How do you measure propeller thrust?

[Guest625101138]

Originally Posted by PetterM

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

Petter
Not that sophisticated. Cannot expect too much for AUD8.

I have found my digital camera in video mode a useful means of collating simultaneous data. Would be nice to have it all data logged but not necessary with a bit of creativity.

So you could set up a GPS beside the scale and using the video mode get force versus speed.

I posted speed versus current for my electric boat in the video here:
http://boatdesign.net/forums/showthread.php?t=20445
The video will take a bit of time to download. You can see the result for two speeds. I have done this with many different variables. Digital cameras are useful data collectors.

Rick W.

[Guest625101138]

Originally Posted by Frosty

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

The scale could also be used to calibrate the trim pressure on land before the boat is in the water. Simpler to set up than known weights and pulley system but would need to be bigger than 40kg unless there is a mechanical advantage.

Rick

[erik818]

Rick,
I'll find a similar scale, and like Petter I'll see if I can find something with an output for a computer. If not, paper and pencil also works. I'm convinced that the thrust measuring method you suggested will work, and calibrating the setup while on land should be easy.

Right now I'm waiting for the ice to settle to allow some scating, and then for the ice to melt again for the next boating season. So there is plenty of time to aquire the equipment for the setup.

Thanks for the ideas and inputs.

Erik

[Frosty]

A load cell between the trim mechanism some how would give out data to the helm??? --Taking trim pressure would read different pressures at different trim angles without altering thrust. I suppose the trim angle should be as said pre calculated and not moved for the duration of the test. As the test is for thrust and not the consiquential speed then resultant speed is not needed.

It might be better to disconect the trim rams in that case all together and just read a load cell. ok ille go away.

[erik818]

Frosty,
I intend to measure speed as a function of thrust, but thanks to GPS measuring speed is the easy part. Using a lever system and a scale is just a low cost setup with easily available parts instead of using a more costly load cell.

Presuming I can get a speed between 20 knots and 30 knots (10 - 15 m/s)from 30 kW, the thrust should be between 2 kN and 3 kN, which shouldn't be too difficult to handle.

The purpose with the measurements is to measure hull resistance at other conditions than full throttle / maximum speed / optimised propeller pitch. I'm interrested in fuel economy at moderate planing speeds, but most of all I simply want to know. There are hull resistance prediction methods that for many purposes are good enough, but at the end of the day I want real measurements. Predictions are based on simplifications and assumptions that never are 100% true, especially when trying something new and different.

Erik

[Guest625101138]

Originally Posted by erik818

........... Predictions are based on simplifications and assumptions that never are 100% true, especially when trying something new and different.

Erik

Erik
I have not actually tested the Savitsky method shown here:
http://illustrations.marin.ntnu.no/h...ing/index.html
However I have done a lot of testing and optimisation at displacement speed using other prediction models. I am confident the propeller models will predict within a few percent and the Savitsky drag calculation is well regarded.

Overall I doubt that your measurement accuracy will be better than the performance models taking all factors into consideration. Things like hull windage and outboard skeg drag.

Ultimately it is the amount of fuel you put through the motor that measures efficiency. A good prediction model of your overall boat will guide you in determining what to do to reduce fuel consumption. The thrust is down the food chain from what you are really interested in if you are chasing efficiency. Maybe you should get hold of a good fuel metering device. For example measuring thrust will not tell you if you have benefited from changing the spark plugs - you might get some indication if you can get a higher top speed but this infers things at lower speed.

Measuring thrust is an interesting exercise but I doubt that it will help you achieve efficiency gains that could not be more easily achieved by using and understanding the available prediction tools.

Rick W.

[dlamprey]

I happen to be searching Google related to a patent that my father obtained about 20 years ago and stumbled upon this post. His idea was to use the pressure gauge to measure thrust on boats using an I/O transom, replacing the use of the often inaccurate / misleading trim gauge:

http://www.freepatentsonline.com/4934970.html

He passed away about 5 years after receiving he patent and nothing ever became of it. Seems like it should be a standard feature or at least an option for newer boats.

I am not sure that this addresses your original question, but thought you might find it interesting nonetheless.

[BMcF]

We routinely measure thrust from outboards as part of our testing programs. We have two transom brackets that cover most outboard motor sizes we ever use and each is simply hinged at the top and reacted at the bottom by a single Strainsert load cell. We do a thrust calibration by pulling tension on a line that is secured to the lower unit at prop centerline and a come-along...with a spring guage to measure the tension.

We've tried to us the trim cylinder pressure. It can give you a rough idea for larger engines but has a heck of a lot of hysteresis in the measurement that can really throw you off.

On the other hand..the waterproof pancake load cells run about 400-800 bucks...

[baeckmo]

Trim cylinder pressure has ben used in practical field testing since the first hydraulic trim device occurred. It is cheap, practical, but sensitive to hysteresis and trim position, just as BMcF says. So how that could even be patented is plain stupidity!

[dlamprey]

baeckmo, I see no need to be rude. The patent is for the device that measures the force, not for the theory. I admit there is no rocket science here.

[baeckmo]

Sorry, dlamprey, no intention to be rude, but the patent summary is describing a procedure that has been in practical use (not just in theory!) looong before it reached the patent office. If there is some kind of device invented, it is well hidden in that summary.

[ShahriarTanveer]

I need a book where can I calculate scantlings of ships