How do you measure propeller thrust?

With great difficulty.

You might be able to infer something from pressure gauge if you have hydraulic trim adjustment. Need to work out the geometry.

Most of the effort is directed at measuring hull drag. This can be done in tow tests but still a lot of variables.

The cost of mounting load cells to do it well would be very expensive.

There are reasonably accurate methods for determing drag under ideal conditions. This is a good place to start. I just keep narrowing in on losses until all are accounted for.

There are reasonably accurate methods to determine propeller efficiency.

You have to have some faith that engine data is accurate and your motor is performing to spec. This can be validated by swapping motors if the outboard is of a easily swappable size.

I can give you some performance calculations if you want to narrow things down.

Rick W.

Rick,
At this time I'm just interrested in measuring thrust, not calculating it, to get a measuring instrument that I can use to see how various parameters effect hull drag. Thanks for the offer to help with calculations anyway.

I'm interrested in any clue how to do this on a limited (=my own private) budget. For the moment I'm leaning towards using a sensor to measure the forces in the motor tilt mechanism and use geometry to get the thrust. Your answer also points in that direction. The problem is then reduced to finding an affordable sensor.

PetterM,
The outbord will be less than 40 Hp, so the forces involved are in my opinion manageable. As I'm doing this on my own and for no better reason than curiosity, including a sensor in the motor shaft is too advanced. I'll have to trade accuracy and do it in a simpler way.

Erik

While there may be too many variables to have a graph that is universal, I think a simple prop value could be created, A shaft of determined length, protruding through a movable "target" wall, in a tank shape that allows for various props to be attached and tested
Size, speed, shape would all be variables. Shape of tank, size of target, distance of shaft through target face would all be constants. The force (thrust) of a prop would be measured by the forward distance the target is pushed in relation to some opposing force (mechanical, hydraulic?) against the opposite side of the target face.
A value of "something" per sq. inch @ so many RPMS would be the assignment given to each prop. Another assignment could be a value using the same RPM for each prop might be helpful also.
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Design a prop, then send it to get it's "Ted's" value assigned.
Props of differing size, pitch, etc, might have the same "Teds" value. Another design tool?

Erik
With a 40HP motor you will not be dealing with huge forces. You could set up a lever system pivoting off the bottom of the transom and pushing on the leg just above the prop. I am thinking of a lever with a mechanical advantage of say 20:1. It would initially set the outboard a few centremeters off the stop to give good travel at top of the lever. Use a calibrated spring or spring balance to measure lever force. If the boat planes with 40HP then the force will only be a few kilograms at the spring.

The spring could have 100 - 200mm of travel so bouncing around would not create too much noise in the measurement.

You could calibrate this lever system or a pressure gauge on your trim (if it has it) using weights and a pulley loading the prop shaft while in land. This overcomes the need to work out the geometry.

Rick W.

erik818, I seem to have seen similar graphs somewhere on the net, can't remember where, they were however limited and just a basic indication.

It may be better to understand the working of a prop, it should be better if you have some kind of idea to your requirement.

The easiest way to determine the thrust of a prop would be to tie a scale to your boat ? and pull away with the scale measuring the force. The smaller the pitch the more torque, the larger the prop diameter the more torque required to turn the prop for a specific rpm.

A low pitch prop has more torque since it 'slips' less and displaces more water positive than a high pitch prop, but at low speeds. The high pitch prop becomes more efficient at a higher speed, allowing less slippage, where the low pitch prop would be out of steam so to speak.

So, the best prop required is a function of the torque available to turn the prop, the pitch of the prop, and the required vessel speed.

There are some tricks to some setups, they are from little holes in the props to cavitation plates to improve pull-away torque while still running a fast prop for top speed. These are however different from setup to setup and depends on the boat hull shape, weight and so on, ie the combination you have. It can become complicated very quickly.

You local prop supplier should be able to loan you a few different props to test. The scale method, complicated as it is should give a good indication of what each prop would do.

Eric
This post reminded me that I needed to get a nice little scale for measuring force. I was intending to get a little spring scale but a search on Ebay came up with a little electronic unit that seems very good value.

The unit I purchased has a 40kg range so I will be able to use for towing tests and other little projects. The unit arrived in a few days and works well. Might not give a steady reading with a bouncing load but it sort of averages. See attached photo - it is about 100mm high and 70mm wide.

Rick W.

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If its got a little smilie face on it then I think it should be ok.

You may be able to tie the boat to the dock with a similar load cell and take the engine up to RPM. Possible but that will only tell you the thrust at RPM or below.

How you going to get it up to RPM and beside the dock method would indicate bollard pull not thrust used to obtain a certain speed.

I dont see how you can do it.

Experiments like this could cause the boat to get out of control, flip or pull the transom off.

To provide Vertiq&#;s partners and those interested in our propulsion modules with the most relevant data possible, we conduct and publish thorough thrust testing data using many different third-party propellers.  

Goto Wing Flying to know more.

First, we must determine what sizes of propellers we want to collect and publish data for each specific module. We&#;ve used all of our previous testing data to develop our own internal program to simulate props on our motor modules at different supply voltages, so this step is relatively straightforward. Props that are too large for the module will hit its previously determined continuous torque limit at a low velocity and low commanded voltage. Props that are too small for the module will hit the maximum commanded voltage and max velocity without creating much thrust or getting near the continuous torque limit. 

Once we have determined the appropriate range of propellers to test for a specific motor module, we prepare the test on our internally developed thrust test stand. Our motor/ESC module gets mounted to an ATI Mini85 Force/Torque sensor. This F/T sensor is sensitive to temperature change, so we avoid mounting our module to the sensor directly. We also avoid mounting it to thermally conductive materials that would transfer the heat from the motor/ESC to the sensor. Instead, we fasten the module to a thick nylon mounting plate that sits between the module and the sensor. This non-conductive surface protects our F/T sensor from drastic temperature changes. It also keeps our data as honest as possible. Because our mounting plate is not a thermally conductive surface, we are not adding any additional heat sinking or cooling to our setup. 

While mounting the motor module to the stand, we also attach a small IR sensor with a mount that is specifically designed for the exact module we are testing. The mount ensures the IR sensor barrel is pointed directly at one of the coils. We use this coil temperature data specifically for tuning our coil temperature estimator model. While this exact coil temperature data is not shared on the website, it is helpful for us to understand everything that was going on while testing different propellers.

We also keep a temperature, pressure, and humidity sensor in the testing room while conducting tests and note these conditions at the top of all our published data. Everything feeds back to a National Instruments DAQ. 

Once the motor module and IR sensor are mounted correctly, we mount the first propeller using the proper adapters. Next, we configure our power supplies to the appropriate voltage for the test we are about to run. For larger propellers, we can supply a lower voltage since they will hit our continuous torque limit at a lower velocity. For smaller props, we typically supply the highest recommended operating voltage for that module in order to spin as fast as possible in our test.  

Contact us to discuss your requirements of motor thrust test. Our experienced sales team can help you identify the options that best suit your needs.

For example, you can see when testing a 28&#; propeller on our 60-08 150Kv G2 Module, we supplied 24V.  When testing a 20&#; propeller on that same module, we supplied 48V.  Both hit the modules continuous torque limit of 1.05Nm, just at different commanded voltage steps/velocities. There was no need to supply 48V during the 28&#; propeller test since the module hit its continuous torque limit around 22V Commanded Voltage. 

We do not publish steps in the test that occurred, but pushed the motor module beyond what we have determined to be its continuous torque limit. These numbers were able to be hit, but would not be sustainable for a customer without the module derating to protect itself from overheating to the point of damage.

Once we connect power and comms to the motor module, it is time to get spinning. For our test script, we use Voltage Mode and IQUART to communicate. We step the Commanded Voltage in increments of 1 or 2V from 0V to supply voltage. We spin at each voltage for a sufficient amount of time to collect smooth data.  

After the final step of the test (either max voltage commanded or derating above max continuous torque ), we run a continuous, steady state test. The purpose of this steady state portion is to collect the Convection Thermal Conductivity Coefficient for that specific propeller on that specific motor module. This value helps keep our internal coil temperature estimator model as accurate as possible and should be adjusted for different props because of the different airflow and cooling they create. We populate the Convection Thermal Conductivity Coefficient along with velocity feed forward values (collected from the voltage sweep) as default configuration files on IQ Control Center.  Because we&#;ve already done the work in collecting these values, the end user can simply select what propeller they are using and have these typically tricky to determine values auto-populate for them. 

We currently have ~100 different propeller thrust tests with our different motor/ESC options published on our website.  As we continue to release new modules, this number will continue to grow. If there is a specific prop/module combo that you would like to see tested, please reach out to us.

If you are looking for more details, kindly visit how much can a drone lift.