Magmotor S28-400 with 8 inch prop direct drive.

I decided to make the trolling motor prop fit onto my 3/4" shaft and test it out to see what kind of performance it has. I did testing at 24 volts and it pulled 3000 watts of energy. Which is of course more then the 5 inch prop testing was done before. Doing my rough efficiency calculates that 2.68HP went to the prop.

The tests were done the small pool, the rpm's seemed decnetly high. Most people think the minn kota trolling motor prop is basically a 10 inch prop, 4 inch pitch. maybe even smaller pitch, there are a few different props they have (that i know of) and this is from their 55lbs thrust motor. The most important part of all this is that it gave off what felt like a large amount of static thrust, more so then any other prop i've used so far.

I ordered a small inflatable boat (partially for fun) to do some testing on. There are 2 lakes near me, one is all low power and closer to me, I think it may be fun to take a small inflatable boat out there. Here is the link to it: http://www.interstatedeals.com/index.php?main_page=product_info&products_id=1916

I plan to test out how much it pushes the boat. Granted the inflatable boat is probably horribly un-efficient, but i don't really care, i think it'll be fun. I'm basically looking for a difference in performance between the trolling motor and my propulsion unit. even 1mph difference is substantial. I'm doubting it'll go very fast, even with my high power unit, if I hit 5mph i'd be surprised, but I do want to see the watts pulled while moving through the water as opposed to statically. Also for testing purposes i'll be able to use both at the same time ,which may allow me to move faster, therefore testing more of a range of speed... although together probably 6mph would be max.. i have no idea really. Trolling motors simply don't push anything faster then 6mph so the trolling motor won't do anything past that, they are rated at 1600rpm though, my motor with the same prop is rated at 4000-4900rpm, at least to deliver the power i'm looking for. So that would put it at 15mph if indeed the trolling motor is 6... granted it's all theoretical, all i'm really interested in, as I said before, is if there is a difference in performance between the trolling motor and my motor.

It's supposed to be delivered monday, and hopefully i'll be able to test it out monday, but i'll probably fill it up in the garage first to make sure I know how to hookup the motor and everything. We'll see.

Ross

 

Ross
If that inflatable has a hard bottom I would not be surprised if you get it on the plane with the Min-kota prop.

I have attached the performance curve for the prop at 4000rpm, 3500rpm and 3200rpm. This is based on the numbers you gave for the prop - 10X4 and 55lbs at 1600rpm. If these are accurate then my numbers will be close.


I expect the drag on the boat at 15kts (say 7.5m/s) to be around 0.4kN. So this would require something like 5kW at 3800rpm. Probably beyond the motor. At 3200rpm you might get to 12kts using 3kW.

You need to be able to get plenty of weight forward to get it to plane easily but once on the plane moving aft should increase speed. Fairing the underwater bits is also important if you really want to get a decent speed.

Rick W.

 

Ross,

The s28 motor have really great power but will not run too long at high power levels. Watch your time under high power as the motor will overheat even with a big heatsink. I went the same route as you in my electric row boat efforts. I used a much cheaper motor but initially did not consider the thermal issues. Unvented motors, even when efficient have a limited amount of constant power they can produce. You should be ok for a few minutes but back off to allow for cooling.

I tried submerging my motor and allowing the water to cool the housing (like a trolling motor) and still burnt the windings quickly.

Joe

 

Joe,

Yea, I am concerned with thermal issues, I was hoping water cooling would be more effective but you said that didn't seem to help with yours. Did you completly destroy the motor when you overheated? Also how much power were you running for and how long before it died?

I've considered switching to a brushless motor, as they are 10 times easier to cool. That's the problem with motors like the s28, the coils are on the inside and that's what heats up, hard to cool if not vented, and technically even if it was vented it's still harder to cool then when they put the coils on the outside. Magmotor does sell some brushless motors, not sure how much they cost though, but they could probably be cooled much easier since the coils are permenantly mounted on the outter most part of the case.

See, my main problem is I need a small motor, at most 4 inches in diameter, and getting enough power in that size is hard. Luckily my project is not a boat and the motor doesn't have to last for an hour, or even half an hour... but i'm hoping for at least 5 minutes of hardcore use... with minimal breaks. The s28 might last, dunno, I was going to use a microcontroller to monitor the temperature of the motor and decrease the amount of power allowed to goto it if it gets to certain temperatures, much how most motor controllers work with their own temperature. At least this would hopefully allow me to not burn out the motor, but also it might make me only run for 45 seconds and then all of a sudden I lose all my power. I'd rather find a motor that can work for me.

I'd love to use something like the Mars brushelss motor, but it's size doesn't fit my project. I was even toying with the idea of using 2 s28 motors and mount them end to end. Some of the larger brushless motors from Magmotor would hopefully only need one. Maybe I should contact them about their other motors.

Ross

 

Other motors

The brushless motors look really good on paper. I know they would be extremely expensive. There was a company marketing a high-end type of trolling motor using a brushless PM motor. Claims were around 5-6 HP. If you could get this motor and package it, you would be done!

http://www.solarnavigator.net/american_marine_electric_motors.htm

I was using the NPS (.7hp) motor which should only be run at .15 hp regardless of cooling.

Some rc boat motors use water cooling. The best plan for PM brush motors would have water flowing through a hollow shaft. Sounds too dangerous to me.

My Lesson PM brush motors are rated 6.5 hp for 15 minutes but are much larger than event the c40. At 5.25" diameter, they would be too large.

I know you plan on a submerged application so brushless might be the best way to deal with the thermal issues.

Joe

 

Check out this motor:

http://www.icare-rc.com/plettenberg_predator.htm

They claim an unbelievable 10kw from that, granted, in an RC airplane they have access to about unlimited air cooling. Water cooling would be hard to compete with in this exact case. Usually water cooling is dramatically better, but would require the engine to be designed for it.

That motor is only 4 inch diameter and 3 inchs long (roughly). Perhaps building a custom external case for it to distribute water cooling would be required, it already costs over a thousand dollars eheh, let's see, it's $1059.. a controller for that would be pricey.

Now lets pretend I did get that motor, really i'd only want to push it at about 5kw, but even then, still the heat dissipation requirement would be tough, creating a case that could be basically dipped in the water like a trolling motor would be the best method.

There's lots of people that use this motor in RC airplanes and have data bout it, but no one talks about how hard they push it for how long... so it may really have about the same issues as a magmotor. Being that the coils are on the outside though, cooling would be 10 times more effective, so maybe it would be able to get 5kw continuous, although I only need it for about 5 minutes for my application (for the prototype).

If I had an unlimited budget i might consider buying this motor and trying it out, however it seems like a gamble.. maybe continuing the prototype with dual s28's just to see how they would perform would be a better idea, and easier on the wallet

Ross

 

Found my S28 datasheet

The motor is rated at 1.8 deg C per watt in thermal resistance. This will allow about 72 watts of power to be generated (losses inside the motor)before you exceed the motor winding temp. Running 83% eff will give you 350 watts of output power for continuous use without overheating the windings. The data sheet says 250 watts is max. Short term use could be way higher especially in the High performance unit that the robot guys sell.

I like the C40 motor better as is has lower thermal resistance, more mass (longer thermal time constant), and is cheaper. It also spins slower which might be handy.

Configuring your motors so the mounting plate is water cooled should help some what. I think you metioned earlier that this was in you long term plans.

Consider your battery capacity and you might not have an actual problem.

With my forklift motor set-up, the batteries will drain before I exceed the motor thermal limits. In your final set-up, you will most likely have the same.

Joe

 

Ross
Look at this beast:
http://forums.radiocontrolzone.com/showthread.php?t=227426
Still need to be cooled but not bad for 4" diameter. The performance of these things (at least for a few seconds) is utstanding.

You need to get together with Amp User here:
http://boatdesign.net/forums/showthread.php?t=12918&page=2

I am not so sure my Mars purchases were the best but that is the way with technology.

Rick W.

 

Well I took out my inflatable boat today, this is kinda unrelated to my s28 motor, well I had planned on testing it but was to difficult. The inflatable boat for the cost is quite nice, but it was pretty cheap for inflatable boats so it really isn't that great. The trolling motor mount wasn't very sturdy at all, although I think I could re-inforce it easily. Using the trolling motor at full throttle (55lbs thrust) it went about 4mph (gps). I really would of liked to see the speed of my thruster, but the weight of the current lead acid batteries I felt might have been to much.. which I think I would of been right. The "hard bottom" they speak of is the place you stand, which is what I though, in fact it is "hard" but it is not sturdy.. it's plastic coated foam, it's better then nothing but not really very sturdy.

The lake I took it to is a 10hp and under lake, and it was so glass smooth testing would be nice out there. Even though the inflatable boat wasn't that great of a success, i'm going to see if I can build a real hard bottom, at least for where I stand, and actually it allows the floor to zip away and I could almost embed it into the entire "structure" if you could call it that. I might be able to make a form for the hull to give is more of a V type hull. So I would design a V-hull bottom with a flat top that would be inserted into it. Building an entire boat seems a bit to daunting at the moment and this might be a good way to go inbetween these 2 things. I'm going to have to look over the boat when I get a chance, but it would make for a really nice test platform I believe. Really the lake I get to use it on is the nicest part, as the low horsepower means not many waves (at least if the wind is down) but the other lake there is no chance of smoothness.

One concern though was somehow water got into the floats, when I was deflating it there was lots and lots of water in them... Now perhaps I did something wrong, but I'm pretty sure there's only one fill hole (going to verify this) and if the thing was airtight, which it was, how in the hell did water get in there. One thing that might of happened, when I came in i unplugged it int he water while I was loading stuff into the car, and perhaps then it dipped under and got water into it... So it may of been my fault, which is really the only logical explanation. The boat did seem slower on the way back, but then again the wind could of easily been to blame for a .7mph difference

Rick,
Yea that predator motor is the one I posted earlier, it's hard to believe they are getting that kind of power out of that size. I wouldnt' feel bad about your Mars brushless motor purchase, the quality of it is far higher, and therefore more reliable. The predator woudl overheat quickly they didn't use them in RC airplanes going 100mph through the air. The mars motor is unfortunatly to large for me though, otherwise I would of bought it already. Perhaps for my inflatable boat Although for that I have an Advanced DC 140-01-4005 motor that should work nicely.

Ross

 

Ross
You can knock up a nice little catamaran from aluminium in a weekend if you want a simple test platform. The hulls would be easily driven so low power performance would be impressive. You could also use it as a battery and test gear carrier for tow testing a person through the water. It would have negligible drag by comparison with the body in the water.

There are a few ways to make it but you need a couple of sheets of aluminium say 0.8mm or 1mm thick. You could just use ply but it would need more time to paint it. It would be suitable for car-topping or stuffing in the back of a truck. Lets say 3ft wide by 12ft long.

My irrigation pipe aluminium catamaran is ideal for testing. Nice and stable. Easily driven so even with low power it does reasonable speed.

You need no special skills. I cut the aluminium with a thin blade on a grinder so the edge is not stretched. You need rubber strips and stainless screws for sealing it up.

Rick

 

Well I tested the S28-400 motor on the lake with the inflatable boat. I used my dewalt 36 volt (which is really 33 volt) battery pack. I have the wires hard wired directly to the A123 cells as the Dewalt pack has a battery management system with an internal 15 amp fuse.

Using my ammeter with the trolling motor, it pulls 50-60 amps from a large 12 volt DC battery, equating to about 1HP before you take out efficiency losses... which put it around 3/4 of a HP.

The ammeter with my unit and the magmotor showed 2700 watts were pulled at the peak, which is a lot of current, around 80 amp. Calculating efficiencies puts it at 2.5HP, and moved the boat a tiny bit faster. Oh yes, I used the same propeller as the trolling motor, adapted it to my unit.

Now, it was more powerful, but it pulled TONS more power for what seemed to be just a little bit more performance, but again this may be as expected. Most claim that trolling motors are designed to run at 1200 to 1600 RPM for the prop and power at hand. Now I put a motor that is supposed to run at 4900RPM with the same prop, which may make it run super unefficiently... It's kinda like a stalled DC motor, you may be pushing 3000 watts to it, but nothing's moving. Basically a stalled mootr is considered 0% efficient, i'm hoping that i'm at some rpm that basically is equating to 20% efficiency because I have the RPM's wrong. Now, this doesn't neccesarily mean it's the wrong prop for the job, as I start moving through the water it may become the right prop, meaning it's not good at the slow speed but works well at the higher speed (which is important to me as well). Only a real world test will determine this, or a full dynometer test of the S28-400 at 33 volts with a ~120amp limited battery source, does anyone have this chart?

One thing I learned out of this test is that a single Dewalt DC9360 battery pack can deliver the power needed for one of these motors, which is quite impressive, those A123 cells are very nice. They peaked for a moment at 140amps according to the ammeter unit, A123's specifications say 120amps for 10 seconds, so it's probably possible, I dunno. Although even at 70 amps the battery will only last approx 2 minutes. But to keep costs down for the prototype, 2 minutes is plenty of time until it proves to work, then the battery pack can be increased easily. And that's 2 minutes of full power, as I start to move through the water I hope it drops at least somewhat...

I'm still not sure if going with 2 motors per thrust unit (quad total) will do what I want. I hesitate for 2 reasons, money obviously and plus i'm trying ot make this really compact and using 2 would lengthen it.

So really i'm not sure what i'm going to move forward with at this point, I think making the unit completly battery operated and able to test it freely in the water. I have basically all the equipment to do this (unless I decide to go with 2 motors..) so I guess I need to redesign my thruster.

Ross

 

I would expect it to perform much better than the trolling motor. It would be good to some pictures and videos of what you are actually doing.

Unless the inflatable boat just slumps in the water it will have LESS drag than a human body. So if you are not getting the boat to move well then it will not do much as a dive thruster.

Your comments on the Dewalt pack is interesting. Did you buy the pack alone or with a power tool? I have been eying off some lithium batteries and it was suggested the DeWalt pack is a good way to go.

Rick

 

Rick,

The inflatable boat is horrible... it is NOT hard bottom and it's hard to determine how much negative impact it could have on performance, i'm assuming a large amount, but have no way to verify this. Plus even the trolling motor's performance has varied dramatically from being able to get 4.5mph (gps) to only getting 2.7mph, and i don't think wind had to much to do with it, but who knows...

I think everyone in the RC hobby agrees dewalt cells are the way to go, goto rcuniverse or something like that, no one has found a cheaper way of getting a123 cells. Your volume has to surpass a million to beat buying Dewalt packs. I had quite the experience getting mine, I initially ordered 3 packs total, 1 from one place (the one i'm using) and 2 from another. All on ebay trying to get a deal. 2 of them ended being bad, luckily the guy allowed me to return them, but it was a pain in the ***, I had to explain to him why they were bad. He claimed they worked on a power tool but I had to explain to him that using the charger was the only real way to see if it's bad or not as it tests each cell individually. Anyway, www.homedepot.com, search for DC9360, the price is $180 with like $8 shipping (for me at least) and you can trust home depot. You can find them cheaper on ebay, but i'm not sure it's worth the risk... I guess you are in australia or something though, not sure how shipping would be with that.

The dewalt packs are limited to 15 amp though if you use their connector, I just opened it and bypassed it, clearly breaching 15 amp's and the batteries are fine.

As far as my inflatable boat, I've been looking for a small catamaran sail boat or something, as they are lightweight and without the sail would basically be what I want. Sometimes you can find good deals on sail boats as well. And heck sailing might be fun to try. Actually when I was out today I saw the perfect catamaran sailboat. It would be fun to build one as well, but this thruster is one of many many projects i'm working on and I don't know if adding another is a good idea. And buying one is just another expense i'm not sure i want eheh. I half thought about buying 6 inch PVC pipes and just capping the ends and joining them together eheheheh, it would probably work There is a youtube video of a guy doing this, quite funny yet interesting at the same time.

Ross

 

Ross
I appreciate the information on the batteries. I get quite a few items mailed from US. THe exchange rate is good right now.

Have you seen the attached V4 video. My youngest son pushing out around 500W on a couple of lengths of 142mm irrigation pipe with pointy ends. He can get to 15kph on this. The boat weighs in at 30kg before adding the seat and various propulsion systems I have tried.

As I said earlier you can make aluminium hulls for a cat from aluminium sheet in a weekend.

Rick W.

 

Rick,

Yea I remember you mentioning that before, do you have any better pictures of your design? Based on what I see it doesn't seem to complex, just make the ends pointy and then put something in the middle. I recall you saying you sealed them with a piece of rubber, i'm thinking of using expanding foam through the whole thing, should increase the structural stability of the pipe as well. So besides making the ends pointy is there anything I should know?

I do weigh 250 pounds, that might have something to do with the bad performance Plus the 12 volt battery I use is a 115AH 12 volt battery for the trolling motor, it weighs a lot as well, at least 50 pounds.

Ross