This is the big question. All the DE pushers out there replace the traditional diesel with an electric motor rated 30%-65% of the diesel rating. Understandably there are losses, but rather than start from the diesel, subtract losses etc and work backwards I want to see what it takes to move my boat, starting from the hull rather than the engine. I want to look only at propulsion efficiency and effectiveness here. Bahia specsLWL: 29ft Disp: 22000 lb , 9.8 long tons. Bahia calculations (Gerr)These are based on David Gerr's "Propeller Handbook" and several accepted equations.
SL = Speed-Length ratio Speed = sqrt(LWL) * SL LB/HP = displacement weight (lb) over HP (per Gerr table) SHP reqd= HP required corresonding to LB/HP (per Gerr table) kW = SHP reqd converted to kW Note (1): this is estimated. Gerr's table does not go beyond 1.07 SL, at 1000 LB/HP But…At first glance it looks like replacing 42shp (31kW) with 20shp (let's call this 16kW). The DE pushers all claim torque is the answer. So let's look at calculated Diesel torque vs. rated motor torque. Rather, for each of the Emotion Hybrids motor, look at the kW, HP as converted from kW (HP), and HP as calculated from rated torque (HP(c)). The formula for this is HP=T x RPM / 5252 RPM for the eMotion Hybrids motors is 1000 For sanity, I'll also take the rated torque and convert that back to a diesel engine using HP= T X RPM / 5252 at 1800 RPM and 2200 RPM, and add 25% to the output HP to take into account system loss. I'll note this as HP(d) –HP(diesel equivalent)
This seems to show some clear advantages to the DE system over a straight diesel engine, but, it takes into account a 25% loss for diesel engines and 0% loss for the D-E systems. The Perkins 4.108 is rated at 55bhp @ 4000RPM and max torque output is 83 lbf-ft at 2200RPM based on the 1983 workshop manual. Of course, this is at the Engine end, and if you assume 95% efficiency on the reduction gear, the max torque falls to 79 lbf-ft. (The Workshop manual prefers "lbf-ft" as a unit of Torque to "lb-ft", but I believe they are equivalent.) So how do all these figures square? I feel I'm missing something obvious here. It's worth noting that with the Perkins, I usually push at 5.5 kts at around 2000 RPM…but I forget if that '2000 rpm' is 1800 or 2200. |