One problem with all these energy density numbers is that they only show how a boat would use the electric motor ONLY or the diesel engine ONLY and compare them. Many of the advantaged to a hybrid, or even pure electric involve ways that you can use then engine other than motoring along at hull speed from point a to point b
I think pure electric systems could be very practical for small boats who sail well and only use their engines to get in and out of harbor anyway. I've been on many a T-bird that departed the dock with only a gallon or two of gas. Pure electric wouldn't be much different.
But pure electric isn't for most. And it's applications are varied and it's merits many. Part of this though is you have to imagine how you would use it differently
to your regular propulsion system. Personally I would kill for the ability to have a hybrid system such as the beta marine system I listed at the beginning of this thread. The advantages for me with just a very modest battery band would be the following:
1: I could add several knots of speed to my light air sailing with very few amps used. This would mean much more sailing and less time with the diesel on instead.
2: If the diesel died, I could motor right up to the breakwater of my marina and use my small battery bank to drive her in to the slip using pure electric.
3: More torque at low speeds means better handling in close quarters.
4: I would have a huge genset (not that I really need it)
5: Regenerative power while sailing at hull speed.
and all this could be done with your regular, or slightly enlarged, house bank. No need to replace your keel with lead acid batteries or spend 20L on Lithium ion batteries. What this system would not do is be the only force driving me long distances. For that, I have my diesel engine, and these funny triangular white things....
The link below outlines the pros and cons of how a hybrid system (of sorts- actually its a pure electric and an outboard) can be used in many different ways. Here is an excerpt from the link below:
Epods in Use
3. Light wind sailing was a pleasure for once. Typically the Gemini is not great in light winds due to her low aspect sail plan and small control surfaces ineffectiveness at slow speeds. However with the e-pods providing just a little push (5 amps) the boat was able to create its own wind to keep the sails pulling and the boat moving. The e-pods providing enough water flow over the rudders to allow you to steer. We sailed past the bow of a 40 foot mono who was hardly moving at maybe 1 knot mostly due to the tide; they were greatly impressed with our speed under sail; they had no idea there was anything helping us along as the outboard was clearly off and up out of the water at the time. With only a breath of wind she was doing between 2-3 knots; just enough to keep me from resorting to the outboard. Keep in mind that this was with 14 people aboard so the boat was heavy and we had been expecting normal heavy summer winds and thus were using the small 100% heavy working jib and full mainsail; not at all the right setup for light winds so the speed was even more impressive given these facts. I am definitely more likely to sail more or should I say motor-sail with the e-pods.