How do you measure propeller thrust?

Hi,
A graph showing propeller thrust vs. speed for various displacements would be valuable to me. I have no problem to measure weight and speed, but how is propeller thrust measured? I tried a contraption with levers, wires and a fish scale to measure thrust on an outboard motor I had. Basically it worked, but the readings were not very precise and quite soon the setup broke down. As propeller thrust is fundamental when evaluating the efficiency of a hull shape I assume there must be reliable ways to measure it. Anyone know how it's done?

Erik

 

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.

 

If AIR were the medium then "Lbs of Thrust" is the norm, right?

Is this a break down or ratio of static pressure and velocity of exiting air?

Would water also have "pressure differences" which could be measured in lbs?

 

A little more gold to mine here maybe
http://www.fao.org/docrep/x0487e/x0487e04.htm

 

I clicked the pdf version and saved it to my computer, and also bookmarked the link.

Lots of good information in there, will take some time to read.

Thanks Ted655.

 

We have done quite a few surface drives were we have measured thrust directly at the end of the propeller shaft.
How large are the outboard motor you are using?
If you have adequate resources you could modify the lower gear case on the outboard and measure thrust there.

 

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.
.
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.

 

Rick,
Nice scales.
Is there any outputs from the scales so that I can record the data?
Petter

 

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 2000 RPM. Possible but that will only tell you the thrust at 2000 RPM or below.

How you going to get it up to 6000 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.

 

Frosty
You need to read some of the earlier posts to get an idea of what was being discussed. It is a 40HP motor and methods proposed included using trim ram pressure or a lever fulcrumed off the transom against the bottom of the leg with a force indicator at the top.

I purchased the scale for some tow tests on smaller hulls.

Rick W.

 

Oh Ok sorry. Trim ram pressure is a cracking idea!!