The efficiency of electric propulsion

T

Tom Hruby

Member
As a scientist I like to collect data and play with numbers. Now that I have data from two electric boats (a 26 ft St. Pierre dory and my new C-Dory 22 cruiser) I want to share some comparisons between electric and gas. At 5mph my C-Dory uses 1600 watts (going with a tide of about 1/2mph) to 2400 watts going against the same tide and a head wind). Averaging that out it comes out to about 2000 watts for 5 mph, 2 kWh per hour, or 400Wh per mile. Data from my old St. Pierre Dory was about 20% higher since it was heavier and had more windage.

Maximum range with my 20kWh batteries comes out to about 50 miles at 5 mph. Maximum speed was 6.8 mph with the two motors maxing out at total 5800 watts. This matches very well with the equation for power vs. speed I adapted from Dave Gerr's Propeller Handbook.

To reach planing speed of 11-12 mph for the 22' C-dory I would need about 12kW of power (based on results from Dave Gerr's equations for both displacement and planing hulls).

COMPARISON WITH GASOLINE
1 Gal of gas holds about 33-34 kWh of energy.
Converting my power needs to gasoline equivalents:
At 5mph I am using less than the power in 1 cup of gasoline (7.5oz) per hour.
My 20kWh battery pack will get me 50 miles for the equivalent of 10 cups of gas (2.5 qts). On a per mile basis that comes out to 1.5 oz of gas per mile.

To get to planing speed I need 12 kW of power which is the equivalent of about 0.35 gal/hour of gas.

Setting up a C-Dory with electric propulsion that will let it plane is no longer out of reach (except maybe $$). There are a number of powerful electric outboards and battery packs available that would get the boat on a plane.
 
Tom Hruby":3myebdfq said:
...

COMPARISON WITH GASOLINE
1 Gal of gas holds about 33-34 kWh of energy.
Converting my power needs to gasoline equivalents:
At 5mph I am using less than the power in 1 cup of gasoline (7.5oz) per hour.
My 20kWh battery pack will get me 50 miles for the equivalent of 10 cups of gas (2.5 qts). On a per mile basis that comes out to 1.5 oz of gas per mile.

To get to planing speed I need 12 kW of power which is the equivalent of about 0.35 gal/hour of gas.

...
Click to expand...

Did you allow for the efficiency of a typical ICE outboard when calculating you gasoline comparison?
 
The comparison is for total energy use and I did not try to compare it with any specific outboard. The numbers represent the actual amount of energy I need to move my boat. It includes all the energy losses in the motors, props etc.

If you want a comparison to a regular outboard, here is an example:
Assume that your gasoline outboard uses 1 gal/hour at 5 mph to move your
C-Dory 22. That means that you have used 34kWh (kilowatt-hours) of energy in that one hour, since that is how much energy is stored in 1 gal.

Now, to travel that same distance at 5mph my two electric outboards only use 2kWh of energy. This is less than 6% of the energy used by an outboard to cover the same distance at the same speed.

My estimate on fuel consumption at 5mph for the outboard is just a guess based on some numbers I found on the web. Please let me know if anyone has some real data so in the future I can be more accurate.

Here are some more numbers: Assume the price of gas is $4 per gallon. That means that 1 kWh of gasoline energy costs about $0.11. This is about what I pay for my electricity. So the cost of energy to travel 5 miles using the electric outboards is about $0.25 (includes losses during charging) vs $4 for gasoline.
 
Tom et al.;

Actually, the real total energy use might not be as outlined above. For many decades, I've tried to teach about energy to students at the University of Washington in an intro Env Sci course. A significant problem with propulsion of anything is that they typically advertise only the end use, and tailpipe emissions. Zero emissions is total BS. Wouldn't your energy use with the electric motor be more like this:

2 kWh energy from batteries

= 2 kWh x some percentage of energy lost as heat from charging source. Note that your battery and charger get hot when you use them. I've seen 64-87% charging efficiency advertised for cars, but my own measurements of "energy-in vs. charge to LiFePO4 battery) are less, so when you charge a battery from the grid, lets say 60%, which my watt meters says is probably pretty good, thus

2 kWh / 0.6 = 2.9 kWh

Now the real crux...original source of your energy. There is really no single way to do this, and trying to find published info is very frustrating. In doing this for home heating, which I used in classes, even the utilities seem to try and confuse you.

Electricity is generally kWh
Natural gas is in therms
Propane is measured in gallons or lbs
gasoline, diesel, is measured in gallons
wood is in cords
wood pellets is in pounds or tons
coal is in tons
water, wind power and photovoltaics are very different since their source is renewable, well actually so is wood and wood pellets
Nuclear, well, I'm not sure how we evaluate it, but it is a resource that can be depleted like coal and gas

Most American electricity, about 60% is from fossil fuels, and conversion is not good, about 34% for coal, and 40% natural gas, meaning 60-66% (I'll use 65% loss or 35% efficient) is lost mostly as heat. Sometimes that waste heat can be used, but usually not much. A good example in a car is how the waste heat from gasoline or diesel is used to heat the car, but that heat must be generated from the battery for electric cars, greatly reducing the miles per charge.

So, our original 2 kWh now becomes

2 kWh to battery / 0.6 = 3.33 kWh to wall socket
3.33 kWh from wall / 0.35 = 9.5 kWh from most common American energy source

9.5 kWh is very different than 2 kWh.

What about CO2 emissions? I won't go there since I really can't get the data.

Battery power is nice, quiet, and no emissions, AT THE SOURCE OF USE! If our grid ever becomes totally renewable, even then, there is a lot of energy use in creating it, and any system will eventually need to be replaced.

Of course, it requires energy to provide gasoline (recovery of petroleum, refining and transportation) for sale. I don't know how much that adds to the inefficiency of a gasoline outboard, but it may be considerable.

That said, I am very jealous of your battery-based based C-Dory. I have a 24-volt MinnKota on my bow, and I've been using my 1103 Torqeedo on the transom mount occasionally to troll and move, and for the dingy. I love the quiet and lack of smell, and I am usually not in a hurry. I did use these to move my CD22 about 4 miles back home once when I ran out of gas, but usually just for trolling and moving short distances.

I also gradually changed other things to a zero-turn Ego electric riding and push mower, Rad electric bikes, Dewalt and Eco electric trimmers, and chainsaws. I rarely use the gasoline equivalents, since I like the ease, quiet and no smell of electric.

I also navigated my canoe through the Everglades about 120 miles last winter with just a paddle, the Torqeedo and a Torqeedo 50 watt solar panel. It was good at first. The bad came when strong wind came from the wrong direction so I couldn't make headway with the paddle, and it became overcast so I couldn't charge the Torqeedo batteries very fast. I spent an extra 3 days camping on a beach until it cleared and I could charge the Torqeedo. I texted my wife to let the park staff know not to rescue me. At that point I wished I'd have had a gasoline motor with gasoline's enormous energy density.

Please take this in the way in which it is offered, just for our polite discussion.
 
If you're going to count the recovery, refining, and transportation of fossil fuels in your calculations, you also need to count the cost of extracting, refining, and transporting the materials used to make electrical propulsion systems, not just the effects of generating the power used to charge them.
 
Rob,
I agree with you that most calculations don't take into account all of the externals, and I was not trying to do a complete energy comparison between the electric outboards and gas ones. There are many factors involved, but in general I can say that here in the Pacific Northwest electric power is environmentally better that gas. I was just interested in figuring out how much less energy is needed at the propeller end for the same speed using either a battery or fuel tank. My charger is rated at 92% so yes the actual energy I use per mile is slightly higher than the numbers I have quoted but I considered that on a similar scale as the energy cost needed to bring a gallon of gas from the refinery to the gas dock.

I am an environmental scientist/ecologist (retired) and have been interested in these issues for much of my career. So, I will try to include some of the externals in the following discussion.

A quick search of the internet gave me the following results: It takes 5-8 kWh of energy to produce 1 gal of gasoline (variations due mostly to source of crude) or about 20-25% of the final product. For solar panels it is about 20% (it takes 5 years per panel to negate the energy used in manufacture; where panels are usually guaranteed for 25 year) . Wind is much better at 2%. I could not find comparable numbers for hydro. So, here in the Pacific Northwest we are blessed with a fairly efficient source of electric power. According to the US Energy information Administration, Washington gets 60% of its electricity from hydro and an additional 10% from solar, wind, and biomass.

The Department of Ecology has been collecting data on this, and here is what I found. In 2020 the production of 1 kWh or electric energy released 0.23 lbs of CO2 per kWh (228lbs/MWh) in Washington. With gasoline producing 20-24 lbs of CO2 per gallon, 1 kWh of gas produces about 0.6 lbs of CO2.

If I use 2kWh from my battery pack to travel those 5 miles at 5mph, my carbon footprint of energy used is about 1/2 lb. If I use 0.5 gal of gas using a small 9 hp outboard to travel the same distance I will have produced 10 lbs of CO2 plus the additional 1 - 1.5 lbs possible from the manufacture of the gas.

I have been cruising electric on Puget Sound for 26 years now. My first was a 24ft St. Pierre Dory powered by a 36V fork lift motor and golf cart batteries. My second was a 26' St. Pierre Dory powered by two Motor Guide 36V trolling motors, and my third St. Pierre Dory was powered by the Minn Kotad 112 36V motors. Now I have the C-Dory and look forward to may more years on the water.

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