Magmotor S28-400 with 8 inch prop direct drive. [Archive]

klick

07-03-2008, 03:23 PM

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

Rick Willoughby

07-03-2008, 06:18 PM

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.

klick

07-03-2008, 09:17 PM

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

Rick Willoughby

07-03-2008, 09:33 PM

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.

klick

07-03-2008, 10:33 PM

Rick Willoughby

07-03-2008, 11:40 PM

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

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.

klick

07-04-2008, 01:33 AM

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

Rick Willoughby

07-04-2008, 01:55 AM

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.

klick

07-08-2008, 05:54 PM

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

marshmat

07-08-2008, 09:57 PM

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.

klick

07-09-2008, 01:02 AM

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-F7-New-Fast-13-7-MPH_W0QQitemZ150257165311QQihZ005QQcategoryZ114236QQcmdZViewItem
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

Rick Willoughby

07-09-2008, 03:37 AM

klick

07-12-2008, 11:17 PM

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

Rick Willoughby

07-12-2008, 11:24 PM

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.

Rick Willoughby

07-13-2008, 12:55 AM

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.

Rick Willoughby

07-13-2008, 09:04 AM

Ross
Here is the shape of 4-blade prop for shrouded operation.
http://www.miwheel.com/MIWheel/products/inboard/com_prop3.htm
The Kaplan prop.

Rick W.

klick

07-13-2008, 12:27 PM

Rick, I see the propeller you're talking about, it almost seems to be to be halfway between a propeller and an impeller. I almost thought about ordering an impeller (for a jet ski or something) and trying to adapt it to see what kind of thrust output I could get. However with impellers I would definitely have to create some type of gear down mechnism because I can't find anything close to an appropriate pitch for the RPM's i'm generating with direct drive. I wonder If I could find an impeller, and basicaly make a jet drive unit, I wonder how that would perform.

I'm not sure pictures or videos would be the impressive, I really haven't built anything yet except a tube with a propeller on the end that I do tests with. At this point i'm just experimenting with thrust output, if I can't obtain what I want then there is no reason to build the rest of the device.

As far as static thrust I realize that it's quite useless in the overall scheme of things for a standard boat design, but I have a spefic reason for wanting good static thrust. It's hard to explain why ehehe, so we'll just leave it at that. One day i'll hopefully show you a video that will explain why i'm doing so many static thrust tests.

I might be interesting to do tests with a more impeller like prop, not sure I want to continue throwing more money into propellers. The 5 inch seems to work alright, I dunno, the kaplan prop looks nice, but I can't find anywhere that sells one anywhere near the size I need. Building a propeller from scratch also seems like it would take probably at least a month, and would be cheaper/easier to buy a larger motor then put so much investment into the prop.

Also, I forgot to ask, the shroud I created was about 3.5 inches long, and the propeller was placed in the middle of it. Is that to long? to short? Should the prop be placed more towards the front? more towards the rear?

Thanks,
Ross

Rick Willoughby

07-13-2008, 05:14 PM

Ross
Making a prop is about 4 hours work if you can weld and use a small angle grinder. The value of this is you can create props much more suited to the application.

I have attached a picture of one I fabricated from 40mm flat bar and also an image of one milled from my 3D drawing file. The latter cost CAD550 without the spinners and bearing housing - CAD700 complete.

Rick W.

ASM

07-16-2008, 03:35 AM

ASM

07-16-2008, 03:37 AM

As for shrouded propellers like the michigan wheel, a friend of mine has develloped a shrouded prop system as well, using a PU based 'shroud' and he always uses special shaped props for tight fit in the shroud. Furthermore he recommends not to use shrouds beyond ~16 knots as their drag becomes too much and you loose performance

Rick Willoughby

07-16-2008, 04:19 AM

Rick / Ross

I have read this thread with interest and one simple question remains..... how do you get to read your ammeter during testing to get actual 'consumption' and settings at a certain speed ? does it have a memory function or do you have to pedal along your model ? O, provided that one would use such a device in model testing....

I actually sit on my models. That is why I started building pedal boats as they are a scale that allows me to get on the water.

I use GPS for measuring speed and these typically have ability to download the data. My latest GPS has inbuilt cadence and heart rate so these are all logged simultaneously.

The easiest way I have seen of getting current readings off a model is using a digital multimeter with a nice large readout. Do passes at constant speed and simple note the current reading as it goes by. The on-board GPS can be used to determine speed after the test.

Another feature of a pedal boat is that it can be done hands free so you can skim along beside a model steering it by radio control to keep pace with the pedal boat.

The video on this link provides a good record of speed and current during testing of a small electric drive:
http://www.boatdesign.net/forums/attachment.php?attachmentid=17436&d=1196419301
The little multimeter cost AUD16 at Tandy. It indicates up to about 14A.

I know you can get radio linked miniature cameras as well as I have seen them used on small helicopters for spying around the suburbs. Could be another option for recording hard data without going to exotic recording devices for each function.

I had a friend who developed input devices to mate to PDAs for all manner of input but he sadly passed away last year and the development has not been taken up by others. There are probably commercial variants of this available.

Rick W.

klick

07-16-2008, 11:26 AM

ASM,

Well, I do have an ammeter which I haven't officially used yet, I showed a link earlier I think, it's this one: http://www.rc-electronics-usa.com/ammeters/dc-amp-meter.html

One feature I like about it is that it saves the max watts/volts/amps in it's memory, so that I wouldn't have to watch it while using it to determine at least the maximum amount of current pulled. I was planning on using it with someone else on the boat to read the data as i had the unit in the water. I can do this because at this point I don' thave batteries or the controller onboard of my unit, therefore it would be easy to place it in the boat. Once I have everything put inside the unit it may not be that useful, except of course for the max load memory, that might be nice to have. The unit however is rated at a max of 100 amps, and in reality I don't see the wires they are using as handling 100 amps for much more then a few seconds. So i'm not sure how much power it really can handle. I'm guessing 2.2HP maximum, but once again it would only be useful for the maximum amp recorded in it's memory, which is useful to me.

The interesting about the shroud, that after 16 knots it creates a lot of drag, this of course is good news for me, as if I hit 16 knots i'll be throwing a big ass party :) 16 knots underwater is probably going to be very difficult, but it would be a hell of a ride :)

My (first) shroud is just wood, cut to 3/10th's of an inch thick. It's 3.5 inches long I think. Also it's of course epoxied, poorly at tihs point, but it's just a mockup for testing. I was wondering if 3/10th's of an inch is to thick, that this might cause issues, and really I don't know, it may. I think the beneifts of the thrust increase are much larger then the drag I will incur on my vehicle.

I think I may try to do a simple static load ammeter test today, after all I bought the ammeter to do that. I'm gonna have to test in my little blowup pool on the porch, making the data not that accurate, but it'll be some amount of data. Technically I believe the voltage will somewhat tell me the RPM of the motor (knowing the windings/rpm per volt data provided by the manufacturer) because even if you apply 36 volt, it's gonna drop down to whatever rpm it maxes the motor out at. I've read about others using this theory and some people who do electric cars go as far as using this technique to basically make an RPM gauge. Perhaps not, but it might give me a lot of data i've been looking for, at least the total watts data.

Ross

klick

07-16-2008, 04:12 PM

Well I finally did some testing with the ammeter, interesting results.

I tested the unit out of the water to determine how much of the power is needed to overcome friction, bearings, flaws in the design.. etc... granted some power goes to moving air, but hardly any. This time around I did all my tests at 24 volt.

So with the unit out of the water:
Total watts at 24 volts is: 454 watts.

Now for the remainder of the tests i'm going to assume the motor has an efficiecy of 75%, which based on what someone did with a dynometer, they got a peak efficiency of 82% at a specific RPM. Now I have no idea what rpm i'm doing, and surely am not getting the exact perfect efficiency rpm, so i'll just assume 75%.

So 454 * .75 is 340 watts towards spinning the device. A little more then I expected, but the PSS (www.shaftseal.com) is big and takes some energy. Also I believe there may be a balancing issue which the unit did vibrate a little, this of course could add to the issues. I also realized that in the water the RPM would be lower, so I tried to get it going the same rpm, which ended up pulling 320 watts, equating to 240 watts of wasted energy (well not wasted, not having a shaft seal would be bad eheh).

So I put it in my awesome $9 inflatable pool and ran 24 volts at full throttle. Now I was surprised to find that it pulled in 2240 watts. With 75% efficiency that means the motor was outputing around 1680 watts, and calculate in my mechanical friction above (subtract 240 watts) that means 1440 watts went to the prop. Basically 2 Horsepower. More then I expected. Perhaps 75% efficiency is completly wrong, maybe it's more like 50% at these rpm's, unfortunatly I dont' have a dynometer to determine what it is at my rpm, and i dont' even know what my RPM is... so this data will have to do.

Now, these tests I did at 24 volt, I did run on the lake at 36 volt so i'm interested how much power it pulled them, but the little ammeter i'm using maxxed out on the 24 volt supply, so i'll have no real way of knowing with that.

This is the first time I tested in my pool with the shroud, it really does help out with thrust (plus it's much easier and safer, don't have to worry about cutting the pool or my fingers off). I need to get a fish scale thing or something, i could probably test it pretty easily in the pool.

Really I think this all comes back to the pitch of the propeller, is it still to much for direct drive? I think it may be, but then again these are static thrust tests (which i know are not all that useful) and once I start moving perhaps it's the perfect pitch, I dunno. Moving to that level of testing is expensive and much more difficult, but I guess that's my next step.

As much as I don't want to, I think i'm going to go with quad motors, 2 props conter rotating. So each prop gets 2 of these motors, which i'll line up straight. If for some reason that's overkill for the prop, it'll just let the motors work easier and the heat will be spread out more, which will be easier to disipate/cool. Granted, it will make my dreams of a super compact unit less likely, but i'd rather go fast then slow...

Ross

Rick Willoughby

07-16-2008, 05:28 PM

Ross
In a small pool you are not quite doing a static thrust test because you have a stirrer that will set up a flow pattern. That will likely increase power and also thrust over a true open water thrust test. The power reading is about what I expect for the 5" prop.

You should do a no-load reading on the motor without any connections. There seems to be a lot of power going into things it shouldn't. Did the seal get hot?

Rick W.

klick

07-16-2008, 05:45 PM

Rick,

Unfortunatly for water proofing needs, I sealed the entire thing shut and can't open it quickly afterwards to feel for things that are hot.. Even the PSS would be tough to touch since it goes in a few inches.

Since i'm moving to a different design, i'm rebuilding the unit to be able to get in and out of easily. The first prototype was just sealed shut to make it easier on me, and i'm glad I did as it wasn't the best design anyway.

I should say that the "slight" vibration as I put it before was probably more moderate then slight. Ironically the vibration is coming from the shaft seal, there's no way to install it "off balance" but it shakes when the rpms go up real high. I remember seeing this on my workbench int he basement and going "hmm, that seems a bit odd" and that was with no propeller, surpringly the shaft seal doesn't seem to lose it's "seal" when this happens. I honestly think my other tiny pump shaft seal that I have from mcmaster is going to work much better moving forward. Rather then put much effort into testing this inefficiency, i'm going to build the second prototype and see what power this uses. I will test the motor with nothing attached when I get a chance, and with just the shaft (no prop, no shaft seal) and with the shaft and the prop (no shaft seal), basically go through all the variables and test which one is causing the issue. Once it's down in the workshop this will be easy to determine with the ammeter.

Also I attached a picture of the propeller and shroud, just so you can see what i'm working with. As I stated the shroud was built quickly and epoxied poorly eheh... i never even got around to sanding, re-epoxing, filling, then painting, but it was meant to be a test anyway :) I'd show the rest but it's enclosed in a non-openable container right now eheh... once i crack it i'll get a picture, but then i'm rebuilding it so it won't lookt he same anyway. Really as I said before it's not exiciting, it's just a tube that has the PSS and the magmotor in it, and the wires come out the side and I connect with the motor controller externally, and currently use large lead acid batteries for testing.

I do have to say I like this little ammeter, for $60 it's nice, upto 100 amps and voltage upto 50, compared to the alternatives it works nicely for the price. I mentioned before I don't see how it could take 100 amps for very long but they use extremly nice wires on here, tinned and high count, they didn't even get warm int he test, but my enourmous jumper cables did get warm... I thought that was quite interesting.

Ross

klick

07-16-2008, 05:48 PM

Oh yea, about the swirrler comment on the small pools, yes I realized this and the tests where done quickly before that occured, once the water started moving the unit would create large vortexes in the water anyway and start sucking in air. So the watts ratings where pulled from when it first started.

I actually attempted to use the swirl to my advantage to try to see how it woudl react to moving water (the pool is circular, tried to get the water spinning in a circle) but once it stirs it up it starts sucking air and doesn't work to well, so these results weren't very conclusive of anything.

Ross

Rick Willoughby

07-16-2008, 06:27 PM

The shroud is a nice fit for the prop. Certainly good enough for static testing./

Once you get this thing moving it will need to have fairing on all leading and trailing faces to get nice clean flow. A lot of power can be lost if the flow is not clean.

I will have to look into the ammeter. I was keen to get a clamp on to avoid need to wire in.

Rick W.

klick

07-16-2008, 10:31 PM

Yea, the ammeter I linked to is alright, the Clamp On you listed before is still nicer, has a higher amp limit, but it 4 times as costly. for $60 the one I got is pretty decent, and the memory function is kinda nice too.

I was browsing the web and found something interesting: http://www.whisper-electric.com/motors/whisper-xt.html The Whisper XT, it's a brushless motor outboard that runs at 72 volts, they claim a maximum of 72 amps. Which equates to 7.2HP, calculating efficiency, lets say 6HP. Now, they have a video showing a 22 foot boat with 2 people going like 8mph or knots, not sure which. This seems like dramatically better results that i'm getting with power input.. i don't see 3 times my input getting the speeds they are getting. I don't know how much their boat weighs, but my thruster didn't even move my 2000 pound boat, except for in circles. We tried making it move it straight, but it was really slow.

If you hit the spec sheet they say they are using a vetus 6 blade propeller, which is made for bow thrusters, based upon their shroud size, which they claim one part of is 12.5 inches, I would assume they are using a decently large vetus 6 bow-prop. I'm guessing it's this prop: http://www.go2marine.com/product.do?no=191488F Which is a vetus 9-7/8" prop, no idea on pitch. Based upon apperance (which can be found elsewhere searching with vetus 6 prop) I would say the pitch is much less then the prop, after all it's meant for bow thrusters. I'm half tempted to get one of these props though, as they seem to be decnet, they are kinda expensive, but the ones i'm getting wheren't exactly cheap either.

The Whisper XT looks like a direct drive system, although I can't confirm this, and I know there are many inline gear boxes available or could be designed compactly, it just seems like it's probably direct drive as the pitch of those vetus props is probably 3-5 inches.

Here's another fact sheet of theirs: http://epowermarine.com/pdfs/whisperxt.pdf

It claims 90% efficiency with the brushless motor... although I can't believe all of their claims, as one of them is "Variable speed control down to 1 rpm"... umm, yea, i'm sure that if I hit the smallest amount of throttle it's going to do exactly one rpm. Regardless though, the video demonstration shows that it's very powerful and moves a boat very nicely. Says it can push a 16 foot john boat at 12.7mph and that it can plane it out.. which is pretty good for a 6-7HP motor.

I wonder what brushless motor they are using, I highly doubt they are building their own, magmotor will build motors for you that are custom if you have a decent amount of volume. Magmotor already produces some decently large brushless motors which are more ppowerful then they use, so for all I know there is a magmotor in that thing. Be interesting to know what's in there exactly though.

Not a bad price for $4500 either, I wonder how many of these things they are selling...

Ross

Rick Willoughby

07-16-2008, 11:32 PM

Ross
You are restricted using the square props. If you really knew what you wanted to do it would be easy to design you the right prop. Can you weld? Have you got a hand grinder?

I have a couple of PMAC Mars motors and Kelly controllers. They have phase locking on the magnetic field using Hall devices for feedback so it is quite reasonable to get very low rpm. I guess 1rpm is realistic. Certainly mine will turnover at a pace that is hardly detectable. If you try to stall by hand it will just generate more torque and keep the same pace.

If you were using the set up in the previous photograph it would not go very well once you get moving. There would be too much drag for water past the motor.

There is some of my test data on this thread:
http://boatdesign.net/forums/showthread.php?t=20445&highlight=electric
There are two videos. One boat has a scooter motor running at about 10A with say 22V (220W) giving 9kph. The other one has the Mars motor. This is set up for 48V but I was testing on nominally 24V. With this I got up to 11.5kph before the batteries started to complain. From memory current peaked at 12A before the undervoltage limit started to kick in. I believe it is set at 22V so 11.5kph (say 6kts) is not bad for 264W.

The Mars motor should drive my proposed solar-wind boat at about 10kts and the design weight is 1.2 tonne. So you do not need a lot of power to push an efficient hull at a good pace.

My latest pedal boat design holds 12kph with 150W. There is a video and performance data on this site:
http://www.adventuresofgreg.com/HPB/2008_06_29_archive.html
It is a matter of knowing where the losses are and designing them out. With readily available design tools it is possible to get within a few percent of design speed for power input.

Rick W.

Rick Willoughby

07-17-2008, 02:38 AM

Ross
I was checking data and realised I did not get 11.5kph using the Mars on the catamaran. I did this speed on my OC1. It was rather cramped and trying to video, adjust speed and steer was more than I could cope with.

The outboard leg also pushed up a lot of water that filled the footwell on the boat so it was not the greatest of tests.

Irrespective I am very pleased with the Mars motor and Kelly controller combination.

Rick W.

klick

07-18-2008, 06:07 PM

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

Rick Willoughby

07-18-2008, 06:55 PM

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.

sparky_wap

07-19-2008, 11:06 PM

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

klick

07-20-2008, 11:26 AM

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

sparky_wap

07-20-2008, 11:50 AM

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

klick

07-20-2008, 02:03 PM

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

sparky_wap

07-20-2008, 02:52 PM

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

Rick Willoughby

07-21-2008, 09:13 AM

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.

klick

07-21-2008, 04:38 PM

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

Rick Willoughby

07-21-2008, 05:39 PM

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

klick

07-28-2008, 08:06 PM

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

Rick Willoughby

07-28-2008, 08:19 PM

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

klick

07-28-2008, 08:37 PM

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 ass, 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

Rick Willoughby

07-28-2008, 08:57 PM

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.

klick

07-28-2008, 10:32 PM

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

Rick Willoughby

07-28-2008, 10:44 PM

The ends are just cones welded on. Believe it or not, adding foam adds a lot of weight relative to the rest.

For the weight you are suggesting you need big tubes. Probably 8" diameter by around 20ft long.

Might be easier to build from flat sheet. Like I say no more than about 20 hours work if you know what you are doing. Probably use two 1/32" sheets (0.8mm) 8X4 would do it. Add a couple of sheets of ply for a bridge deck.

Rick W.

klick

08-05-2008, 09:09 PM

Well going back to the roots of this thread, a s28-400 magmotor with 8 inch prop direct drive i've now realized is not realistic. Even a 5 inch prop is proving to not be realistic. For the past few days i've been in contact with magmotor getting efficiency/torque/everything data for the approx RPM's I was hitting and they match up exactly with my results. At approx 1500 RPM this motor is only around 30-40% efficient (at 36 volt). Basically without going into a lot of detail, the 3000 watts I saw pouring into the motor and only about 700-900 watts coming out, is about right with every efficiency calculation. The big one is that the motor simply is not efficient at that RPM, around 3500 it becomes a much better motor. So needless to say I think the S28-400 is not going to be my primary driving motor.

I'm now talking with them about their C40-500 motor with a special windings setup to obtain better performance. It still has about the same efficiency rating at lower RPM's, but with a smaller pitches prop and about 3 times the available power I should be able to hit the appropriate RPM's to output some serious power. Granted they are made to order and will take 6 weeks to get, then probably take me a few weeks to test, then it'll be winter time probably eheh.

They do offer some nice gearboxes that I may look into, since direct drive is looking to be more and more difficult.

How much more efficient is it to go with a gearbox and lets say a 8x8 prop as opposed to a 4x4 prop? The new motor could probably direct drive a 4x4, perhaps even a 5x5, (not being efficient until it starts moving a bit..) But would it just make more sense to use a large prop with a gearbox? I liked direct drive also because i'm trying to keep this compact, and a gearbox just adds to the unit.. Plus it is simplier.

Ross