Great posting. Very informative.
George
Greetings,
I have recently posted pictures of the new 5 KW electric drive installed in the Catalina 30, Kapowai. More pictures are available at www.propulsionmarine.com/kapowai.htm
This group is the first to see and learn about this new electric drive.
We have performed many tests on this drive which have been well documented in pictures and videos which we will be posting to Utube.
What we found out will interest everyone in this group.
INTERMITTENT VS. CONTINUOUS RATINGS
The most notable concern that we have with electric drives entering the marketplace is most, if not all, ratings of motors are intermittent. Comparison of intermittent ratings to diesel engines is not realistic because diesel engines have continuous ratings. Intermittent ratings are fine if all you are doing is navigating your marina because the motor has time to cool down after being used for 5 minutes. However, if you are planning on cruising across a large distance, or using a system backed up by a DC Generator, an intermittent rating is of no value at all.
Continuous ratings need to be rated at winding temperatures. Winding temperatures of air cooled motors can get as high as 150 degrees Celcius. Generally speaking, if you can't hold your hand on an electric motor continuously, it will probably be getting close to its maximum winding temperature. Our 5 KW electric drive consumes 5 KW at 100 degrees C winding temperature, well within the temperature capability of the windings varnish. Continuous ratings and their associated temperatures should be made after at least one hour of continuous operation. I have noticed that motors running in excess of 100 degrees C may develop thermal runaway so for our purposes 100 degrees C is where we rate this motor which allows enough headroom in the design for inclement conditions. The addition of force air cooling will further increase power output (see below).
EFFICIENCY
Electric drives are also rated at power consumed not power created. Power created can be measured in terms of thrust. Thrust per kilowatt is a good measure. However, as an electric drive consumes more power, the thrust per kilowatt decreases. This is because the propeller gets less efficient the faster it turns, which is a result of more propeller slip. Also, the electric motor becomes less efficient as it creates more power. The only way to truly compare one electric motor to another is by testing a system on the same boat and graphing the results on a graph of thrust per kilowatt. To determine thrust, the boat is pulled through the marina at different speeds and the tension on the pull rope is recorded. The Boat is then powered through the marina at the same speeds and the Kilowatt consumption is recorded. Here are the results from Kapowai.
Thrust per Kilowatt
2 knots: 76 lbs thrust / KW
3 knots: 52.8 lbs thrust / KW
4 knots 40.4 lbs thrust / KW
5 knots 34.2 lbs thrust / KW
GEAR RATIOS
The only way to properly match a propeller to a motor is with a reduction gear. Slow turning large propellers with substantial tip clearance are more efficient than fast turning propellers. Yet an electric motor produces horsepower proportionally more with increased rpm. So a reduction gear is necessary. After an electric drive is installed it is important to measure the AC current going to the motor. If the reduction gear is to low (closer to 1:1) then the AC currents going to the motor will be too high. If the reduction gear is too high (closer to 5:1) then the system will not produce enough power because the propeller will not turn fast enough. Gear ratio changes of just .5 : 1 make an enormous difference in the performance of the boat. For example, Kapowai with a 2.8:1 will make 5 knots of boat speed, but with a 2.2:1 gear ratio will make 6 knots of boat speed. Higher gear ratios are more efficient, and lower gear ratios are more powerful. We have 10 different gear ratios available with differences of .3:1 between most ratios.
PROPELLERS
The most efficient propeller has a pitch to diameter ratio of 1.3 :1. That is more pitch than diameter. Pitch is what allows a propeller to bight into the water. Kapowai uses a 12 inch diameter propeller with 14 inches of pitch. Tip clearance is necessary especially when using the brakes. A boat at full boat speed when shifted to full reverse will cavitate if there is not enough tip clearance. When choosing a propeller we try to fit as large a propeller as possible onto the boat. Tip clearance should be at least 15 percent, and preferably a minimum of 2 inches.
CONTROLLER EFFICIENCY
Propulsion Marine pioneered the use of Sevcon's Gen 4 controller on boats. This controller is very efficient! Standby currrent is less than 10 watts. Some controllers only reach maximum efficiency at full power and consume as much as 750 watts in standby.
The true magic of an electric boat is in using the electric motor as an auxiliary to the sailplan or motor sailing. When crossing the Santa Barbara Channel, a distance of 22 miles, we use the motor all the time. The electric motor gives us a guarantee of sailing performance. When the wind dies down and the boat slows down the electric motor rises to the occasion keeping apparent wind high and the sails full. Motor sailing on a continuous basis is only possible with a highly efficient system otherwise you will run out of batteries! Kapowai recently returned from a 3 day trip to Santa Cruz Islands, total 50 miles and motor sailed the entire way. She returned with a resting voltage on her batteries of 48.6 volts, or roughly half of her batteries.
One test that we will be posting to Utube will show the impact of the electric motor on sailing performance. While sailing along at 3 knots we added 200 watts of electric power to the motor and the speed increased to 4.8 knots!
Kapowai consumes roughly 700 watts to travel at 4 knots under motor alone and 2.7 KW to travel at 4.8 knots with motor alone or 2 KW more power.
That is a 1,000 percent return on energy invested into the electric motor while motor sailing. There are three reasons for this. Firstly a still propeller has about the same amount of resistance as a bucket of the same diameter dragged behind the boat. However, it takes very little energy to turn a propeller at the speed of the water column, in most cases, only 100 watts or less. The second reason is there is a vacuum or low pressure area behind the boat in the water caused by the lines of the boat coming together. This low pressure zone is mostly filled in by the propeller wash and transfered from behind the boat to in front of the propeller. It takes a very small amount of energy to accomplish this. The third reason is the creation of apparent wind in the sail plan which increases the efficiency of the sails.
On Kapowai we run the electric motor at small amount of power (100 - 200 watts) all the time. We never turn it off. Unlike a diesel, where a sailor hums and haws about when to turn the motor on and then when to turn it off, with Kapowai the electric motor is always on. This is possible because of the low end efficiency of Sevcon's Gen 4 controller. Always running the motor makes Kapowai a very fast, easy to sail and controllable sailboat.
CONTROLLER INTELLIGENCE
Sevcon's Gen 4 controller is CANBUS compatible, allowing the addition of other nodes in the CANBUS. Another node may be an additional motor but it also allows for the Clearview display. The Clearview display shows relevent motor operating data including motor and controller temperature, motor rpm, current, torque, and voltage, and battery voltage. Future enhancements will include battery current. The display also allows for dealer programming of acceleration and deceleration rates. This is important to protect the flexible coupling, especially when shifting from full forward to full reverse. The Clearview display is a small computer with LCD screen and is also highly efficient. The system standby current of 10 watts includes the operation of the Clearview display.
Motor Protection
The Gen 4 relies on feedback from the motor's hall sensor and now also the motor's temperature. If the motor reaches a certain temperature setpoint which we have chosen as 130 degrees C, then the controller will cut back output by 10 percent for every degree higher than 130 degrees C. This protects the motor from overheating while allowing the operator of the boat to still use his electric motor at lower capacities.
ELECTRIC MOTOR DESIGN
Our new double stator motor is the result of extensive testing and design by Mars Motors, Propulsion Marine, and Sevcon. To perform efficiently a motor should run on voltage rather than current. In order to slow a motor down we add more windings to the motor. The issue though is that a longer winding has more resistance and slower turning motors running at lower rpms reach the limit of their power when the current through the winding exceeds the ampacity of the winding wire. Hence a slower turning motor has to be larger in order to develop the power. Enter Mars new double stator motor. Two stators working in parallel draw half the current in each stator resulting in over twice the power of a single stator motor. This is because the amount of heat created in each stator is proportionate to the square of the current consumed. This new double stator motor runs at a temperature of approximately 1 degree C for each amp of DC current consumed by Kapowai. 80 amps of current is approximately 80 degrees C winding temperature and 4 KW total power consumed. (100 amps, 5KW, 100 degrees C winding temp). These power levels for this motor are for the motor when totally enclosed in an engine compartment without the addition of forced air cooling. We are in the process of developing forced air cooling and will be reporting on the higher power outputs of this motor in the near future for larger sailboats. For a Catalina 30, forced air cooling is not necessary. In most conditions over 6 months of testing, the motor at most was warm and never hot to touch.
The double stator motor has other advantages too. In an axial gap motor as the stator pulls the rotor around, the stator also pulls the rotor towards the stator resulting in axial forces on the bearings of the motor. With a double stator motor, these forces are cancelled out and the rotor levitates between the two stators. Less forces on the bearings results in less friction losses and higher motor efficiency.
BATTERIES
Kapowai has four 4D batteries and four 8D batteries. Each 4D battery has 2.44 Kilowatt hours of energy and each 8D battery has 3 KWH's of energy for a total of 21.68 Kilowatt hours of energy. We can use up to 80 % of the energy, so Kapowai has 17.34 kilowatt hours of useable energy.
Large battery packs work more efficiently than small battery packs. When used at over their 20 hour rating the battery pack does not have to be derated by Peukerts exponent calculations. Doubling your battery pack more than doubles your range and also increases the life expectancy of your batteries.
PERFORMANCE
The overall result is that Kapowai can run continuously at over 5.5 knots and the motor is never hotter than you can hold your hand on.
We can motor sail at 200 watts for 86 hours enhancing our sailing speed by 1.8 knots.
In order to establish a baseline, we tested Kapowai in the Santa Barbara Marina with no wind and no waves. We tested in both directions then averaged the results. The following we call theoretical range, or bias, because we rarely encounter complete calm at sea. These calculations were performed by Lifeline Batteries.
Theoretical Range
2 knots: 262 watts 5.3A @ 50V - 87hours - 175nm
2.6 knots 392 watts 7.8A @ 50V - 56hours - 146nm
3 knots 663 watts 13.3A @ 50V - 30hours - 90nm
4 knots 1.485 KW 29.7A @ 50V - 12.07 hours - 48.3nm
5 knots 3.211 KW 64.2A @ 50V - 4.97 hours - 24.9nm
5.7 knots 5.5 KW - 110A @50V - 2.68 hours - 15.3nm
We have now tested this new electric drive for 6 months in Kapowai. Over the next month 4 new systems will be installed. The results from these installations will be posted to our website.
More information will become available on electric boat design, batteries, motors and controllers, also posted to our website.
CONCLUSION
In 1898 the Mary Gordon was built in England. She was a 70 feet long capable of cruising at 8 knots for 6 hours, a range of 48 miles. Electric boats preceded fossil fuel boats. Then came the fossil fuel boats which have dominated pleasure boating for over 100 years, (due to the development of fossil fuel engines and electric power being ignored.)
Advances in electric propulsion are now closing the gap on range with diesel and gasoline propulsion. 100 years of fossil fuel domination of the pleasure boat industry is too much. Electric motor sailing is fast and quiet and can be built with enough battery power for the majority of sailors.
Electric boats are back and better than ever.
James Lambden
Propulsion Marine
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George Johnson
GeoMar Logistics
Jomtien, Chonburi
Thailand
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