the higher engine load factor reduces NOx emissions.

Standby & Low-Speed Transit Mode Standby & Low-Speed Transit Mode
Standby & Low-Speed Transit Mode

High-Speed Transit & Light Towing Mode

Here, the hybrid system operates in full power mode with the main engines powering the thruster and the generator sets powering electrical load. Full main engine power is available for propulsion.

High-Speed Transit & Light Towing Mode High-Speed Transit & Light Towing Mode
High-Speed Transit & Light Towing Mode

Full-Power Operation

To achieve full power, the hybrid system’s main engines and booster motors operate together, delivering the maximum available performance and the best possible acceleration response from both the engines and the generator sets.

Full-Power Operation Full-Power Operation
Full-Power Operation


  • Higher average engine load: The hybrid system runs only the engines/generator sets necessary for a particular operating mode.
  • Significant fuel savings: Allowing for low-load operation with one small generator set while the vessel is in standby or position-keeping applications may result in fuel savings of as much as 50 percent.
  • Reduced maintenance costs: Operating hours on the main engines are reduced.
  • Increased uptime: A swing program allows generator sets to be serviced and tested in the shop rather than on board the vessel.
  • Increased redundancy: Engines or motors can power azimuth thrusters, and generator sets or alternators can be used for electricity production. In addition, the main engines can be used to power a water pump for firefighting applications.
  • Better low-power maneuvering: Running the electric motors at very slow speeds improves fine maneuvering, particularly for vessels with fixed propellers.
  • Improved load response time: The high-torque electric motor supports the main engine with RPM increase.
  • Reduced emissions: Higher average engine load results in cleaner fuel burn. (NOTE: This may not be true for all emission types if the main engines use an aftertreatment system but the generator sets do not.)
  • Smaller DEF footprint: Because the main engines are not running during the majority of operation, not as many DEF storage tanks are required.



Harbor Docking and Towing was seeking a propulsion solution for two new tugboats that will operate in Lake Charles, Louisiana. To help the company make an educated buying decision, Cat Marine and dealer Louisiana Cat entered engine load history data from Harbor Docking and Towing’s existing vessels into the Caterpillar Engine Value Analysis tool to provide an accurate lifecycle cost analysis.

The Caterpillar team also entered vessel requirements, operational and resistance profiles, propulsion system data and power generation/conversion information into Cat Select — a new performance and efficiency evaluation tool that compares propulsion system options.

Vessel Requirements

  • Minimum of 80 metric tons of thrust
  • Minimum of 12 knots of free running speed
  • ABS, no ice
  • Meet U.S. EPA regulations for January 1, 2018

Operational Profile

  • 4,000 operating hours per year
  • 50% in standby or position-keeping mode
  • 35% in transit mode (70% at 6 knots and 30% at 11 knots)
  • 15% in towing or ship-assist mode (65% at 5 knots/30-ton thrust, 30% at 2 knots/60-ton thrust and 5% at 0 knots/80-ton thrust)

Assumptions included twin screw Z-drives, 100kW maximum hotel and winch load, and hull resistance based on model tests of 30m and 80 metric ton bollard pull. Maintenance costs were considered, and the financial investment was distributed over 10 years with a 4% interest rate.



Using this data, Cat Select generated a report showing the differences in utilization, fuel consumption and total cost of ownership between a conventional diesel-mechanic system and a hybrid propulsion system.

Utilization Comparison

With the conventional system, utilization time for the two 3516C engines and C4.4 generator set is 100 percent. Average load for the two main engines only reaches 12 percent. The hybrid system consists of two smaller main engines (3512Cs) plus three larger generator sets (two C18s and one C7.1). Utilization time on the main engines is just 25.5 percent with average load reaching 52.9 percent.

Utilization Comparison Utilization Comparison
Utilization Comparison

Fuel Consumption Comparison

When the tugs are in transit, fuel consumption is expected to be about the same for the conventional and hybrid systems. In work mode, the hybrid system likely will consume slightly more fuel. However, in standby mode — where these tugs will spend 50 percent of their time — the hybrid system should reduce fuel consumption by more than half.

Fuel Consumption Comparison Fuel Consumption Comparison
Fuel Consumption Comparison

Overall Cost Comparison

Although the initial cost of the Cat hybrid propulsion system is slightly higher (5 percent), the anticipated savings in fuel and maintenance over time should more than outweigh the upfront investment. The Cat Select tool’s estimates indicate that Harbor Docking and Towing will reduce its annual costs by 6 percent by choosing the Cat hybrid propulsion system over the conventional diesel-mechanic system.


Overall Cost Comparison Overall Cost Comparison
Overall Cost Comparison


As a result of the Cat Select study, Harbor Docking and Towing made the decision to purchase the Cat hybrid propulsion system for its two new tugboats. This “stem-to-stern” solution will consist of 3512E main engines, C18 generator sets, C7.1 generator set, shaft lines, MTA 628 azimuth drives, booster motors, VFDs, switchgear and a fully integrated, simple-to-use control system. 



To learn more about Cat hybrid propulsion systems, and to take advantage of the Cat Select tool to make an educated decision for your operation, please contact your local Cat dealer.

Ask the experts at Caterpillar Hybrid Propulsion Systems