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Further Marine Hybrid Drive Thoughts for our Explorer Yacht

Updated: Apr 28, 2023

Selecting the correct marine hybrid drive configuration for our Explorer Yacht was a learning curve, the biggest hurdle for us? Presupposed knowledge; we did not know what we did not know. With that in mind, selecting a good solution was about becoming educated by the various potential suppliers as it was about the final price. As in all walks of life, the lowest-cost answer is not necessarily the best. Here is an example of why.

Marine Hybrid Drive - Boundaries

Firstly let us set a few boundaries.

  1. 30kW electric drive on each shaft giving 60kW in total or about 6/7 knots hull speed.

  2. 120kWh battery capacity

  3. 120kW diesel at 2300 RPM, minimum speed at idle about 500 RPM.

Marine Hybrid Drive - Electric Motors

The first consideration is to select an E-motor that can produce 30kW. More importantly, it must do this at the speed at which the propeller will absorb 30kW, in our case, about 1000 RPM (engine) before the main gearbox. Have a look at the enclosed graph. The correct speed is where the solid propeller curve line intersects the chain dotted 30 kW E-motor curve.

Look also at the other dotted 30kW E-motor curve. The difference a result of the PHT or Parallel Hybrid Transmission. The function of this device is to step down the E-motor RPM meaning a smaller, faster running, and less costly E-motor can drive the shaft. The torque required is reduced by PHT Ratio, in our case 1.5:1. You can see also that up until 1000 RPM, the available torque from the E-motor is higher than the power absorbed by the propeller at that speed. There is sufficient reserve to accelerate the yacht up to its design speed at 1000 RPM (engine).

Can you also see the two solid lines? One is the propeller power absorption curve, and the other is the available engine power for various speeds. Again it is higher than the propeller curve up until the maximum rated power (120kW at 2300 RPM (engine)). The engine curve starts at 500RPM as that is the idle speed for the engine; below this, it will stall. However, the E-motor can operate below this speed; there is no need for a trolling function on the gearbox. In Hybrid mode, we can work as slow as we wish for extended periods with the diesel engines turned off.

Propeller and power curves.

Advantages of a step-down PHT drive

There are two other advantages of this arrangement. We can drive the E-motor as a generator. At 1000 RPM (engine,) it will produce slightly under 30kW output. However, the engine can rotate at up to 2500 RPM, resulting in over 90kW generation capacity from each E Motor. With 120Kwh of batteries, we now know we can recharge the batteries from 20% capacity to 100% in about an hour. Given we can run 4 days at anchor on these batteries without running the generator, that means 1 hour or so of engine operation every four days at anchor: near-total silence, all the time!

Fitting a PHT was a cost addition we believe is well justified as the advantages in this setup are manifest. It is not the correct solution for all problems, but with our power needs, battery capacity, and operating profile, it should serve us well.

As I said, the lowest cost solution is not necessarily the best one. Our Parallel Hybrid Drive solution and Twin Disc main gearboxes are supplied by Esco Power in Belgium

Credit is given for the illustrations taken from their web site.

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