It doesn't make sense if the only consideration is saving money.
But consider a boat with a bunch of solar, a bunch of LiPO batteries, and a parallel hybrid system:
Household appliances with no worries about electric usage
No generator necessary
No shore power necessary
Redundancy (get home on solar)
Charge the batteries (when solar doesn't get it done) in a short time
Quiet and fume-free when using electric propulsion (canals)
Lots of torque at 1 rpm using electric propulsion (docking)
Diesel power when you need it
Does it save money? Who cares.
I think there are two points in response to this.
First is that you are describing a number of what I would call life-style features, and I don't mean that in any negative way. They increase your enjoyment of the boat, and that's the primary goal, not efficiency. And in some cases there are efficiency gains as well. Solar, for example, to offset generator run time, has both lifestyle and efficiency gains.
The second point is that when it comes to propulsion, the numbers just don't add up. Very few boats can fit enough solar to cover their full house electric load. A few people here have come close, and one or two have completely pulled it off. But they are the exception, not the rule. There just isn't enough space on the typical boat, compared to the power loads.
And that's just for house loads, which are a tiny fraction of propulsion loads. Even if you have space for say 2kw of solar, that will yield about 10kwh of power harvested per day. Let's assume you use half of it for house loads, leaving 5kwh left over for propulsion.
A typical trawler might operate with 50kw (about 75hp) of engine output for normal cruise. That will consume your 5kwh of remaining power in 6 minutes. For reverence, the Chevy Volt motor mentioned in the original post is 111kw, so twice the power of my example, and half the run time at full load.
The Chevy Volt battery pack is 17kwh. With the 50kw cruise example, the battery would be drained in 20 minutes. That means the diesel gets started up pretty much right away, and the solar is pretty much useless for propulsion.
To put this in perspective, a gallon of diesel contains 38kwh of raw energy. That has to get converted into usable energy by the engine, and it does so at about 25% efficiency, so each gallon of diesel is equivalent to about 10kwh of stored battery power. That means the volt battery pack hold the equivalent of about 1.7 gal of diesel.
Let's look at it another way. Say you trawler holds 400 gal of diesel. That's 4000kwh of stored, usable power. Now let's pretend we have a magical battery pack that can store 4000kwh of energy, and see how long it will take for the previously mentioned 2kw solar system to charge the batteries. With 100% of the solar output charging the battery, it will take 400 days (yes, that's over a year) to charge the batteries. Another way to look at it is that a 2kw solar array makes diesel at about 1 gal per day.
With all this in mind, you might wonder how the various hybrid boats are working out, and the answer is not very well. If you look closely at all of them, they operate at very slow speeds when on electric drive - on the order of 3-4 kts. That's like running around with a 15hp outboard pushing your trawler, and at that speed you can run long enough to not be a total embarrassment. If you want to operate like that, then it might work and you can enjoy a quiet ride. But if you want to run a trawler in any sort of typical operation, electric drives just don't make any sense and you are much better off with a simpler, less expensive, and more efficient gear drive.
