Thank you for an excellent, well thought out post. There are volumes of information, leading would be electric boat owners to make more informed decisions.
On Thu, Mar 13, 2014 at 9:03 PM, James Lambden <james@electroprop.com> wrote:
I have been following some of the threads lately with much interest which leads me to this contribution. I'm looking forward to your comments and edits after which it will be published to my website. May as well get the group to chime in and make it perfect first. Here goes:SYNOPSIS1. Torque is more efficient at transfering power than rpm - A larger motor running at lower rpm will have less rpm related (friction) losses in the motor bearings, seals and speed reduction system.2. A larger motor will run cooler than a smaller motor for any given power level and will be consequently more efficient3. A double stator motor has over twice the power of a single stator motor.4. A double stator motor can make the same amount of power as a single stator motor at less than half the rpm. Less rpm in the motor will reduce rpm in the speed reduction system and further contribute to higher efficiency.5. A water cooled motor will run at a lower temperature with higher efficiency and higher output than an air cooled motor6. A motor geared to make hull speed will be less efficient than a motor geared to go at a lower boat speed ( 90 % of hull speed) because for any given rpm the motor will be running on more current and less voltage. Voltage is always more efficient than current.7. Lower speed reduction ratios will be more efficient than higher speed reduction ratios because the speed reduction system is doing less work. Lower speed reduction has a smaller drive pulley so the propeller can be closer to the hull and more horizontal so more of the force is pushing the boat forward rather than up. The exception to this rule is planetary gears which work at higher efficiencies at higher gear ratios.9. Propeller slip determines propeller efficiency. More slip, less efficiency. Larger diameter, higher pitch propellers turning slower are more efficient. Propeller efficiency varies with rpm and propellers get less efficient the faster they turn.9. Dripless shaft seals are more efficient than packing glands10. Batteries run more efficiently when using less power. Higher efficiency in the drivetrain requires less current from the batteries so total system efficiency increases even more.11. Lithium batteries are the most efficient followed by Odyssey TPPL Batteries, followed by AGM or Gel. Flooded lead acid batteries are so inefficient and dangerous they don't belong on an electric boat.POWER AT THE PROPELLER IS WHAT COUNTSWhat matters most in choosing a drive is the efficiency of the drive. Efficiency determines how much power gets to the propeller shaft which determines speed and range.Inefficiency shows up as heat. If the drive is running cool, then it is efficient. If the drive is hot, then the energy is going to heat and not to propelling the boat. Heat can be contagious spreading between parts and decreasing system efficiency. Heat generating parts should be spread out to avoid heat contaminating other parts in air cooled systems and cooled by water in water cooled systems. Different air cooled components should not be enclosed in the same enclosure.ACTUAL RESULTS ARE THE ONLY AVAILABLE BENCHMARKElectroprop (previously Propulsion Marine) , posts actual power used at various boat speed of drives that we have put into boats. I encourage the other vendors to do the same. We use calm conditions, with no wind and no waves so we can get a baseline for comparison purposes. These tests are performed independently by the boat owner and are completely verifiable. I am sure that any one of our customers would take you out and show you first hand how efficient their drive is. Efficiency, along with safety and durability are the primary reasons why our customers are choosing Electroprop for their propulsion motor.PMAC MOTORSPermanent Magnet AC motors are 88 to 98% efficient, with efficiency varying slightly with rpm. The most popular motor for small boats is the Motenergy motor. It is reliable and cost effective. Other solutions are on the market but few give the value of the Motenergy motor. Motors are most efficient if run at cooler temperatures.PMAC and BLDC motors are essentially the same. The difference is in how they are controlled. Brushless is important in the marine world, especially if you have propane on board because it is spark free.SINGLE STATOR VS. DOUBLE STATOR MOTORSA single stator motor has to spin at over twice the speed of the double stator motor to make the same power. It is actually more than twice the speed because efficiency is related to the current (i) squared multiplied by the resistance of the wire. So a single stator motor at any given rpm has less than half of the power of a double stator motor. There will be less i squared r losses on a double stator motor at the same speed. The double stator motor will make over twice the torque of the single stator motor so for any given power level it can turn at less than half of the speed which decreases efficiency losses.CONTINUOUS VS INTERMITTENT RATINGSIntermittent ratings are only useful for braking and accelerating. The intermittent rating could be as high as the stall torque which will only last for a second. Electroprop recently changed to peak ratings so we can be easily compared to the competition but we always list our continuous rating. The difference is, our intermittent rating is determined by how a motor performs in a boat. Typical peak power during acceleration and deceleration is 40% higher than the continuous rating at full throttle, so that is the number we use. Hopefully the industry can agree on an intermittent power rating for these drives and the conditions of a continuous power rating which should include: motor rpm, torque, ambient temperature or water temp and flow rate. What matters is how the boat is geared which should not exceed the continuous rating of the drive at full throttle.All ratings should give the conditions of the rating. Heat from inefficiency is the restricting factor. Torque and rpm determines overall power. If a drive is rated it should be rated at a specific rpm at a specific torque at a specific ambient temperature or water temperature and flow rate. Giving a rating without the conditions of the rating is little information to the boat owner. What good is a rating of a motor at 5,000 rpm when you are using a motor at 2,000 rpm? Your use will be 2/5 of the motor rating. This is done commonly and it should stop so the playing field can be levelled.Ten years ago when I started this business I bought a single stator motor advertised at 8.5 KW and put it into my Catalina 30 to find out it overheated at 1.5 KW for my application. This kind of misleading advertising should stop in the marine world. The fact is that boats run at continuous ratings, and not intermittent, which is only good for small periods of time accelerating and decelerating. Every vendor should give their continuous ratings and conditions of that rating so customers know what they are buying. Boats are not like go carts or cars that operate very intermittently and depend largely on acceleration. Boats run continuously over time and this is even more so if you are considering a hybrid conversion.A boat operating in Alaska will have higher output than a boat operating in the tropics because the ambient temperature or temperature of the water is lower. When you head south cruising, you should derate your drive for the warmer climate. This is one more reason to overbuild any electric installation.POWER DEVELOPEDPower has to be developed by an electric motor. A motor with no load will draw no current. Getting the right load is critical to an efficient drive. If the load is too high, the drive will run into a current (torque) limit and will not make rated power and the power the motor makes will be made more from current rather than voltage which will cost efficiency. If the load is too light the motor will run into the voltage (rpm) limit before it makes rated power.Power in Kilowatts or Horsepower is Torque x rpm There are different formulas for the different units but it is the same equation.CONTROLLERSThe Sevcon control is used commonly for good reason. These controls are 95 to 97% efficient.The programming of the controller can affect the efficiency of the motor. I would like to tell you more about this but this is proprietary knowledge based on many hours of trial and error and testing and running the latest firmware. Most changes in the program result in incremental increases in efficiency . There is one variable that makes a huge difference in system efficiency. Software does improve over time so getting the latest software does increase efficiency.DIRECT DRIVESDirect drives are often many times more expensive than smaller motors with speed reduction and quite often cost is the limiting factor to an electric conversion. However, if efficiency is the ultimate goal, a properly designed direct drive will be the most efficient solution because there are no speed reduction losses.SPEED REDUCTIONSpeed reduction is necessary because a propeller works most efficiently at low rpm but a motor makes power from high rpm. Speed reduction has the added benefit of being able to match a propeller exactly to a motor. Increasing the gear ratio decreases the ratio between AC currents and DC currents. AC and DC currents should be similar at nominal operating rpm for a balanced system.SPEED REDUCTION LOSSESIt doesn't matter if the gear reduction is belt drive or gear driven the losses all work the same way. There are three types of losses to consider - losses proportionate to rpm, losses proportionate to torque, and losses proportionate to thrust.Losses proportionate to speed are easy to calculate. Just run the motor at full rpm and see what the DC current draw is at any particular speed. A motor running at a higher rpm will have higher rpm proportionate losses for the same amount of power. Seal losses are proportionate to rpm as they are determined by the friction of the seal. Different seal materials produce different amounts of friction and the seals can be optimized. Belt drive systems may have two bearings and 4 seals producing friction. Gearboxes have two seals for less losses and some gear drives can function with only one seal. Friction (Speed losses) are higher with higher input rpm as well as higher output rpms.Losses proportionate to torque are more difficult to calculate. We can judge different systems by looking at similar boats and the power required to drive them in calm conditions. As the industry matures, different products will be tested on Dynos and we will be able to get a better take on the different solutions.Losses proportionate to thrust are determined by the thrust bearing. Ball bearings are more efficient than tapered roller bearings because they have less surface area. Ball or Tapered Roller bearings may be installed with pre loading on the bearing. The amount of preloading affects the overall efficiency. Thrust bearings lubricated with oil are usually better than those lubricated by grease because the oil is more viscous. This does change as the bearing heats up. We would have to look at this in closer detail to do an accurate comparison. Thrust bearings in a gearbox contribute to overall gearmotor heat so this must be optimized or cooled to prevent contamination of the motor by heat. Thrust bearings on a belt drive could produce heat and would affect ambient air temperature.ALIGNMENTAccurate alignment increases overall efficiency. Each Electroprop comes with a flexible coupling with accurate alignment indicator. If your system is not aligned correctly then it will bind on every revolution. With a 4 inch coupling it is recommended to have alignment within 4,000th of an inch. This is easy to do with a flexible coupling and very hard to do without one. Mechanics get skilled in this job over time. You would be getting very lucky if you had accurate alignment with a sleeve type coupling on the first round through.Alignment also matters where the shaft goes through the shaft log. If it is not centered - most are not after years of use - then the shaft will not be aligned in the cutlass bearing which will cause wear over time and looseness, as the shaft will not be fully supported over time within the cutlass bearing. If the shaft wiggles back and forth in a loose cutlass bearing you will get efficiency losses.SHAFT SEALA dripless shaft seal is more efficient than a packing gland. A dripless shaft seal runs on a film of water between a carbon tube and stainless ring. A packing gland tightens down on a shaft and if overtightened acts like a brake. A packing gland is also supposed to drip one drop every 60 seconds as a minimum. Who wants a leaking boat? Be very aware of a packing gland as it could rob you of ten or twenty percent or more in efficiency. They are supposed to leak one or two drops a minute and the leakier they are the more efficient they are. We have stayed away from packing glands with all of our conversions (to my knowledge) for this very reason.EFFICIENCY IS A MAP NOT A CONSTANTEfficiency changes with rpm and with torque which is different for every boat, propeller and gear reduction combination. You can't just say an electric drive is 85% efficient because this has no relevance. Efficiency is a moving target and for any particular application is best represented as a map rather than a finite number. In general overall efficiency of different electric drives is in the 75 - 85% range and is represented by a multiplication of all component efficiencies together. Efficiency usually increases to about 1,000 rpm and then decreases after that but this is just an observation and this changes with every different application. Generally speaking efficiency decreases with torque, rpm and thrust. This is why electric motor sailing has such incredible results because these small amounts of power are at the right end of the efficiency spectrum. This would be a good place for Nigel Calder to chime in as he is the expert on efficiency and getting actual, verifiable statistics.VOLTAGE IS MORE EFFICIENT THAN CURRENTTo increase power levels you can either choose to increase the torque or the rpm or both. Increasing the torque by increasing the AC voltage is the most efficient way of increasing the efficiency of the motor. This is accomplished by building the motor with more turns of wire around the stator which slows the motor down while at the same time increasing the torque. The longer, thinner wire will have more resistance so wlll not be able to develop the same amount of power as the shorter, fatter wire. The key is to pick the right winding for the application so the motor can run on voltage instead of current and this is done by realizing the current limitation of any winding and operating within those limitations. Picking a motor that is rated at 10 KW will be less efficient than the same motor rated at 5 KW, if 5KW is all you need, because of how the stator is wired. All motors are current limited.For the most part you can consider all motor losses directly proportionate to current with almost no losses directly attributable to voltage. So reduce the motor current and you reduce the losses. This is why there is a tendency towards higher voltages in every application.A motor geared to go at hull speed will be less efficient than a motor geared to drive the boat at less than hull speed. This is because at all other speeds, the motor is running less on voltage and more on current.DRIVELINE EFFICIENCYA typical driveline efficiency formula might look like this. ( I welcome your comments on this formula!)Motor (90%) x Controller (97%) x Contactor (99%) x Speed Reduction (95%) x Seal losses (98%) x Shaft Seal (99%) x Cutlass bearing (99%) = 79%.Then you have to factor in the propeller:Multiply by propeller at 80% = 63 %Multiply by propeller at 50% = 39.5%As you can see by the formula, if any one component decreases in efficiency, the entire system efficiency decreases by a like amount. If two components are each 2% less effiicient you get 4% less overall efficiency This 4% loss will then be increased by the battery efficiency loss to a 5% loss. (more on this below) Every component is therefore under the efficiency microscope. The efficiency relationship relates to all of the components in the system. EVERY COMPONENT IS INTER-RELATED.PROPELLER EFFICIENCYThe greatest variation is in the propeller itself which can vary between about 50 and 80% - debate of these numbers is encouraged.Boat speed is determined by pitch x propeller rpm less slip.Thrust is determined by pitch, diameter, blade area ratio and cupping.Diameter, blade area ratio and cupping need to be optimized to the size of the boat. These features need to be slightly larger than the boat requirement to make allowance for difficult conditions. Exceeding the optimum size results in efficiency decreases due to increased surface friction on the propeller blades. Theoretically, less blades are more efficient but in actual practice more blades are more efficient provided that surface friction has not become a problem from a larger blade area.Efficiency of the propeller decreases with rpm and is a map. A slower turning propeller will be significantly more efficient than a faster turning propeller. Propeller efficiency increases with diameter.Our experiments go beyond any other information that I could find and determine that increase in pitch also increases efficiency and adding cupping increases efficiency. But beware of cupping because optimizing for forward will decrease performance in reverse so a customer needs to understand that the brakes might not work quite as well with a cupped propeller. It is perfectly ok to have a cupped propeller but we all need to know what to expect so we can keep out of harms way. In other words, slow down when you are in tight quarters when using a cupped propeller.BATTERY EFFICIENCYOverall system efficiency is also determined by the efficiency of the batteries.If everything else in a system loses efficiency then this is compounded even more by losses in the batteries, because batteries get less efficient with higher loads and an inefficient system will draw more current for the same performanceA regular AGM battery is 75% efficient when run at 1 C.So if we look at our overall efficiencies using the different propeller efficiencies and multiply by battery efficiency we findSystem Efficiency using an 50 % efficient propeller ( 39.5% ) with flooded batteries x .50 = 19.25% ( just do me a favour and don't even go there )If we look at the optimized system using an 80% efficient propeller with Lithium Batteries our overall efficiency will be 63 % x .97 = 61 %HULL EFFICIENCYWe can go one step further by optimizing the hull by eliminating through hulls and flush mounting the remaining ones, using low profile zincs, fairing out the bottom, keel and rudder - just ask your local racer what he does. Easier than that, make sure both your bottom and propeller are cleaned regularly and especially before any long trip.As you can see, if you consider all the component efficiencies and optimimize them, then the total system efficiency quickly increases from a boat where these basic steps have been ignored.TOTAL BOAT EFFICIENCYA fully optimized system can have three times the range of a system that has not been optimized.POWER REQUIREMENTS: GO A LITTLE SLOWER FOR MUCH GREATER EFFICIENCYFor each knot increase in boat speed, power required roughly doubles. Back it off a bit and your range increases exponentially!POWER AND EFFICIENCY INCREASE WITH COOLINGPower and efficiency are almost always inversely related though with technology this is changing. Just think of a Prius compared to a muscle car. There is one area of design that increases both power and efficiency and that is providing adequate cooing. The best possible way to do this is to utilize the worlds largest heat sink only inches away from your drive - the ocean. But this is not always necessary as the amount of heat to dissipate is finite if dealing with a battery powered vessel and can be easily accomplished by a radiator or water heat sink (fresh water tank) or keel cooler. Heat exchangers are recommended for hybrid application that can run continuously for many hours.END RESULTInstalling a motor that has been optimized for efficiency can increase range by 300% or more Since batteries have limited amount of range and range is often the determinant to whether you choose electric over diesel, efficiency counts!It is my opinion that a gear drive is more efficient than a belt drive based on the observation of our customers for the actual power used to propel a boat at any given speed and the fact that we are the only company that publishes this information ( to my knowledge. ) Gear drives have no lateral forces placed on the motor bearing of the drive bearings.If belt drives were more efficient, then those numbers would be forthcoming. Various communications I have had with the other manufacturers lead me to beiieve that we could make an efficiently optimized belt drive as it is the optimization of each component that results in the final efficiency. Belt drive efficiency could be enhanced in many different ways and the place to start would be with cooling.Ultimately there is a sweet spot between every component and the size, shape and displacement of the boat that gets the greatest efficiency. The final measure will always be watts to knots in calm conditions and I encourage everyone to post their numbers so we can learn from our collective experience.COMPARING VENDORSAll the vendors are providing power input levels which are rather irrelevant to the performance and range of a boat. It is the output power that determines performance and range and this is determined by input power multiplied by system efficiency. As the industry matures, these numbers will start coming to light.The different systems have different attributes. Belt drive ratios can be changed easier than changing a gearbox. Gearboxes are tougher, low maintenance and have no belts so no cowlings. A gearbox drive has to be measured prior to ordering where a belt drive can be installed after it is ordered.An optimized drive requires pulling the boat out of the water to fit a new propeller and change the shaft seal and align the shaft. There are many more comparisons to be made - just look at the websites of the different competitors and find the one you think will give the best match to your needs. Ongoing service and warranty are also very important considerations.How much work you put into an electric conversion ultimately determines what your range and performance will be.Sooner or later, Practical Sailor will get a hold of a dyno and test all of our systems and at that point, let the best system win!RATINGSRatings have no value without the conditions that achieved the rating. Is the rating continuous or intermittent? What was the ambient temperature of the air or water temp and flow rate for water cooled drives? What was the rpm? Look carefully at ratings because there is no industry standard for ratings. Be careful and get what you need for your boat. Ask how the rating has been determined by the vendor. Make sure that a rating is realistic. Make sure that you have the batteries to back up the rating that you are choosing by having more amp hour capacity in your batteries than the amp rating of the motor - and that is a bare minimum recommendation. Otherwise a very high rating is inconsequential to how the boat will be used.The best way is to see other boats and their performance numbers before you make a decision.BOAT EFFICIENCYAll of the above are system efficiencies. Boat efficiencies matter as well. Flush mounted through hulls, filling dings in the hull, location of the propeller, angle of the propeller shaft - even the software of the controller and the autopilot will affect the overall range your boat has using electric propulsion.OPTIMIZE YOUR DRIVEThe bottom line is we should all do what we can to optimize efficiency. When optimized, electric and hybrid propulsion can go up against the reigning super power of boat propulsion - fossil fuels - because with a highly efficient electric drive we can provide the ranges that typical sailboats operate within.The ultimate achievement is optimizing efficiency so the whole boat can become sustainable on alternative energy. This has been my goal and achievement with Kapowai which can reach up to 3 knots on solar power alone, and has been completely energy independent for years.The good news is we are winning the effiiciency war with fossil fuels which are inherently inefficient. After all, the original name of an internal combustion engine was the heat engine, and that pretty much says it all.Thanks for listening and I look forward to your comments.JamesJames LambdenThe Electric Propeller Company625C East Haley Street,Santa Barbara, CA93103jlambden: Skype
Dominic Amann
M 416-270-4587
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