This has been covered in the past in the
engine forum. To sum it up, electric is only an option if you plan on daysailing and never straying far from your electric hookup. It is the same problem as with electric cars, range. I have built 3 electric cars and they make great commuter vehicles but none of them would
work for a multi state drive.
The reason this is a problem is energy density of your
power storage. Diesel
fuel is incredibly energy dense and it can be moved quite quickly through a hose.
Batteries have poor energy density and cannot be charged very quickly. To figure out how much
battery capacity you would need is actually quite easy. For most boats, a reasonable approximation of the power draw would be to divide the horsepower of the diesel
engine by 2. Then convert that to kilowatts and multiply it by the maximum number of hours you would ever want to be able to
motor for. Remember that this is the maximum number of hours between
shore power hook ups not just the maximum amount at any one time. This gives you the number of Kw Hrs that you need to store in your battery bank. If the number of
batteries this requires hasn't scared you yet, you need to start taking into account things like reduced capacity due to age/temp, reserve capacity etc. I ran a rough calculation for a 35' boat a year or so ago and came up with 18 Trojan T105s. I can't remember how long it could power the boat for off the top of my
head but it wasn't exceptional.
There is a common misconception that you can greatly increase your range by adding a
solar panel,
wind generator, or gas
generator. It comes back to simple energy conservation, if you don't put as much energy into the system as you take out, you will be depleting your batteries. These things will increase your range but not by much. Considering that a 30' boat motoring at a conservative speed is consuming 5kw of power, that would be a large
generator to keep up and doing it with
solar panels or a
wind generator is out of the question even in ideal
weather conditions.
This brings you back to needing to store all of the energy that you will need. Since it is hard to change the power
consumption, the only way to change the number of batteries you need is to use it for less time. For this reason, it only really makes sense for daysailors that are kept at docks with
shore power or cruising boats without a schedule to keep that avoid bad
weather.
There are many other design considerations if you actually want to go ahead with an electric drive system like voltage, gearing, ac vs dc, etc that I would be happy to talk about. At this point in time, it makes much more sense for the vast majority of people to use an inboard diesel.