Swimmy Thang!: Testing the Thrust of Your Motors

Bollard testing is a useful tool both for testing the thrust of your motors and also for making comparisons between the thrust generated by different types of propellers. The rig is elegantly simple! You all know that any action produces an equal and opposite reaction right? That is Newtons law of motion. SO if you take some PVC and a piece of pipe or a broomstick you can build a device that will let you see how much thrust your motors are giving you, and swap out propellers to find out which one works best with your motor.

The supplies you will need are a PVC cross:

two 3 foot lengths of PVC of the size that matches the cross

one 6 to 7 foot length of wooden or metal rod that will fit through the cross

you will also need a PVC tee that is the same size as the cross. You will modify the tee to use as a motor mount

2 hose clamps to attach the motor to the motor mount, (you may only need 1)

a drill

a fish scale

a hacksaw

a vise (everyone should have at least one vise!) OK bad pun! bad, bad pun!
Cut a slice off of the long segment of the tee: (you may also need to clip one end down)

Now take your hose clamps and attach your motor to the tee, place one 3 foot section of PVC into the top of the tee and put the other end of the 3 foot section of PVC into one of the slots in the PVC cross.

Drill a hole through the other section of PVC and put a loop of twine into it. This is where you will attach the fish scale to measure your thrust.

Put the other three foot section of PVC into the slot in the cross directly opposite the the one holding the first 3 foot segment.

Now put the broomstick or metal rod through the open holes in the PVC cross and put the motor end in the water with the propeller facing away from you.

Have a person hold each end of the broomstick/metal rod and attach the fish scale to the loop that you put in the PVC or clamp the rig to the test tank if you have a smaller tank like ours.

Apply power to the motor and measure the amount of thrust you motor is giving you.

It is important that the PVC arm holding the motor and the PVC arm the scale is attached to be the exact same length. If one is longer or shorter than the other than the data you get will be meaningless.

While you are doing this test,measure the amperage draw of the motor. You can do this using a multimeter. If you are in a school situation and have access to an oscilloscope and someone who knows how to use it, that can also give you some interesting information about how well the thruster works and what the amperage spike is upon start up of the motor.

We recently conducted a bollard test on the 500 gph bilge pump motors with the Octura propeller and the Master Airscrew prop adaptor I talk about in How to Build a Motor Controller Assembly. This is what we found:

The 500 gph motors with the Octura prop and prop adaptor that I list in How to Build a Motor Controller Assembly draw 3.13 amps going either in the forward or reverse directions and they generate .31 kg (.68 lb)of thrust in the forward direction and .27 kg (.59 lb)of thrust in reverse. When the motors are completely stalled they draw 5.2 amps.

Inexpensive meters are available at Harbor Freight:

If you haven't used a meter before it can be a bit daunting but it is really not too difficult. I am going to attempt to post the tutorial from the MAKE blog.

How to Solder!

Click To Play

In this Make: Video Podcast, Joe Grand shows us how to solder and de-solder
electrical parts.

Yea! It Posted right! My apologies to those of you who caught me over the weekend! I had the multimeter video up by mistake!

Measuring amperage is a bit dicier because you have to place the meter in series with the circuit. I'm going to have to figure out how to make mp4s so I too can make pod casts of tips and techniques I think might be of interest to you. Telling you how to measure amperage with a meter just doesn't have quite the degree of usefulness that a little video would have. Anyhow, cheers for tonight!

For the true geeks among you here is a little introduction to oscilloscopes again compliments of the MAKE blog.

An Introduction to Oscilloscopes!