April Propwash 2018 web.pub

Since most races are over in around 2 minutes or 1/30th of an hour, the battery can deliver 30 times the one hour current. That would be 150 amps in this example. If you want reasonable battery life, 80% of this value or 120 amps will be safer. The battery C rating gives an idea of how much current the battery can supply. A 60C 5000 mah battery can supply 60x5 or 300 amps. I’ve seen burst C ratings of 130. That would mean that the burst current from a 5000 mah battery could be 650 amps! The real question is how much power do our gas and nitro engines really produce? From a lot of gasoline engine testing I can give the estimates below: Stock 26 cc Zenoah or clone engine with tuned pipe: 4 hp, 3000 watts Modified 26 cc Zenoah or clone: 6.5 hp, 4800 watts 26 cc Quickdraw: 7.5 hp, 5500 watts Modified 30 cc industrial style engine: 7.5 hp, 5500 watts The nitro power estimates are a lot more approximate and variable: 15 cc nitro engine: 5 hp, 3700 watts 11 cc nitro engine: 4 hp, 3000 watts 7.5 cc nitro engine: 3 hp, 2200 watts 3.5 cc nitro Engine: 1.5 hp, 1100 watts Conservative electric power plants give great performance at 200 amps current draw. That will take around 10,000 total mah for a race. That means two of the usual 5000 mah packs. Power estimates with shaft output power at 80% of the input power gives the following: 2S battery: 1200 watts 4S battery: 2400 watts P limited: 1200 watts (Only needs one 5000 mah pack at 100 amps) 6S battery: 3500 watts 8S battery: 4700 watts 10S battery: 6000 watts Electric power plants have been built with custom speed controls and motors to deliver 14,000 shaft output watts or 18 hp from lithium polymer batteries. That’s why the current ultimate speed record for RC boats is almost 200 MPH. A more realistic setup with 4S batteries and a commercially available motor and speed control still has gone 140 mph. A Year of Racing District 20 (Continued from page 17) I look forward reporting on how well this will be doing in the next Propwash article. My goal for the district this year is to get back to a place where FRIENDS and FUN come long before a heat race win or the “big trophy”. I will also continue to search for ways to keep what we do fresh and interesting. Happy Boating! Electric Power Plant Calculations By Lohring Miller NAMBA Safety Director Electric motor powered boats are a mystery to nitro and gas racers. How do we figure out what fits into the engine categories we are accustomed to? Actually, electric power calculations are easy and give an honest idea of real power output. Online calculators for much of this is available at https://www.offshoreelectrics.com/conversion.php See the table below for examples. Real data can be found in Darin Jordan’s article on the P limited motors. Input power to the motor is easy to calculate. The power in watts is volts x amps. There are 746 watts in one horsepower. The nominal voltage for lithium polymer batteries is 3.7 times the number of cells. Thus a 4S pack has 14.8 nominal volts. Since lithium polymer cells have very low internal resistance, this nominal voltage will be pretty close to the voltage supplied under load. The current (amps) is a little harder to estimate. Either your motor or the speed control is usually the limiting factor. Low cost motors and ESCs used in the P limited classes usually can’t stand much over 100 amps. High end controllers can operate at 300 amps and more. A 5000 mah (milliampere hour) battery might be good for 500 or more amps, at least for a short time. From the above, a P limited power plant has around 1500 watts or about 2 input hp. Actual power to the prop shaft is probably around 80% of this or 1.6 hp. The next question is how fast will the prop turn? The motor’s KV rating determines this. The no load rpm is the KV x volts. A 20000 KV motor with a 4S battery should turn 2000x14.8 or 29,600 rpm. Under load that will be more like 80% of that or 23,600 rpm. The problem is that the real KV may not be close to the listed value. See Darin’s tests for examples. His data confirms the 80% approximation for most motors if you know the real KV. The real KV can be found with a data logger. Run the motor at full throttle unloaded and note the voltage and rpm. Divide the rpm by the voltage for KV. How long will the boat run? The battery capacity determines this. Capacity is measured in milliampere hours. If you divide by 1000 you get ampere hours. A 5000 mah (milliampere hour) battery will deliver 5 amps for an hour. PROPWASH Page 18 April 2018 Symbols V=volts A=amps W=watts KV=rpm/voltage constant mah=milliampere hours (battery capacity) C=currentxcapacity constant hp=horsepower rpm=revolutions per minute To Get: Formula Example Power VxA=W 14.8 volts x 100 amps = 14800 watts Horsepower W/746=hp 14800 watts / 746 = 19.8 horsepower Maximum Battery Current Cxmah=A 60 x 5000 = 300 amps Shaft RPM KVxV=rpm 2000 x 14.8 mah = 29600 rpm (unloaded) Run Time mah/1000/A=hoursx60=minutes 5000 / 1000 = amphere hours; 5/100 amps = .05 hours .05 hours x 60 = 3 minutes