Mixing ESCs and brushless motors with water

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Mixing ESCs and brushless motors with water

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20 January 2016 at 19:52 #62916
david fillingham 1
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@davidfillingham1
Hi,

I am new to this forum as my main hobby is rc aeroplanes, but I think this forum might have more experience relating to my question.

Posted by Delta Foxtrot on 20/01/2016 19:34:19:

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.
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20 January 2016 at 19:52 #2548
david fillingham 1
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@davidfillingham1
What happens when ESCs and brushless motors are submerged
20 January 2016 at 20:15 #62918
Dodgy Geezer 1
Participant
@dodgygeezer1
1 – why would you want to submerge the components – particularly the ESC? While you CAN waterproof anything, it's a lot easier just to not put it in water…

2 – are you going to test water propellers? or air? Or something else like a centrifugal pump…?

3 – people have run brushless motors underwater and reported that they work **LINK**

but I suspect that you need to clean and dry them after each outing, and they will probably corrode at some point.

I can't see any problem extending ESC wires to a foot or so, and keeping the whole thing out of the water.

I would make a wooden base with the prop underneath, driven by a shaft poking up through the wood, and mount my motor, esc, battery, watt meter and thrust gauge on the wood, nice and dry. The motor can be near the ESC – it's just the prop that needs to be a bit further away…
20 January 2016 at 21:03 #62919
david fillingham 1
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@davidfillingham1
Thanks DG. Here is a bit more background I posted on the rcme forum

Posted by Delta Foxtrot on 20/01/2016 20:39:50:

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
20 January 2016 at 21:41 #62920
Dodgy Geezer 1
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@dodgygeezer1
Hmm… I've seen pictures of those things, and they do have long blades – but the blades are also typically oar-shaped – that is, quite wide, with a large chord.

I think that the issue is that humans are quite good at torque, but poor on rotational velocity. You don't want many gear trains eating up the energy, so you design low-speed, high 'water movement' props. I suspect that you will find that props designed for air need to have a high pitch and low diameter to work well in water…

You will need a strut and a drive to get the power down there. If you don't have the time to make a proper strut, I think you could bolt a brushless to a piece of wood and attach a prop – with nice fat wires to the ESC about 12-18" long. make a cavitation plate so you don't get soaked, and bathe the brushless in a light oil before running. Keep the volts low if you can. Dry it out well afterwards in an airing cupboard.

You might have problems with cavitation and associated drag inside the brushless rotor if the speed is fast enough. Incidentally, a brushed motor also works OK under water, for a while, though the magnet is much more likely to corrode, and that might provide better torque/speed match to your expected final use…

Them's my thoughts – anyone else want to pitch in…?
20 January 2016 at 22:01 #62921
david fillingham 1
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@davidfillingham1
This a quick and dirty look so I am thinking of making up a pylon and attaching the motor and prop directly to the bottom of this. It does not need to last too long. The cavitation plate sounds like a good idea.

If this works than we will look to do a proper design which we can either test more off the shelf props or my own designs.
21 January 2016 at 09:33 #62923
shipwright
Participant
@shipwright
Hello David,

A few years ago I was undertaking experiments with model boat propellers in a test tank (I was measuring electrical power in and mechanical power out but I was unable to measure thrust). I looked on the internet for simulation programs for propellers to see if I could predict the performance if I had the basic geometry (diameter, pitch, number of blades). My internet search led me to a US society which whose objective was human powered boats. I have a pdf copy of an article written by a Swiss researcher (who at that time – late 1990's – was based at Bangor University, Wales) named Theo Schmidt. I eventually managed to contact (in 2011) Theo who by then had moved back to Switzerland. He had developed a simulation program called "PropSim" and he supplied me with a copy of the program with the proviso it was for my own personal use (ie not for commercial exploitation).

After a quick search I have located the "Human Power" article on the internet :

**LINK**

Theo Schmidt is no longer involved in researching the powering of boats via human power.

Ian
21 January 2016 at 13:08 #62924
ashley needham
Participant
@ashleyneedham69188
Something like a rubber band type air prop may be worth trying, at a low speed of rotation.

I would have thought that normal air props do not have sufficient surface area viz. their diameter.

Another thing to consider is the torque effect of a large diameter prop, imparting a lot of twist to the driving (whatever) body.

Ashley
21 January 2016 at 19:31 #62934
david fillingham 1
Participant
@davidfillingham1
Shipwright,

Thank you very much for forwarding that publication, it looks very interesting. I will have a good read later.

I did spend a year studying propellers for aviation application a few years ago and I have access to several codes with which to do design work, but I will certainly have a look at propsim.

Edited By david fillingham 1 on 21/01/2016 19:33:34
21 January 2016 at 19:44 #62935
david fillingham 1
Participant
@davidfillingham1
Ashley,

You might be correct about the air props, but I think that it is reasonable to try them. Some of the HPB props I have. Seen look not dissimilar in that they are unswept and of a higher aspect ratio than ships propellers. I think what is needed for this project is a relatively large diameter 2 bladed prop with high aspect ratio blades.

Never having studied ships propellers I am not sure why they look like twisted clover leaves. I would guess that they have relatively high disc loadings and need a lot of chord to keep the blade lift coefficients down to a level which avoids cavitation.
21 January 2016 at 22:10 #62938
Colin Bishop
Moderator
@colinbishop34627
Boat/Ship props need to be sufficiently small to fit under the hull below water but not protrude below the keel line and so require relatively high blade areas..

Colin
21 January 2016 at 22:12 #62939
Dodgy Geezer 1
Participant
@dodgygeezer1
…I suspect that strength is an issue as well – it's hard to make strong long objects…
22 January 2016 at 10:05 #62945
ashley needham
Participant
@ashleyneedham69188
I think it also down to the blade tip speed. A large prop would have a very high tip speed which would cavitate very easily.

The London science museum used to have a fantastic collection of early props (Colin must have a photo somewhere) and there are shapes and sizes of all and any imaginable configuration. BUT we end up with the ones we are used to seeing, which rather leads you to believe that the shape we have is as it is because….

Also for consideration, "propellers" in desk fan type devices, a different shape altogether, and perhaps worth trying underwater!!!! as they are designed to move large volumes of air at relatively low speed.

Ashley
22 January 2016 at 20:40 #62970
david fillingham 1
Participant
@davidfillingham1
Posted by Colin Bishop, Website Editor on 21/01/2016 22:10:25:

Boat/Ship props need to be sufficiently small to fit under the hull below water but not protrude below the keel line and so require relatively high blade areas..

Colin

Thank you Colin! I am getting a good idea now from this and other forums plus text books as to why marine propellers look as they do and why they are not optimum for ur purposes.
24 January 2016 at 20:29 #63033
Andy C
Participant
@andyc56856
Hi David

Brushless motors are widely used by the ROV guys / girls for propulsion. They seem to work okay under water, see this link;

**LINK**

Not sure any of the other electronics would do so well though.

Andy
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