Hybrid Marine Propulsion - Why Bother?

Vanguard will have twin hybrid diesel-electric drive systems. New technology, is it simply fashion or do they have a practical use? Their facility on yachts is not the often quoted but incorrect assumption of operation over long distances on electric power. We are not going to save any puffins this time. For the true answer, read on.

Illustration - an early engine room layout showing position of John Deere 4045, step down drive from Esco Power, drive motor/generator and Power battery from Praxis Automation. Note battery positions later moved forward of the engines to improve trim (they weigh 1300kg each!).

Hybrid Marine Propulsion System Design Criteria

In the case of Vanguard, XPM-078, what we needed was three fold:

substantial power storage capacity to limit engine hours under light load. This also allows us to remove two dedicated diesel generators, relying instead on the hybrid motors and 6.5kW of solar panels.
easy and quiet implementation of dynamic positioning with flexibility to drive two propellers from a single diesel engine or no engine at all.
access to environmentally sensitive areas that intend to ban operation under diesel power (think Scandanavia).

In sympathy with our readers, we will divide the design considerations over several blogs as it's quite an involved topic. For this blog, let us consider issues of redundant system design, flexible shore power capability, and system resiliance. In this discussion, we will refer to the system drawing attached, N21.4051-F01, provided by Praxis Automation, our partner in this project.

Visiting Praxis Automation - Part 2

To engineers or those who cannot sleep, the full installation file is here:

Hybrid Marine System Redundancy

Commercial systems lean heavily on redundant system design to improve overall system availability. This consideration goes hand in hand with an understanding of MTBF or Mean Time Between Failures. Thus, there are no point in onboarding two mutually redundant but intrinsically unreliable critical systems. Essential components in both systems need a high MTBF, typically above 25-50,000 hours for each assembly. The effects are cumulative, reducing the overall system MTBF. So, referring to the drawing, we have a system split port and starboard, wholly duplicated and able to run independently. To increase overall availability further, we can connect the power batteries or generator on either side to the motor on the opposite shaft. We can also link either power battery to the vessel's 3-phase AC Grid.

Hybrid Marine System Shore Power

Onboard Solar power is converted to 24VDC and fed to the house batteries.
Shore power at 230V 50Hz and 120V 60 Hz single phase is converted to 24VDC and fed to the house batteries.
Shore power 60 Hz 3-phase is converted to DC and charges the one 60kWh power battery. 50 Hz 3-phase is connected directly to the 3-phase AC bus bars.
In case of failure, both 50 and 60Hz shore power connections use the same components; they are interchangeable, negating the need to carry critical spares.

Hybrid Marine Propulsion Flexible Drive System

Vanguard has 30kW Praxis DC drive motors connected in parallel with 120kW John Deere diesel engines. The motors drive at 1000RPM shaft speed through a 1:4 reduction gearbox from Esco Power. At 1000 RPM, they provide 30kW of propulsion power. But, disconnected from the propeller, at 2300 full engine speed, they will generate 90kW of power to charge the batteries. 125kW capacity inverters connect to nominally 30kW motors that provide 90kW when running as generators. This gives us a degree of future-proofing and a short charge time should battery capacity improve over time.

Hybrid Marine Propulsion Battery Storage

Two by 60kWh power battery banks can charge at 3C or three times its maximum discharge rate, so we can fully charge both power battery banks within 2 hours. We can also drive one shaft line, propeller, and generator from a single diesel. The other motor can use generated power to provide 30kW propulsion power to the opposite shaft at sea. So two propellers can operate from a single diesel engine though at slightly reduced capacity. 30kW is a reasonably modest power, but with a slippery hull shape, we should achieve 5 knots, ample with about 4 hours running or all day keeping station near the shore.

Marine Propulsion Control

A duplicated Ethernet (RS485) backbone connects the engines, inverters, and controls. Standard commercial/military practice can survive multiple fault conditions in replicated locations yet still function. Ethernet connects the helm to the engine room & forepeak bow thruster. All other system cables are run centrally to the engine room. This dramatically reduces the weight of cables needed to connect various points on the vessel and to that extent parallels the move to digital switching on systems like Maretron running on NMEA2000 protocol.

Read also: A Novel Water Glycol Cooling System for our Explorer Yacht

Read also: Wet Exhaust System Design - Developing Basic Design Rules

So there you have it, assuming everyone is still awake. Hybrid yacht design is a "battery-centric" system and provides convenience unavailable in a traditional drive line. A robust, fully redundant, flexible hybrid drive design by Praxis Automation, powering XPM-002 Vanguard to the ends of this earth.

Chris Leigh-Jones

One Grandfather learned to sail on his father's topsail schooner, dependent on the wind and tide. The other learned on steam turbine driven transatlantic liners. My training was on double acting B&W or Dofxord's and Alfa Laval steam turbines. But time influences all, today we have hybrid drives and skysails, my firstborn now teaches sailing to soldiers. "Plus ça change, plus c'est la même chose", it always sounds better in French.