Five Propulsion Strategies for the Marine Sector

There is a growing effort to utilize electric propulsion in marine recreational sailboats and powerboats as well as commercial vessels. There are many benefits to going electric and incentives/funding to help boats convert to electric propulsion. If you are considering electric propulsion, you will need to choose one of the following propulsion strategies:

This page provides information about these five strategies for propulsion systems available to commercial and recreational vessels and each of their advantages and disadvantages.

Diesel/Gas

100% of propulsion and energy loads are derived from combustion engines sized to the propulsion and energy loads needs of a vessel based on vessel displacement, speed requirements and hotel loads.

Advantages :

  • Diesel/gas is the least expensive system to install and diesel fuel has the highest energy density available to marine vessels.
  • Diesel/gas is what boat operators, captains and mechanics are familiar with.

Disadvantages :

  • Diesel/gas creates the most pollution, even with cleaner engines.
  • Cleaner diesel/gas engines reduce NOx, ROG and PM10, but not CO2, which causes global warming. Commercial diesel vessels collectively contribute more global warming emissions than 190 of the 195 countries in the world.
  • Diesel engines operated at low RPMs/speeds run dirty creating soot build-up and increased maintenance costs. This is particularly a problem with cleaner engines such as EPA Tier 4 diesels

Recommended uses: Recreational power boats and long-haul commercial shipping. Electric propulsion is too costly and does not have enough energy storage for recreational powerboats, with a few notable category exceptions such as pontoon boats and houseboats. Commercial shipping goes too far and too high a percent of their time above cross-over speed – the speed below which an electric propulsion system is more efficient than a combustion engine. Of note, commercial cargo vessels can reduce emissions in port by plugging in at the dock rather than running their engine to provide energy for the systems onboard such as what the Port of Los Angeles is doing

Electric-assist Hybrid

A diesel propulsion system that use booster motors or slow-speed motors powered 100% by a generator or the primary engines. This strategy is marketed by engine makers like Caterpillar. It is not a true hybrid system, but more of a diesel engine efficiency strategy.

Advantages :

  • Significant reduction in maintenance cost because the primary engines don’t run at low speeds
  • Slight decrease in fuel consumption and emissions and slight decrease in cost compared to traditional diesel engine

Disadvantages :

  • Electric-assist only achieves minor improvements as electric-assist would cause commercial shipping to create more global warming emissions than 188 of 195 countries in the world, meaning the impact on our environment is negligible
  • Does not satisfy the California Air Resources Board definition of electric propulsion hybrid requiring greater than 30% of operation to be zero-emission
  • Recommended uses : cargo ships operating in higher traffic zones or at slower speeds, tug boats

Parallel Hybrid

A diesel engine and an electric motor are coupled to the shaft/propeller that has been used commonly in busses over the past decade and is promoted by makers of heavy-duty land vehicle systems such as BAE Systems or electric propulsion system providers like Danfoss, or ABB The diesel engine is used at higher speeds and the electric motor is used at lower speeds. The electric motor uses energy from a battery bank that intermittently is resupplied with energy from a diesel generator.

Advantages :

  • Significant reduction in maintenance cost because the primary engines don’t run at low speeds
  • Electric propulsion capable of powering vessel with zero-emission at low-speeds
  • Ideal for vessels that go long distances at high speeds and then operate at low speeds for an extended period of time or that have high energy needs for greater than 50% of a vessel’s operation and low speeds for 5-50% of the time

Disadvantages :

  • Requires two propulsion systems and an investment in large, heavy-duty diesel engines
  • Requires a transmission, which causes gear reduction inefficiencies necessary for a diesel engine to reduce electric motor efficiency
  • System depends primarily on diesel engines making it difficult to integrate future technology advancements easily
  • Represents only minor reductions in global warming emissions as commercial shipping would still create more global warming emissions than 188 of 195 countries in the world, meaning the impact on our environment is negligible.
  • May not satisfy the California Air Resources Board definition of electric propulsion hybrid requiring greater than 30% of operation to be zero-emission.

Recommended uses : Commercial fishing, long-runferries, dredge boats, cruise ships, especially those operating in Norway

Series Hybrid

The electric motor is the only propulsion that turns the shaft/propeller. It gets energy from a battery bank that intermittently is resupplied with energy from a diesel generator. Series hybrid systems are promoted by electric propulsion providers like or EP Technologies, Danfoss and ABB for commercial vessels and Oceanvolt, ElectricYacht, and BellMarine for recreational vessels uses when pure electric won’t provide enough range. Series hybrid systems are best for vessels that operate at high speeds for short distances, or medium/low speeds for medium to long distances.

Advantages :

  • Significant reduction in maintenance cost because system is electric at high and low speeds
  • Reduced fuel and operational costs
  • Quiet, no-fumes operation is better for passengers and crew
  • Zero-emission operation 30-99% of the time depending on use and energy storage capacity
  • Enables emission reductions based on shore charging and reduced engine hours as generators providing power to batteries run intermittently based on need
  • Future proofed against new regulations and technology as the system is modular and any component can be replaced or upgraded without changing the rest of the system
  • Satisfies the California Air Resources Board definition of electric propulsion hybrid requiring greater than 30% of operation to be zero-emission

Disadvantages :

  • Electric motors are not as efficient at high speeds above cross-over speed as a diesel engine
  • Current battery technology does not allow low weight to energy storage options for significant range on batteries alone and power generation beyond battery capacity generally requires fuel-based power generation

Recommended uses : Medium run ferries, pilot boats, commercial passenger fishing vessels, excursion vessels, heavy displacement sailboats, racing sailboats that have a 100 nm emergency motoring requirement, and houseboats.

Full Electric

The electric motor is the only propulsion that turns the shaft/propeller and gets 100% of its energy from a battery bank that is charged by shore power. Pure electric propulsion systems are available from electric propulsion providers like EP Technologies,Danfoss and ABB on the commercial side and Oceanvolt, ElectricYacht, Lynch Motors and BellMarine. for uses when pure electric provides enough range on a battery bank alone. Pure electric systems are best for vessels that operate only at low speeds or go short distances.

Advantages :

  • Significant reduction in maintenance costs
  • Significantly reduced operating costs as electricity costs less than diesel fuel
  • Quiet, no-fumes operation is better for passengers and crew
  • Zero-emission operation 100% of the time
  • 100% future proofed against new emission regulations as it is already zero-emission
  • Future-proofed against technology as the system is modular and any component can be replaced or upgraded without changing the rest of the system
  • Satisfies the California Air Resources Board definition of electric propulsion hybrid requiring greater than 30% of operation to be zero-emission

Disadvantages :

  • Current battery technology does not allow for sufficient range at high speeds or medium to long distances yet

Recommended uses : Short-run ferries, short-run excursion vessels, hydrofoiling recreational power boats, cruising sailboats, and performance sailing catamarans

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