3lbs thrust @ 5 amps

[scubersteve]

One of the students on the rov team approached me with a technical report from a MIT rov team where they clamin to have gotten 14.5 Newtons (3.1 lbs) of thrust @ 5 amps from a 1000gph bilge pump motor spinning a 44mm prop.
Does that sound right to any of you?
You can find the report here under Massachusetts Institute of Technology:
http://www.marinetech.org/2010-tech-reports/

[a_shorething]

I don't see why not.

Here's a listing for some professional thrusters with 3" props with 6.4 lbs of thrust

http://www.seabotix.com/products/auv_thrusters.htm

VideoRay lists 21 lbs of thrust from theirs

http://www.videoray.com/homepage/thrusters-3.html

Check out the report from Jesuit High School from the same page:

http://archives.marinetech.org/rov_comp ... Report.pdf

This one lists 4 lbs of thrust at 10amps and 6lbs at about 18 amps from their custom designed (and very impressive) thrusters.

[scubersteve]

Well taking those three in order..

The seabotix thruster operates on 19v usinv 4.25 amps (80 watts total) to produce 4.85lbs of thrust.

The video ray thrusters were listed as a pair. Taken individually, each made 10.5lbs of thrust on 12v while drawing 17amps (204 watts total) This info available in the download brochure.

The Jesuit high school thrusters operated on 22v and according to the thrust to amperage chart made 2.4lbs of thrust at 5 amps. (110 watts)

The significance of the MIT claim was that they were making such high thrust numbers with such low power requirement. (60 watts)

[a_shorething]

Good point on the VideoRay thing, I remember noticing that but didn't mention it in the post.

The MIT claim is the 'outlier' in a lot of respects. Most notably the fact that it is an 'off the shelf' motor that they just added a prop to. They list numbers in forward and reverse and the ratio was interesting (14N in fwd, 11 in rev) and makes sense to me since the design wasn't optimized in any way by streamlining it or adding a nozzle, they just cut off the outer housing and put a 44.8mm prop on it.


Now that I'm thinking about it...Those are the rated specs of the bilge pump itself aren't they? All except the forward and reverse numbers. I wonder if they put the prop on it, took some readings of the thrust (on a test stand they don't describe at all) and the just posted the motor's voltage and amp draw numbers as they were listed on the box (when it was in bilge pump mode).

This is from their write-up:http://archives.marinetech.org/rov_comp ... 202010.pdf



One way to read the write-up would be to say that everything from before the 'from force testing' was taken off of the rated specs, everything after was actually from real testing. To me that looks like they may not have actually done a power draw test with the final configuration.

[scubersteve]

Hmmm...
Good point and it makes sense. Although it would be very sloppy of them.
It seems extremely plausible though.

[a_shorething]

Hmmm...
Good point and it makes sense. Although it would be very sloppy of them.
It seems extremely plausible though.

In the MATE competition there is a limit to the amount of amps you can draw. If you design your own system you have to test and prove it. Maybe if you buy one you can just (re)publish their numbers. Even if they weren't tested in the same configuration or even for the same purpose. All's fair in love and competition man! Second place is the first loser!

[SSN626B]

The picture of the MIT setup indicates that they are using one of the Johnson Mayfair 1100 GPH bilge pump motor cartridges and not a reworked Rule 1100 GPH bilge pump.
I have a Johnson Mayfair 750 GPH bilge pump motor cartridge attached to a Robbe 50 mm 42 three blade prop and Steve's testing produced 1.8 lbs of thrust at approximately 6 amps.
Regards,
TCIII

[scubersteve]

That is excellent information SSN626B.
I think you should post those specs over on the "draw, thrust, and components" thread.
I'm trying to get everyone to put that stuff over there so we can have kind of a database for thruster specs.
Kind of one stop shopping for those trying to decide what they want to do for thrusters.

[SSN626B]

@scubersteve,
I am just repeating Steve T's testing results with the Mayfair 750 GPH bilge pump cartridge and a Robbe 50mm 42 3 bladed propeller. This combination seems to be the most efficient using a bilge pump and a three bladed propeller. One thing to remember is that Steve was running wide open which may have exceeded the most efficient rpm of the Robbe propeller. I believe that there is a formula for propeller efficiency based on the propeller characteristics. There is also a term called the ideal blade speed which is inversely proportional to the number of blades on the propeller.
I have purchased a HK 30 amp car ESC with which to control the bilge pump motor to see if a lower propeller speed will result in more thrust. I believe that there is a point in propeller rpm where the thrust will actually drop off or not increase with higher rpms. The idea is to operate the propeller within its thrust range to be able to conserve power. I wonder if Robbe publishes the ideal blade speed for its line of propellers?
I am also going to try a HK DT-750 brushless motor with the same Robbe propeller and see what kind of thrust results I can achieve over its rpm range. My feeling is that it is best to run the brushless motors in the open water and replace them when either they quit or the bearings freeze up. The DT-750 is cheap at only ~$11 USD and I plan to only run in fresh water at depths no deeper than 50ft.
Regards,
TCIII/SSN626B

[scubersteve]

@scubersteve,
I am just repeating Steve T's testing results with the Mayfair 750 GPH bilge pump cartridge and a Robbe 50mm 42 3 bladed propeller. This combination seems to be the most efficient using a bilge pump and a three bladed propeller. One thing to remember is that Steve was running wide open which may have exceeded the most efficient rpm of the Robbe propeller. I believe that there is a formula for propeller efficiency based on the propeller characteristics. There is also a term called the ideal blade speed which is inversely proportional to the number of blades on the propeller.
I have purchased a HK 30 amp car ESC with which to control the bilge pump motor to see if a lower propeller speed will result in more thrust. I believe that there is a point in propeller rpm where the thrust will actually drop off or not increase with higher rpms. The idea is to operate the propeller within its thrust range to be able to conserve power. I wonder if Robbe publishes the ideal blade speed for its line of propellers?
I am also going to try a HK DT-750 brushless motor with the same Robbe propeller and see what kind of thrust results I can achieve over its rpm range. My feeling is that it is best to run the brushless motors in the open water and replace them when either they quit or the bearings freeze up. The DT-750 is cheap at only ~$11 USD and I plan to only run in fresh water at depths no deeper than 50ft.
Regards,
TCIII/SSN626B

I agree with your optimal propeller speed conclusion. In my own testing with a DT700, I found that the thrust only increased up to about 3/4 throttle and then stabilized while current draw continued to increase beyind that indicating i was not utilizing the best possible prop for that motor if I wanted to run at WOT (wide open throttle)

I'm a big fan of the HexTronik DT series of motors. Very strong and they give you lots of prop mounting options with that long threaded shaft.
Please post your results when you around to testing your DT750 and Robbe prop combo at varying speeds.
I would be interested to see them.