Additional electric motor while leaving original in place and using original ax

Was working on the assumption that the goal was reduced speed on electric. Was assuming the lower prop loading would allow some fudge room in required power.

 

It’s all speculation without any real numbers to crunch!

 

Perhaps the first question is what the goal is? Many people want electric for its own sake, even if it is very expensive and not efficient. The OP is not considering the extra weight and volume of the batteries, chargers and power management.

 

Weight, cost, outcome?

What is the objective?

Put a swivelling 3HP outboard on the bow
(and you get a bow thruster).

 

Outboards are always my first line of thought when people talk about emergency propulsion systems.
They are modern miracles with lots of power, and exhaust, cooling and steering systems all built in.
Unfortunately diesel outboards have not flourished yet, although some are available, not much selection.
Storage of gasoline onboard is another can of worms too.

 

Yes, there is simply no system that can be installed as a hybrid on this size drive that can operate at the same efficiency as the Cat diesel at 20% load. The diesel fuel efficiency might be down by 30% or so, and that might seem like it opens up a lot of options, but it isn't and it doesn't. The actual fuel burned is quite small, there simply isn't enough fuel savings to justify the cost of these systems, particularly on twins. If you double the size of the boat and consider a single shaft setup, and expect a lot of slow travel time or bimodal operation, then things change enough that they are worth a look.

The get-home drives are not hybrids. They have terrible efficiency, but they work. They are often required by regulation on certain single shaft commercial boats. You wouldn't choose to run on one if the main was available.

You could probably save more fuel by looking at the genset and the accessory loads on the Cats - by figuring out how to best manage the timing and gearing of those loads - than you could ever hope to save on propulsion. And rejiggering that would cost very little compared to hybrid propulsion systems.