Magmotor S28-400 with 8 inch prop direct drive.

Discussion in 'Hybrid' started by klick, Jul 3, 2008.

  1. klick
    Joined: Apr 2008
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    klick Junior Member

    I'm trying to come up with an electric drive system using a Magmotor S28-400 and was hoping to utilize this in a direct drive scenario.

    For more information on the motor, here is a link: http://www.robotcombat.com/products/MAG-S28-400.html

    Basically it's claimed (by many, search the web for people using it) to be around 4.5 horsepower and it's very small. If you click on the Tech Specs from the above link, the main chart says 4900rpm, but the chart is very vague what this is refering too. Most DC motors if they post an RPM is unloaded RPM value, whereas this I really don't know, i'm not sure if that's when they got 4.5HP, or just some RPM they felt like posting.

    Anyway, I have this motor, I hooked it upto a prop which is 8" diameter 8" pitch. I floored my motor controller with it and although it produced power, it wasn't very significant. If I had to guess pounds of thrust I would say from 10 to 30 (clearly I need to hookup a method to measure this). Now, without hooking up an RPM guage, amp meter, all that stuff, is this prop just way to big for a direct drive? I was thinking of getting a 6" prop and trying that, also maybe a 7", although I hate to just waste money buying props if it's useless, perhaps I have to gear down to get this to work, I dunno, that's why i'm asking evyerone here :)

    Also, the props are actually industrial mixing propellers, available from mcmaster, here is the 8" prop I have: goto mcmaster.com and search for: 8004K251 The reason I used a mixing propeller is because I wanted a source where I could get both right and and left turning propellers, because eventually I plan to use 2 of these motors in dual, using counter-rotating props to eliminate the torque.

    Ok, I think that's enough info for this time, so to sum up, does an 8 inch prop seem to big, to small? Am I going ot have to gear down to get any level of efficiency?

    Also i've seen people with videos using DC amp guages that are digital, where are those available? I was hoping locally but I can find one online i'm sure.

    Thanks,
    Ross
     
  2. Guest625101138

    Guest625101138 Previous Member

    The motor will produce design power around 4000rpm. This is likely to be a bit high for the ideal boat set up to transport people. If it is a large planing model it will be OK.

    So a lot depends on the type of boat. The 8" prop will give good efficiency at 20kts. To get this speed using 4.5HP will be a significant challenge if you want to travel in the boat with a bit of gear. Not a problem if it is radio controlled model boat.

    Going to the smaller prop will give OK performance at 16kts.

    If you explain more about your objective then it might be possible to give a better answer.

    If you are serious about testing all of this then this could be worth the money:
    http://www.powerstream.com/DC-clamp.htm
    Way better than shunts. For low current you can get a $16 multimeter from Tandy that reads a bit over 10A. Only suitable for no-load testing though.

    Rick W.
     
  3. klick
    Joined: Apr 2008
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    klick Junior Member

    Rick, appreciate the response, i've watched your video on your electric boat, very nice project.

    At this point i'm just goofing around, but if I can produce the kind of thrust i'm looking for I want to build something like a seabob: http://www.seabob.com/ So it's not really a boat, so not sure how much power will be needed. The seabob's pricing start at $30000 and you can't even buy them here, I figured it might be a fun project to try and re-create one.

    Now I can say that at 24 volt, it produced no where near enough power to get anything upto any amount of speed. I understand it changes as it starts to carve through the water, the motor begins to spin faster and it may hit a better part of the electric motors power curve, allowing it to push farther, but it just wasn't enough static thrust even. I do possibly intend to use 2 of these side by side, but the seabob is supposedly 4.5HP, so I would think I could get similar performance, well at least half the performance hehe.

    I may re-try my test at 36 volt, that may make it start to pull more current. As of right now though, at 24 volt I have no idea if it pulled 1HP of electrical energy (746 watts) or 4HP during my initial test, perhaps it only pulled 300watt, i dunno. That's why I need to get some kind of amp gauge to determine what power it's actually pulling.

    I think I may order a 6" and 7" prop to test with. Do you think that the "mixing propeller" I am getting from mcmaster is possibly a bad design for boating? Is there something special about boat props as opposed to the picture they have next to it on mcmaster? I may search for some small composite boat props, not sure if 6" is available or not...

    Ross
     
  4. Guest625101138

    Guest625101138 Previous Member

    Ross
    You are confusing power and thrust. A stalled prop like you have will operate poorly. Power is thrust times speed.

    I estimate that the vehicle you are looking at might get to 6kts. I have not looked to see what the thrust required is but 4.5HP can produce a lot of grunt with the right prop.

    Your best result without gearing will be achieved with one of the 5" Mcmaster props operating in a shroud. The efficiency is OK. Under these conditions it will produce the sort of thrust that will take some hanging on. Of the order of average person body weight. So all that can be endured for any reasonable period.

    Thrust at stall (zero speed) will probably be around 1/3 of this but this depends on the shape of the blades and I do not have this detail.

    You will need decent batteries to get this power level at 24V - say 40Ah. At say 200A they will not last long.

    Rick W.
     
  5. klick
    Joined: Apr 2008
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    klick Junior Member

    Ok, I ordered the 5" prop, we'll see how that does when I get it next week.

    I do realize that static thrust versus how much thrust at speed is very different, although hard to test. Based on what you said though, the 8 inch prop seems oversized no matter what. I do plan to shroud the propeller eventually, although not initially.

    Would it be more efficient to go with the 8 inch prop and gear it down? I'm looking at a working speed between 0 and 15 knots (at best) I guess, which is along the lines of the seabob. Honestly i'd like 20, but i know that's truely pushing it. I may go dual with this setup, well most likely I will, but still i'm not doing that unless I can get some decent performance out of one prop, hopefully the 5 inch prop will show me some performance to excite me and therefore invest more money into the project.

    As far as batteries, the eventual idea is to use lithium ion, which i realize will cost thousands, but if I can get this device to do what I hope, i'd be willing to spend that kind of money. For the final version I expect batteries to be around $4000, however i'm not where near to that point yet, so i'm not going to get them until I can prove that I can get the rest of the system as I need.

    That ampmeter you showed is very nice, although not sure I want to invest over $200 just for that. It would be nice to have though.

    Ross
     
  6. Guest625101138

    Guest625101138 Previous Member

    You cannot just select a speed and expect it can be achieved with a low power motor. You need to work out what drag you get at given speed. Getting dragged through the water at 15 knots will need super strength. If you donned a dolphin shaped suit you might be able to hang on. So first step in the design is to determine the hull shape and calculate the forces involved in getting it to a certain speed.

    You then see where the thrust curve and drag curve intercept for a given power level. This becomes the design speed. All of what you are proposing can be calculated with reasonable precision.

    The larger diameter prop running at lower rpm will be more efficient but there is very little difference if they are both shrouded. The constraint you have with the props is the pitch. The props you are looking at are "square" or over square. 8 X 8 for example. The best prop and rpm will depend on the design speed that can be achieved with the available power.

    You need to be aware that many lithium-ion batteries (if not all) are an explosion risk when placed in water so this is a matter that needs careful consideration. If your head happens to be in the vicinity of the unit when it goes bang it could cost more than your bank balance.

    The ammeter would probably not be too useful in a craft designed to dive. Any testing at zero speed will not mean much. It really only tells you something about the shapes of the blades.

    If you have access to a boat to do some tow tests you should determine the forces it takes to drag your body through the water. I think you will find 15kts is an absolute joke.

    I once made a 6ft long planing hull that would easily get to 20kts with a 6HP outboard. It was a challenge to make it plane as I needed to lean right forward but once on the plane it was a flyer. I have seen a small boat plane with a 12 year boy and 3HP outboard. So you can go fast with low power but it takes good design. The human body is not highly evolved for moving at speed through the water. Even dolphins prefer to fly in air above 3m/s (6 kts) as often as possible.

    The thrust available from an unshrouded 5" prop is just over half what you will get from the shrouded prop at 6kts. It will need to be shrouded in any event so you may as well sort this aspect out now rather than later. Unshrouded you should get around 60lbf force at zero speed and full rpm - it will depend somewhat on the shape of the blades though. I can tell you how to change them to get higher thrust but this only applies to static thrust, which has no bearing on your end objective.

    Rick W.
     
  7. klick
    Joined: Apr 2008
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    klick Junior Member

    Rick,

    I didn't realize a shrouded prop would be such an improvment, I knew it would be some improvement but you seem to think it will be quite significant, so I will definitly get on that.

    I was also not aware of water causing lithium ion cells to explode, and cannot find any information on the web about this. I will be utilizing Lithium Nano-phosphate batteries, actually the exact cells described on www.a123systems.com. I have 10 cells of their 26650 cells. Fully charged I dipped them into water after your post, they didn't seem to do anything, I left them in there for a few minutes then pulled them out. I dryed them off and then re-hooked them up and the cell tester says they are all fine. I know there have been many lithium ion related fires/issues, but never with lithium nano phosphate cells, which is why they are being mass produced for the automotive industry.

    I do have access to a boat, testing will be tough but I see that it will probably be worth it, at least to understand how it acts at speed.

    I'll post the results of the 5 inch prop once I receive it next week.

    Ross
     
  8. Guest625101138

    Guest625101138 Previous Member

    The benefit of prop shrouding varies depending on application. In the range of operation you are considering with the props you have chosen it will provide a substantial benefit. To get the results I am calculating you may need to modify the blades slightly to suit the shroud but the application responds well to shrouding. Shrouding the 5 X 5, 4000rpm prop lifts efficiency from just under 50% to almost 80% at 6kts. A benefit of 60% over the unshrouded case.

    Here is an example of a small shrouded prop:
    http://www.youtube.com/watch?v=m2aMX2lW0zA
    Prop around 4" and motor about 1HP. Will take a lot of effort to paddle a small sit-on at the speeds this achieves.

    The gain comes from lowering the induced drag of the heavily loaded low aspect blades. For my applications I use high aspect blades and there is no benefit in shrouding because the small gain in efficiency is lost to dragging the shroud through the water. You will need a shroud to protect the user so you already suffer this cost irrespective of any gain in efficiency, which just happens to be substantial.

    There is plenty of information on the explosion risk. Google lithium battery explosion. Add "water" to the search if you want to be more specific. Here is a sample:
    http://www.nerc.ac.uk/about/work/policy/safety/documents/guidance_lithium_batteries.pdf

    The A123 cells reportedly have the lithium stabilised so do not present the same risk as others. There are still issues though in the way they are used to get good life. I have corresponded with Bill Dube of Killacycle on the issues and he recommends using DeWalt batteries and their charger unless you really understand what is going on. There are lengthy discussion on the topic. I have not priced the DeWalt gear.

    Rick W.
     
  9. klick
    Joined: Apr 2008
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    klick Junior Member

    Well i got the 5 inch prop in and tested, at first it looked tiny and wasn't sure how it would act but it did surprisingly well.

    I decided to do some testing on land with a small fillup pool versus off the back of my boat where i almost fell in. Also I re-ran the 8 inch prop tests, also I tested both at 24 volt and 36 volt to see if there was any difference.

    At 36 volt the 8 inch prop still didn't produce that much thrust, but did alright. At 36 volt the 5 inch prop produced roughly twice as much static thrust as the 8 inch (rough estimate). So that was encouraging. I really don't know how many pounds of static thrust, but it took quite a bit of muscle to hold it back, so i'd have to say anywhere from 30 to 60 pounds (obviously I need a guage). I'm half tempted to goto Dicks and buy a trolling motor and see how their rated thrust feels in comparison. Plus it might be interesting to do some other testing with one of their motors.

    Next I need to build a shroud for the 5 inch prop, get that increased energy and efficiency. I'd really like to also test a 6 inch prop, but not sure it's worth it at this point. I believe i've gained enough confidence to move forward and try making this a more usable unit.

    And yes I was planning to use A123 cells, or ones with the same chemical composition, as really IMO there are no other lithium ion batteries worth using besides the safe ones. It's just not worth it to me. Plus really they are cheaper and better documented.

    It will be interesting to see how the thurst output changes when it starts to move through the water, oh my how difficult that'll be to test eheh. Also I need to figure out a better way to seal this shaft, I have a dripless shaft seal from http://www.shaftseal.com/ but it's quite bulky for my application, other devices just seem to use some type of O-Ring to keep the water out, I found an O-Ring with a stainless steel pre-loaded spring in the middle, keeps pressure on the shaft, I might try that, but not sure how effective they are.

    I'm gonna get to work on some type of shroud and hopefully get some testing in with that, hoping for a little bit of a static thrust improvement, we'll see :)

    Ross
     
  10. marshmat
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    marshmat Senior Member

    Trying to figure out just what your motor is doing with that output shaft is proving interesting.

    From the spec page it looks like it should be giving you about 19 ft.lb torque and 4.5 hp continuous, at 24 V. The peak current rating of 390 A would suggest about 9.3 kW or 12.5 hp could be achieved for brief bursts, although I don't think I'd want to sustain that for more than a few seconds.

    Still, though, I don't think direct drive is the optimal way to go with this unit. The discussion so far has given some good insight into optimizing your thrust in a direct-drive setup. Have we completely ruled out a reduction (belt, gear or chain) though? If we really are talking six knots, a larger diameter / lower pitch prop, swinging at lower rpm, might be worth comparing.

    I've gotta echo the safety concerns about Li-ion cells. I have worked with several varieties of these and have seen what they can do if things go even the slightest bit outside of normal range. They absolutely must have dedicated fail-safe protection circuitry to cut the pack off from everything else in case of overvoltage, undervoltage, overcurrent, imbalance, overheating, etc. The explosions that can result from mistreatment are spectacular but also very scary. Accidentally crossing terminals for even a millisecond gives you not a spark, but a superheated plasma arc carrying hundreds of amps. Undervoltage can short them out, exceeding any of the maximum ratings can puff them up like party balloons. Puncturing or rupturing one releases a nasty cocktail of highly toxic fumes. If you use these, good commercial protection circuitry is essential, and don't tamper with it.
     
  11. klick
    Joined: Apr 2008
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    klick Junior Member

    Well the design speed is not 6 knots, more like 15 knots, if 4000rpm is the "butter zone" with with magmotor then a 5 inch prop with 13% prop slippage (not sure if that's right, found it on some site) then the speed it outputs would be 16.4mph... which is like 14 knots, so I think this prop is good. Granted I do realize how unlikely it may be to obtain that speed, I want to try ehhe.

    I am most definitely going to use 2 of these motors with two 5 inch propellers counter rotating. So my power may be along the lines of 9HP, although I doubt i'm going to push these motors to their full potential. I'm actually contimplating using 4 of the motors total, joining them end to end (as the shaft comes out the top of the motors) But still only driving 2 props from them, grnated this thinking is way down the line. I believe i'm going to move ahead with developing a unit that is more testable with just a single motor, and go from there.

    Here are some seabob specifications I found on their fastest model (which is what in some ways i'm trying to create). I pulled this info from here: http://cgi.ebay.com/SEABOB-Cayago-F...165311QQihZ005QQcategoryZ114236QQcmdZViewItem
    Seabob specifications:
    Performance up to: 5.2 kW (7 HP)
    Maximum torque up to: 22 Nm
    Maximum thrust up to: 734 N
    Speed over water up to: 22 km/h (13.7 mph)
    Speed under water up to: 16 km/h (9.9 mph)

    734 newtons converts to 165 pounds of thrust, which is interesting to see what thrust they are getting from 7HP. Granted as you can see, their design speed is somewhere between 9.9mph and 13.7mph. They use a large prop (if you can call it that) that is geared down, it's appears to be like 12 inches in diameter but is multi bladed, like 20 blades. Kind of like the ducted fan on a high bypass jet engine.... kinda. I have no means to design a prop like this, so the standard prop is what I have to work with.

    I did buy/order a ammeter, http://www.rc-electronics-usa.com/motorized-bicycles/electric-bike-meter.html really it's one that was designed for the electric bike community, it shows the voltage and amps, total watts hours, all sorts of neat things. Was relatively inexpensive compared to other ammeters and it has a realtime LCD display which is really nice. Hopefully get that by the end of the week and i'll be able to tell how much power the magmotor is really pulling with the 5 inch prop. Hopefully i'll have the shroud by then, i'll be interested to test with and without the shroud. Would be nice to know the RPM, the only thing I have doesn't work underwater ehhe, so haven't figured out how to get that info yet.

    How important is the tolerance of the shroud to the edge of the blade? You think I should be 1/4 inch from the blade tip to the edge? Closer? Really I should be able to get it pretty close, I think i'll go for 1/16" tolerances from the prop tip to the inside of the shroud. Unless there is no reason.

    Ross
     
  12. Guest625101138

    Guest625101138 Previous Member

    Ross
    If you can get the shroud clearance to 1/16" it should be reasonable. To get the best result you will need to machine the blade tips down so more of the blade is near the shroud. To do this you could use sat a trimmed 6" prop within the 5" shroud. It does then look more like a ducted fan.

    You can buy low cost electronic scales from Ebay that are suitable for measuring force.

    If the shrouding on the prop is effective you should be getting a thrust around 70lbf at zero speed. Power demand will be around 1.5kW. Once you get the unit moving the thrust will get up around the 140lbf mark if the shroud is effective.

    The motor speed can be determined by the motor current and voltage using the data curves. If these have not been supplied they can be worked out quite easily. You will probably find the curves on the www.

    Rick W.
     
  13. klick
    Joined: Apr 2008
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    klick Junior Member

    Well I did goto the lake and do some more tests... I had a few issues with the motor controller but was still able to do some adequate tests. I built the shroud around the 5 inch propeller, the tolerances are quite close, the 5" propeller from mcmaster is not exactly 5 inch and was hard to determine the exact size, regardless the shroud is within a 1/16 of an inch all around.

    The test on the boat determined a good amount of thrust output, although once again I have no real way to determine the exact amount of thrust. I did buy a 55 pound of thrust minn kota trolling motor. Using this motor we tested how long it took to spin the boat around in a circle. There wasn't a huge difference between my propulsion system and the trolling motor. The magmotor propulsion unit did seem faster, probably 25 to 50% better, which is along the lines of what Rick had mentioned, that it woudl be around 70 pounds of thrust. Granted spinning the boat around isn't a very scientific test I think it does provide some information.

    So the tests IMO were encouraging, this is along the lines of what I expected. I plan to create a much better setup at home for testing thrust output and also determine watts being pulled.

    I did get my new gauge for determining amps/watts pulled but didn't get a chance to use it today. Hopefully i'll get it setup later at home.

    Also even though I did tests on the boat, I didn't do any drag tests, at this phase they are quite difficult and really we had other complications today that limited this from happenening. I plan not to focus on boat tests but on home tests for a while to get a few things ironed out.

    Total static thrust is around 70 pounds per motor, which I plan to use 2 motors total, so 140 pounds of static thrust is pretty good... I'm half tempted to use 4 motors total, although that's quite cost prohibitive, it might be the only way to provide the performance i'm looking for though, not sure at this point.

    I need to figure out if my active water cooling system will disipate heat from the motors quickly enough for them to run at full speed continuously, well, maybe not full speed, but at a decent amount of load, around 1.5HP. so lots of testing will follow.

    Ross
     
  14. Guest625101138

    Guest625101138 Previous Member

    Ross
    Where are the photos and videos? Very disappointed you are having all this fun without giving us a bit more to look at it!!

    Rick W.
     

  15. Guest625101138

    Guest625101138 Previous Member

    Ross
    It is worthwhile noting that testing static thrust is next to useless for the type of unit you want to build.

    I can generate about 200lbf in a pedal boat with the right prop. I will not go very fast using that prop though.

    Static thrust is only meaningful if you want to have tug-of-war competition. It is a meaningless measure if you want the boat to actually move with some speed.

    So doing thrust tests in a tank may be useful to test water sealing or some other parameter but thrust will not correlate well with tests on an actual craft. Even higher thrust in the tank will not mean better results in a moving condition.

    The thrust figures given for electric outboards is just marketing hype. Has no bearing on how the thing will perform on a boat that is intended to go forward.

    Rick W.
     
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