Originally Posted by bvander
Ref electric hybrid here is one example of a cruiser who did just that.
With modern DC Stator electrical
motors you do gain some efficiencies.
The diesel gen set is typically smaller than the diesel propulsion unit. Also an electric motor has a better torque profile than a diesel engine, which is usually optimized at cruising RPM. Big advantage for Cats, as one diesel motor vice two, and DC motors in the hulls much less space requirement.
No real weight advantages as need extra batteries.
There is typically a fuel saving.
For the same available power, the diesel gen set is NOT SMALLER. It must be larger both in HP and size to produce the same HP at the prop.
The main criteria driving the choice of HP is being able to maintain a steady speed while fighting a strong headwind or bashing into waves. In that situation, the conventional diesel is putting out it's peak continuous HP with relatively small losses in the transmission. A diesel electric would have greater losses converting mechanical energy to electrical energy and back to mechanical energy, so the generator would need to be rated for more HP. (shaft and prop losses would be roughly the same)
When people put in electric motors and use lower HP, they are accepting less capability but that lower capability could just as easily be achieved with a smaller HP conventional drivetrain.
The instant torque is nice around the dock
(assuming you add expense by providing a battery
to create a true hybrid, without the battery
to provide the surge, the electric motor can only put out what the generator is producing) but since it is exceedingly rare to use max HP docking
, it doesn't typically drive the HP requirements.
Now if you were designing a ferry
that crosses a 100m channel dozens of times a day, top speed may not be critical but acceleration and deceleration could benefit from that instant torque with a modest battery bank that fills back up while the cars are loading.
This is also why electric cars often have lower HP electric motors. It may only take 50-60hp for a small car to maintain freeway speeds but a 50-60hp conventional drivetrain would be horribly sluggish in stop & go traffic as peak torque typically requires the engine to wind
up to a higher RPM. With electric motors, peak torque is available from a standstill, so even with lower HP, it can still feel peppy off the line.
Hydraulic has been tried but it's more complicated and expensive and as others have suggested, likely to reduce resale value as it will be an oddball system.