The Nigel Calder Science of Hybrid propulsion article is an excellent in-depth study of parallel and serial hybrid propulsion, but it is not up-to-date.  His study is missing two important aspects of current day hybrid systems: (1) regeneration while sailing and (2) lithium batteries.  Therefore his fuel consumption for electric only propulsion is off if one is using lithium batteries, which can accept a much higher current (and charge more quickly) and more than half their battery charge comes from regeneration .  He even hints this when his says things would be different with solar power charging the batteries in the bottom of Part II of the article.  I also think that the generator input was AC, not DC, which gives higher loss at the charger b/c of the inverter.  Because he didn't have lithium batteries, the diesel could only trickle charge the batteries or direct power.  While this added some efficiency, it means the diesel was probably not operating at peak efficiency.  While the Polar Power DC generator specs g/kWh is about 290 (peak diesel efficiency is 230), they say RPMs and windings could still be changed to make it more efficient. 

He does makes several important points:
  1. An electric motor is most efficient at high RPMs, so a smaller prop is sometimes better.  However this counters what the guy in Top Secret said
  2. Most of the time a diesels are revving well below their peak efficiency when used in sailboats.
He also defines a term crossover speed: a theoretical speed below which a hybrid propulsion system is more efficient at moving a boat than a conventional propulsion system.  Above this speed, a conventional system is more efficient.  His biggest point about crossover speed: " This is the Achilles heel of a serial system.  If vessel operation requires sustained running above the crossover speed—and often, cruising speed will be above the crossover speed—the losses will rapidly overwhelm any gains below the crossover speed, for a net loss of efficiency. "

Graphs from the Nigel Calder Article

Diesel Fuel graph
Diesel Fuel consumption vs speed on 48ft Monohull
crossover graph
Crossover speed estimates.

The chart to the right above shows the smaller prop outperforming the larger prop when it comes to crossover speed efficiency, but no one wants a grossly undersized propeller on their boat, especially since an electric motor has more low end torque to drive it. 

In all his analysis is sound, I just wish he would redo it with an OceanVolt Servoprop and lithium batteries. 

Estimations for our system

Using Calder's info above and OceanVolt calculations (confirmed by our experience with the hybrid Voyage 480), we can do some estimations that look pretty good for a SHPS.  If we put two 15kW OceanVolts on 44ft (svelt) lightweight performance Catamaran (~22klbs loaded) we should be able to drive it at 6knots with ~13kW battery draw (yes estimates by OceanVolt, but in the ballpark with our observations when we were on the 2ft longer and 4,000lb heavier Voyage 480 with the same motors).  So with a 30hp Polar Power DC generator to pump 14kW into our batteries we would consume 1.2gal/hr (290grams/kW * 1/3255grams/Gal * 14kW).  The Nautitech 40 we just sailed on used 2.5gal/hr at 6knots (two engines at 2100rpm), so we should be able to run at cruising speed on a SHPS using half the gas.  So that's double the range right there, not even at a slow speed but decent cruising speed.  If we slow down to harbor speed (~2.5Knots), where the electric motor is best, the battery burn is 1kW on the SHPS (1/13th of cruising speed).  Unfortunately I don't have diesel data for that speed, but extrapolating from the Nigel Calder data above, it looks like the diesel consumption will go down to a little less than ⅕ of the 7knot burn or ½ gal/hr.  So when slowing down to harbor speed the electric motor reduction is 2.6 times greater than the diesel, which means the electric consumption savings is 2.5 times more than a diesel (1/13th vs ⅕). 

So even if the harbor speed extrapolation is off by a factor of two, there would still be nearly ⅓ less energy consumption by the OceanVolt SHPS in the harbor vs diesel.  This kind of calculation is what is missing in the OceanVolt whitepaper.  Therefore, even without factoring in regen during sailing, an electric motor should be more efficient as long as we stay under 6.5knots