Please help me understand what I seem to mis-understand.
I’ve been thinking about my sailboat cruising time and the possibility of using an electric motor
. Here is the data I am using to make comments and observations about electric propulsion
- Four trips from Seattle
to San Diego
- Two trips from San Diego
to Puerto Vallarta
and back ( 2,000 NM)
- Three years cruising in the Sea of Cortex and Western Mexico
- One trip from Annapolis
to BVI (1,500 NM)
- Many overnight and multiple day trips from San Diego to Catalina
, Ventura, Santa Barbara, Newport
, Oceanside (~1,000 NM)
Assuming an average speed of 5 knots (lots of slow sailing) that is about 3,000 HOURs underway.
My Caliber 40 cutter
needs about 8 Horsepower at the prop to motor at 5.5 knots in flat, calm water
. That is about 6 Kilowatts of power at 100% efficiency. I know my 18” x 15” Maxprop is not 100% efficient but I’ll assume the max possible efficiency in order to minimize electrical
energy demand. I will also ignore the need for more power needed to motor into currents, swells, and waves.
I keep very detailed records about wind
, waves, motoring, and sailing. I sail almost any time I can keep the boat moving at 3.5 knots or greater. I have a big Code 0 and an even bigger lightweight spinnaker
I love to fly. My records show that I sail about 53% of the time. But, what is more interesting is that my records show many, many, YES – MANY, intervals where we motor for more than 12-hours at a time due to there being no wind at all – NONE!
For example the last two days before arriving in the BVI we had not a single
breath of wind while motoring at 7 knots (~330 NM). This was in a very high tech lightweight 53’ sloop
with a huge array of light air sails
The problem AT SEA with calm winds is the constant swell rolling by. In my experience, it is not possible to comfortably just sit in that swell with no way on to keep the boat aligned to the swell. Therefore, the boat must be kept moving. My boat needs about 3.5 HP ( ~2.6 Kw) to move at 3.4 knots in calm seas and wind.
The data above shows that I need sufficient battery power to produce about 3 Kw of propulsion energy for up to 24-hours. That means I need to store 72 KwH of energy and, to keep from drawing batteries below 50%, I need about 144 KwH of energy.
Additionally, I need sufficient battery capacity to pull up to 6 Kw (5.5 knots) of power for many hours without damaging the battery. If I want to use a 20-hour withdrawal rate I therefore need about 120 KwH of energy storage
The batteries being discussed here are $3,500 for 10KwH so I would need 12 10KwH batteries at a cost of $42,000. Assume they weigh 310 pounds each – that is 3,700 pounds of batteries. ( A 300 amp hour 8-D LeFePO4 weighs 93 pounds and can deliver 3 KwH when discharged 80% so 10KwH would weigh about 310 pounds)
Now, how do you recharge that big battery bank?
Assuming you can put power back into the batteries at the same rate you withdraw it you will need a 3.5 Kw generator to recharge the batteries. The lightest I can find is a NextGen 3.5 which weighs 160 pounds and burns 0.2 GPH. If I need 20-days recharging then I would need 96 gallons of diesel (0.2*24*20) or 672 pounds.
My generator, diesel fuel, batteries, and 48V traction motor (40 pounds) would weigh a total of 4,572 pounds (3700+160+672+40). All that equipment
would cost about $50,000.
4JH2E weighs about 500 pounds with it’s transmission
and fluids. One Hundred gallons of diesel ( 750 miles ) weighs 700 pounds.
So just the batteries needed for electrical
propulsion weigh more than three as much as my current
diesel and fuel. Even if I add in the 500 pounds my two 8-Ds weigh the electric power system weighs twice what my diesel and big house battery bank weighs.
The bottom line is that it takes about 1.5 KwH to move my boat one nautical mile at a normal cruising speed. The Yanmar
burns (measured over thousand of hours) 2 ounces of diesel to accomplish that. The NextGen burn 1.4 ounces of diesel to put 1.5 KwH back into the battery at 100% efficiency or 1.6 ounces at 90% efficient conversion of diesel energy to torque at the prop .
I can put in a new Yanmar, propshaft, and Maxprop for less than 1/4 the price of the electrical system
What then is the advantage of diesel electric power?
I have 500 watts of solar panels
and in Mexico
in the bright summer sunshine I can put about 225 amp hours back into the batteries at 14 volts. That means I am recharging at about 20% of the rate I am using power – OR – for each hour I use the electric propulsion motor (2.6 Kw) I need 5-hours of maximum solar power to replace the energy.
I also have a Ferris towed generator, which produces 7 amps at 5.5 knots sailing and 12-amps at 7 knots sailing. That means I need about 20 hours of sailing and towing the generator to replace ONE hour of motoring. And, the generator slows the boat by 0.3 knots when sailing at 7-knots.
Tell me how electrical propulsion makes sense for a cruising sailboat.
What is wrong with my analysis based on 13,000 NM of cruising and 15-years of living with only solar power on the boat?