Mixing brushless motors, ESCs with water!

[Delta Foxtrot]

I need to make a test rig to measure propeller performance underwater. I would like to test the concept with a quick and dirty prototype. The basic idea is to mount a brushless motor off a pylon with esc, along with speed controller and watt meter and submerge to measure thrust and power input at various water speeds. I could design a pylon to house the esc etc to keep this dry, but for a quick look see I wondered what would happen if the components were submerged. I suspect that the motor would run ok for a reasonable length of time, but not sure about the esc. The test would be in fresh water.

I also would like to know how much I could extend the esc to motor wires as I may have to do so to keep the watt meter out of the water.

I know that this is an odd query, but I know that there is a huge amount of practical knowledge on this forum.

[kc]

You could ask the question on the Model Boats forum......

[Delta Foxtrot]

Posted by kc on 20/01/2016 19:37:22:

You could ask the question on the Model Boats forum......

Which one would that be?

[Delta Foxtrot]

Posted by Delta Foxtrot on 20/01/2016 19:39:11:

Posted by kc on 20/01/2016 19:37:22:

You could ask the question on the Model Boats forum......

Which one would that be?

Found it! I did not know that there was a forum just like this, but for boats.

[Allan Bennett]

The motor would be fine. Many people used to bed-in the brushes on their brushed motors by running them at full throttle in water, for cooling.

In theory, if the water is mineral-free, electronic devices also shouldn't suffer by being submerged. But I wouldn't like to put that to the test with tap water. So best to keep the ESC clear of the water. You can extend motor-to-ESC wires as much as you want (within reason) without any problem. Just use a thicker gauge wire than the original if it's a long extension -- say 18" or more.

[Delta Foxtrot]

Thanks Allan! 18 inches is a useful extension that may just be enough for the present.

It will be tested in a lake from a boom attached to a boat.

[Don Fry]

Delta Foxtrot, you are as mysterious as Mata Hari on a Saturday night. What is this about, have you found a route to cold fusion?

[Colin Carpenter]

DF - I used to race fast electric boats with brushed motors and ran them in , in a bucket of water . A wet boat in a race never bothered the motor but the ESC would normally fry ! Colin

[Pete B]

Flite Test did a video on waterproofing a leccy setup a while back.

A rather tongue-in-cheek video here, too;

Pete

[Delta Foxtrot]

Posted by Donald Fry on 20/01/2016 20:16:05:

Delta Foxtrot, you are as mysterious as Mata Hari on a Saturday night. What is this about, have you found a route to cold fusion?

Didn't she get shot by firing squad?

I am not aiming for cold fusion, just doing a bit of research into propeller performance for a human powered boat. I am looking at how well off the shelf aeroplane propellers fit the bill. Marine propellers usually look very different, but a look at some data on large APC props suggests that they might do the job. Looking at HPB sites the propellers used on similar craft look more like aircraft props.

One potential problem is cavitation, so I want to check this out using a simple test rig. If the propeller performance is close to that predicted in wind tunnel tests after due allowance for the difference in fluid properties then this might just be good enough, if not then I will have to design one. The prop does not need long life, just enough for 24 hours use, but it does need to be quite efficient.

The propeller will be turning much slower in water so ideally I need a low Kv setup. The test rig doesn't need a long life, just enough to get the job done.

More than that cannot say or I will suffer the same fate as Ms Hari

[Delta Foxtrot]

Pete,

Thanks very much, that video was very useful and answers all of my questions.

[Delta Foxtrot]

Colin,

Thanks for that. The video Pete linked to clearly shows ESCs must be kept dry as you say.

[Piers Bowlan]

I used to fly an Ace-RC Puddlemaster from the lake at Kempton Park and it frequently got 'dunked'. In fact keeping water out of it was virtually impossible. Consequently the Jeti BL inrunner was always wet as was the ESC which was attached to the motor pylon. Once or twice the model would finish its landing run inverted (which doesn't say much about my water landing technique!) yet the motor would continue to run under water and be controlled by the ESC. When I got home I would dry the motor and ESC with paper towel before putting it on a radiator to dry out. I never had any problems with this power train in fact, I think it liked this abuse!

Just don't try this in salt water!

[Stuphedd]

almost on the topic !! the weather at present is a bit snowy so my ski equipped planes are at the ready ,

But a year or so ago , flying off the snow with a fairly high powered aircraft , I ended the day , taxiing it around the patch just having fun doing donuts , Eventually I ran into snow that was a bit deep , the prop kicked up a snow storm , and my 60 amp ESC bit the dust !Got wet !( Hyperion ouch !!!)

the motor and the esc were mounted out at the front , no cowling !! It has a polly bag over it now .

cheers

[Piers Bowlan]

Perhaps the Jeti ESCs PCBs were lacquered after being assembled so were effectively water proof, but it was about twenty years ago, so perhaps standards have slipped .

[Simon Chaddock]

Delta Foxtrot

I presume from your description you are intending to use an air prop under water?

It will work but I believe the huge difference in their relative densities does alter the 'ideal' prop shape quite a bit to something much more like... a water prop!

Do let us know how you get on!

[Bob Cotsford]

I water tested the setup in my Multiplex Dogfighter, but not entirely intentionally! It just happened to end up in the river, near the far bank of course. I did manage to taxi it a short way but even on low throttle it wasn't long before things got smokey and smelly. Water provides a lot more load than air, just as Simon predicted

[Dave Hopkin]

Relatively clean water shouldn't affect the electrics - but the density of air at 0 degrees is 1.2 Kg per cubic meter, water at 0 degrees has a density of just under 1000 Kg per cubic meter - so spinning a air prop under water require about 833 more energy than in air, that increase can only come from one place, current out of the Lipo........ hope you have a REALLY big ESC!

[Delta Foxtrot]

Posted by Dave Hopkin on 20/01/2016 23:31:38:

Relatively clean water shouldn't affect the electrics - but the density of air at 0 degrees is 1.2 Kg per cubic meter, water at 0 degrees has a density of just under 1000 Kg per cubic meter - so spinning a air prop under water require about 833 more energy than in air, that increase can only come from one place, current out of the Lipo........ hope you have a REALLY big ESC!

Dave,

yes, however you do not spin the prop that fast in water. power scales with density and rpm cubed, so given propeller operated at a given input power in water will be spinning much slower than in air.

The guys pedalling this 3 man boat are likely to be able to provide around 400 watts continuously so the challenge is to match the prop to that sort of power, so the esc requirements is not as large as you think.

[Simon Chaddock]

Delta Foxtrot

My concern is not that an air prop won't work in water but with its efficiency when doing so.

An air prop is designed to work well in a very low density medium and has relatively thick section high aspect ratio blades. whereas a water prop has very wide low aspect ratio blades that have a very thin section.

A water prop is that shape for sound hydrodynamic reasons.

With only limited 'human power' available you would be well advised to use the most efficient design of prop for the medium in which operates.

[PatMc]

DF, have you considered using oars ?

[John Olsen 1]

Actually the ideal shape of a marine prop depends on the speed and torque of the power plant, as well as the available space. Usually boats are quite limited in the diameter that can be accommodated, due to the maximum draft that is acceptable. So to accept the required power, the blade area has to be quite large, and they often have to be run quite fast as well. This leads to cavitation problems, so the blades are kept quite thin.

If the space is available, a shape more like an aeroplane propeller will actually give more efficient results. However the speed has to be low enough to keep the tip speed reasonable. One form of power where this applies is with small steam plants. Steam engines usually run quite slow...my own engine is considered a high speed one as it runs up to about 600 rpm. The propeller I need to make for it will be 21 inches diameter and 21 inches pitch, and will look more like an aeroplane propeller than the usual boat type. It will have two blades. Note though that it will only be absorbing about 10 hp, which should, all going well, push the boat along at about 7 or 8 knots. This is actually about as fast as the hull would go, since this is a displacement hull, not a planing one. (The hull is 30 feet long)

I think in fact a human powered boat would have similar requirements....the power plant is capable of good torque but only at low rpm. To get the best out of it, you want a large diameter high aspect ratio propeller, turning at low speed. But of course, said propeller must be fully submerged and also must not be touching the bottom of the water, so the maximum diameter may be limited. There isn't enough power to plane, so a long displacement hull will minimise wave drag and give the best performance for the limited power. (that is also why rowing eights are so long.)

Even on aircraft, where possible we would use a larger propeller turning at a lower speed if we want high efficiency. However we must often sacrifice efficiency for practical concerns like not having a grasshopper like undercarriage. We can't turn a large propeller fast or the tips will exceed the speed of sound. In order to absorb more power in a limited diameter, we add more blades, although this leads to interference like on a biplane, and so reduces efficiency.

John

[Piers Bowlan]

Very interesting post John. Presumably your boat is man carrying? Do you have a photo of its current state of development or is it on the drawing board at this stage? Some info on the steam engine would be interesting even if it is a tad off topic.

[kc]

For very slow revving props you would do well to see what the rubber powered free flight people are using. It's not my type of flying but I have seen many articles on prop design - they carve their own props. . I think they have turned it into a science with lots of data somewhere.

[Delta Foxtrot]

John,

Thank you for your considered response, your thoughts are very close to my own. I have some professional experience in the aerodynamic design of aviation propellers, but not marine propellers. My initial thoughts were to design to the largest practical diameter to achieve a high Froude efficiency and design the blading to achieve minimum induced drag. some compromise on diameter may be needed if the blading looks non optimum or impractical and I may need to drop the blade loadings if the efficiency characteristics looks too peaky. The codes I plan to use are very similar to the methods used to design the propellers for successful human powered aircraft.

The idea of testing model aeroplane propellers is perhaps unlikely to deliver the optimum solution, but it will provide a link to some existing wind tunnel data which could help with design trades and it will allow us to shake down the test rig which will be needed to test a bespoke design.

It would be good if there were off the shelf propellers for human powered boats to have a look at as a bespoke design could be costly and take some time to develop. I would be very interested if anyone knows of anything that might fit the bill

Your comments of the form of marine propellers is very helpful and confirms some of ideas I have picked up from text books and others forums.