All were direct drive except the Astro 01 motor which has a factory 3.3:1 gear. I don't have thrust values, but I've been thinking about ways to measure thrust when I test. I can tell you that a 1.75" prop turning at 3600 RPM is hard to hold with one hand.
A simple fish scale works pretty well. A dedicated rig would be nice but I dont have a true workshop/lab so no space for one at the moment. Ideal would be be a dedicated rig that was also an endless pool type system so you could actually speed test and do dynamic thrust measurements etc. Then again I mulled over the idea of doing a PhD simply to get access to the tow tanks at Stevens....decided that building my own would be more practical/realistic. You'd probably be surprised how little thrust our boats actually need to get to speed. For Fast gun the majority need under 2lbs of static thrust/bollard pull.
How do you 'spec' motors like that. You estimate 2lbs.. if I want more torque(thrust?) but delivered at 6volts at a low rpm, how do you test for that? How do you even determine the thrust requirments at all? As I posted on the email list long ago that I wanted to get the most torque I could get out of the motors and stay at low rpm (28 sec) without reducing power from the ESC... I want to target 80% (does that mean the ESC will put out 80% of 6.4v?). How would I test or determine the thrust requirements from that? Or what my target RPM for that matter?
Easy answer: Big props, geared way down low. Without more data on components, you can't get too precise an answer. I had been tempted to build a double-reduction gearbox for Baden, but decided that in the end, simplicity trumped technological awesomeness.
I'm pretty sure Chris did testing at home to determine how much thrust was needed to get his Bismarck to 24 seconds. He can explain more on that. As we collect more and more data from actual brushless setups in use in ships, we are finding out that prop RPMs tend to end up around 2000 - 4000 rpm depending on prop size, ship size and weight, the number of powered shafts, and even the required speed. With prop rpm in mind, a brushless setup can be put together using system voltage, gearing, and motor kv numbers to get us in the ball park. Right now, there simply isn't enough data to put together a defininative formula for a motor/gear/voltage combination for all the different types of ships, much less try to figure out how much thrust a particular ships needs and how much thrust a drive system combination will make. By the way, the first post of this thread says the target rpms we are discovering. In the model aircraft world, there is software that will calculate thrust and amperage of an electric power system by taking all the variables to include prop pitch, diameter, blade width, prop efficiency through the air, battery type, etc.. But we are talking of literally decades of data from people dedicated to making and selling a software product whereas we are just starting to scratch the surface for our ships. So for the moment, what we know is getting posted in this thread. That knowledge will grow as more and more data becomes available as people switch to brushless. And do not doubt that brushless will eventually be more common than brushed motors. It is already getting difficult to find good brushed escs whereas brushless stuff is all over the market, above and beyond the performance and efficiency advantages brushless motor systems bring to the hobby.
You're thinking about it backwards, get the thrust requirement and then build/optimize towards that. So.. 1) Get the boat on speed 2) Measure the thrust 3) 3'x9' pond in the shed, 1-15V 60Amp Power supply, 8-21T 5mm gears, 4 Props, 20~ Brushed Motors, 5 Brushless motors, 4 gear boxes, ~3 ESCs, 14 dead mice/chipmunks, and uncounted hours 4) Kick ass ship Basically find the thrust you need and then optimize towards that, it really isn't that hard there just isn't a magic formula or a "no think" method to get it done. (Yes it is hard to get thrust data before you actually build a ship but normally there is someone that has a similar ship you can use to get in the ballpark.) Tugboat and Mike are right, there are too many factors to easily just saw exactly what you need, but generally speaking lower kv (
Let me relate some information and lessons learned from this past weekend ... 12 days before this past weekend's battle, I decided to refit a WW2 Sharnhorst. Taking a cue from Chris K's amazing Bismarck drive system, I put together a 42mm 400kv outrunner and a BC single motor gearbox on a 12.8v system to drive the ship's single centershafted 2" x 25'ish Rabeoash prop. Although the gear ratio was 3.2:1 (versus Chris's 2.5:1) and the motor had 180 less kv than his, I felt that would be enough to push the ship to a 28 second Treaty format speed. Boy was I wrong! We didn't even bother to measure the ship speed because it was so slow. At a guess, it might have been doing 35 seconds and acel/decel was laughable. Crunching the numbers, the prop RPM was 1600 rpm due to the high gear ratio, lower kv motor, and a rather low pitch prop. The ship needed at least 2x more prop rpm than that. As a comparison, Chris K's Bismarck motor system was capable of around 3000 prop rpm for 24 seconds speed. Luckily, the Verite PDN was in the car and graciously donated it's 2600kv motor. I knew the kv was too much but it was all I had on hand (the other motors being home 600 miles away). After the motor swap, the ship woke up big time. The throttle had to be reduced to 27% to put it on the 28 second Treaty speed. Crunching the numbers, the prop rpm at that throttle setting was around 2800 rpm. Not to far off the Bismark's possible prop rpm. In hind sight, I should have known better than to try the 400kv motor. I did not crunch the numbers ((voltage * kv)/gear ratio=prop rpm) and relied upon guessing. Obviously it didn't work out for that ship and prop combination. A better motor for best performance should be around 800kv for a 28 second Treaty speed. If the ship needs to bump up to 24 second speed for MWC/IRCWCC, I will adjust the throttle of the current setup to figure out the prop rpm needed for 24 seconds, then crunch the numbers to find a better matching kv motor. So a little more has been learned so to speak. By the way, the brushless drive system powered the Sharnhorst amazingly well. It turns as tight as the Erin at the same 28 second speed and acel/decels incredibly good. I can't wait to try it at 24 second speed just to see what it can do. Heh.
I for one am shocked that someone of Darth Kessler's scientific acumen would resort to sacrificing chipmunks and mice in order to obtain data from the Omnissia (His cogs be praised). He should know that at the bare minimum, you need to be sacrificing squirrels (And large ones, at that).
The problem with only comparing prop RPM is that it is only relevant if you're using the same prop. Prop Size is HUGE! As for the chipmunks, it isnt my fault they can't swim forever....
True on prop rpm. Heh. But at least we have a range to start out in. If a person was converting a ship from brushed to brushless, they could possibly measure the prop rpm of the brushed setup to get a target rpm, then work backwards from that to select a brushless motor and a gear ratio if a gearbox is used. That is assuming the ship is usually at speed on the brushed setup. I had always wanted to try using a model aircraft tachometer to see if we could measure prop rpm on the bench. Another thought on the Sharnhorst a couple posts back. Instead of changing the 400kv motor, I could change the gearbox ratio from 3.2:1 to 1.8:1 to get a prop rpm of around 2950. That would have put it on 28 seconds with a little to spare.
Ok so I get the ship to speed and measure the thrust. Does that mean that no matter what method/means I use to get to speed, the thrust would be the same. Are thrust and torque the same? I am not sure what the thrust test would be but I can make my ship get to speed with a fast motor but reducing the pinion or just adjusting the esc.. (Like at NATS.. to 35%).. or I could use a brushed motor with more winds and less rpm and get the same speed with the same pinions but with the esc set higher. Would the thrust be the same in each case? If thrust=torque the I would think it wouldnt be. I heard from bob that if you have the ship underpowered you can lift the stern out of the water and run full speed and if it has low torque it will stop the prop or significantly drop the rpm when you dip the prop back in the water. If you have a lot of torque then the same procedure would result in much less drop in rpm on the prop. (If I am describing it right). My goal is not just to get to speed.. but to have every extra ounce of torque I can possibly get while doing it. I am not sure how to get to that right now.. unless thrust = torque ... I have the recipe in the Kess post. Thanks
For me, the goal is to make speed with the throttle at 90%. That would get the best performance from the motor. Of course, the prop might not be the best prop for the performance I like and might require trying different props, but we have that issue already with brushed systems. Heh.
Thrust doesn't equal torque, but more torque can in some situations help you get more thrust. More torque lets you spin a larger prop at the same speed or the same prop faster, either of which will give you more thrust. Moving more water or moving the same amount of water faster gets you more thrust. remember F=ma So for the forces to balance and the ship to run at a steady/constant speed the thrust must be equal to the drag (acceleration is zero so the sum of the forces has to be zero too). Drag is mostly a factor of hull shape and draft but other things also play into account. Thrust is supplied by the propeller. And yes dynamic thrust and static thrust aren't exactly the same but until we have a good way to measure dynamic thrust static thrust is close enough, heck even if we ever can measure dynamic thrust they'll still be close enough. If you are running at a given speed with a set drag you will ALWAYS need the same amount of thrust. Realistically lots of things can cause your drag or thrust to vary which is why we need to tweak speeds differently at different lakes but if you measured under lab conditions and were able to hold drag constant then you would always need the same amount of thrust, doesnt matter where it comes from, prop, rocket motor, oars, jet engine, flippers, etc. I'm with Mike, I try and get my system to run ~85% to be on speed so I can adjust either way as needed without hassle, after that it comes down to reducing the current/power draw.
There is software available to calculate boat performance with electric motors. Unfortunately, it's not even close to being optimized for our situation. But even with all the limitations, it is a good place to start because it can at least help eliminate some combinations that will not work. The software is called FE Calc, for Fast Electric Calculator. So it's for racing boats, rather than heavy ships. You can download it. I have added some info for props and batteries that we use. There is a text file that is easy to modify to add your own motor, batteries, props, ESCs, connectors, and wire. Limitations: The software is only for direct drive motors. However you can get good results for geared setups by dividing the Kv of the motor by the gear ratio. For example: 1000 Kv motor, geared 2.5:1 = 1000/2.5 or a 400 Kv motor. If you have a way of measuring current draw, measure how much the motor draws with gear box connected. This will give you a new no-load (io) value for the motor that accounts for the gearbox drag. If you can't perform this measurement, assume that the motor will draw about 5% more than the program will calculate. It makes its calculation based on one motor and prop. To estimate speed for a twin shaft setup, multiply the calculated speed by 1.25. Since drag increases by the cube of the speed, doubling thrust only nets a speed increase of the cubic root of 2. The software apparently doesn't account for 4 blade props. Not surprising, you rarely see 3 blade props in racing, and a 4 blade prop doesn't make sense. I don't have a good fix for this problem, one of my experiments required almost 50% more power between the 3 and 4 bladed props, so use that as a start. The prop calculations are based on different profile props. Our standard props have flat blades and no pitch progression, which doesn't match commercial racing props. This means the software is over predicting the power absorption of the props and rpms are less than actuals. Also, calculating what pitch to use is going to need a bit of trial and error. For the props I entered, I figured pitch based on the blade angle at the middle of the blade. It's close enough to give useable results, but it may not be the best estimate. No throttle calculations. However, it does allow inputting cells in less than whole numbers. So to see what partial throttle will get you, multiply the number of cells by the throttle setting. For example: 2 cell LiFe battery, at 60% throttle - 2 * 0.6 = 1.2. So set your cell number as 1.2 instead of 2. Very limited input parameters for hull drag. This is the big problem. You can select from 3 different types of racing hulls and each has three sub-configurations to select. However, I don't know how long the default hulls are or how much they displace. Start by selecting the highest drag hull - "Mono" and "Wet". We can take the resulting calculated speed and apply a hull slip factor to get actual speeds. On my Henri IV, this slip factor is about 60%. This is significant, but until something better comes along I am willing to put up with a program that gets me within +- 100%. This hull slip number will have to come through testing. Examples: Ship: Baltimore heavy cruiser Props: 1.25" 25p 3 blades - the largest prop that will fit, with low pitch to give the most low end thrust. 3 blades is just for the program limitation. Battery: 2 cell LiFe First motor selection: Generic 17 turn car motor. Results 18.8 amps, 15.4 MPH, 62% efficiency. This motor is clearly unsuited, the speed is way too high and it is drawing too many amps. 62% efficiency doesn't look good either, but it not the % that matters, it's the absolute power loss you need to worry about. In this case it is almost 40 watts. I don't want two 40 watt soldering irons under my deck. Second motor selection: Lenhner 2280/11 geared 2:1. Results: 5 amps, 2 MPH, efficiency 0.8%. Wow! Only about 33 watts lost per motor this time, but obviously a poor set up and too slow. Third motor selection: Turnigy 2836/11 . Results 1.1 amps, 4.8 MPH, 29% efficiency. Current draw is good, speed looks fast, but I know actual speed will be less, so it's a good start. An efficiency of 29% doesn't really look good, but only 5 watts is being lost, so not a big problem. This looks good for a start, and if it is too slow I can go to a 4 blade or higher pitch prop while keeping the same motor, battery and prop diameter.
Hey guys, I am thinking of putting a brushless system in a Gearing. Yes, the destroyer. What would you recommend for the drive motor. I am planning a gear box with one drive motor. Also maybe one for the pump but might just keep the standard brushed motor. Thanks Kim
I'd suggest picking a motor or two and then testing with different gear ratios to tweak the output to something which suits your needs. Trying to limimt yourself to one gear ratio isn't wise.
I'll second the comment on the gear ratio. 3/4" props in particular have to spin pretty fast to make any thrust, so a 2.5 gear ratio is going to make for a high RPM motor. For your setup, my calculator gives me a needed prop RPM of around 12,000. To get that with a 2.5 gear ratio you need a motor with a Kv of 4700 to 5700 RPM/volt. That will let you use a short 20mm inrunner or even a 12mm inrunner. The 12mm inrunner actually makes sense, I use a 12mm brushed motor with a 6.2 gear ratio driving two 3/4" props in my Golo. The Golo easily makes the 100' in 30 seconds. It probabally would not make 22 seconds, but it is a short ship plus it just has too much gear ratio. I recommend that you go up to 1" props if you can make them fit. I'm going to 1" props even on my Gridley, not because I need them to make speed necessarily, but because they will give me more low end thrust. Even going up to 7/8" props would give you an advantage. The 1" props only need about 6400 RPM to make the same thrust, so with the 2.5 gear ratio your motor Kv would be 2700 - 3200.
