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Author Topic: Propeller speed - In the water  (Read 4949 times)

geoff p

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Propeller speed - In the water
« on: August 23, 2008, 04:06:33 PM »

Does anyone have ideas how to measure the rotational speed of a propeller when the boat is under way?

I have a tachometer which shows my prop-shaft turns at 4500 RPM on the bench (at full 'throttle') but I wonder what it makes out on
the pond.  This could be useful information for selecting/designing new propellers.

Fairly recently there was some stuff on this Forum about transmitting videos back to shore - is there similar technology for remote data acquisition?  Or at least, some way of adapting the technology?

Food for thought ....

Geoff
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toesupwa

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Re: Propeller speed - In the water
« Reply #1 on: August 23, 2008, 04:57:42 PM »

Does anyone have ideas how to measure the rotational speed of a propeller when the boat is under way?

I have a tachometer which shows my prop-shaft turns at 4500 RPM on the bench (at full 'throttle') but I wonder what it makes out on
the pond.  This could be useful information for selecting/designing new propellers.

Fairly recently there was some stuff on this Forum about transmitting videos back to shore - is there similar technology for remote data acquisition?  Or at least, some way of adapting the technology?


Seeing as the prop shaft / motor is attached to the prop, measuring the RPM at the shaft / motor when in the water would give you some idea..

Some of the 'fly' boys use measuring equipment for altitude and other stuff. Maybe something like that could show RPM too?...
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Colin Bishop

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Re: Propeller speed - In the water
« Reply #2 on: August 23, 2008, 05:52:58 PM »

The problem in measuring the RPM when the boat is static in the water is that the RPM at any given power setting on the TX will change once it starts to move, that's why some sort of telemetry is needed for accurate readings.
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geoff p

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Re: Propeller speed - In the water
« Reply #3 on: August 23, 2008, 06:08:40 PM »

Exactly, Colin.
I could measure things, sort-of, in the bathtub but that would only be static.
Geoff
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Garabaldy

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Re: Propeller speed - In the water
« Reply #4 on: August 23, 2008, 08:54:58 PM »

i have no idea of any of the terminolgy on this one but i'l attempt my suggestion anyway using lots of "thingys"

You can get them infrared reader things which meaure the rpm of a shaft.  sureley it would be possible to purchase one which will tell you the highest rpm read abit like the trip on a car or bycicle speedometer.  You could sellotape it position inside your hull to give you a reading.  Whether such a device exists i am not sure ::)
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Ghost in the shell

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Re: Propeller speed - In the water
« Reply #5 on: August 23, 2008, 09:08:29 PM »

that may throw the propshaft out of balance as it requires a magnet placed on the shaft
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malcolmfrary

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Re: Propeller speed - In the water
« Reply #6 on: August 23, 2008, 09:45:58 PM »

There should also be optical devices that resond to, say, black and white, or light/no light.  I'm fairly sure that theres a pair of them in my mouse.
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Garabaldy

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Re: Propeller speed - In the water
« Reply #7 on: August 23, 2008, 10:27:55 PM »

that may throw the propshaft out of balance as it requires a magnet placed on the shaft
i believe theres a small tab (very small) which sticks to the shaft  It shouldnt set the shaft off balance...
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nick_75au

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Re: Propeller speed - In the water
« Reply #8 on: August 23, 2008, 11:07:15 PM »

An Eagle tree data recorder will do what you require along with a whole lot of other useful things, they also do a live system as well, quite pricey.
the basic data logger is on my wish list.
Regards
Nick

http://www.eagletreesystems.com/
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andyn

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Re: Propeller speed - In the water
« Reply #9 on: August 23, 2008, 11:09:52 PM »

that may throw the propshaft out of balance as it requires a magnet placed on the shaft

The entire shaft is magnetic....
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GG

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Re: Propeller speed - In the water
« Reply #10 on: August 24, 2008, 12:36:48 PM »

If it's an electric motor then the rotational speed is proprortional to the current drawn.  If steam, then you could perhaps count the "puffs", see "Model Boats" July 2004.
Unless it is a very fast model, I doubt that the difference between the shaft speed when the model is held stationary in the water and running free is that great.
GlynnG
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geoff p

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Re: Propeller speed - In the water
« Reply #11 on: August 24, 2008, 01:12:58 PM »

I don't agree with GG that the speed is proportional to current drawn - on that premise, at stall i.e. maximum current, the motor must be doing maximum revs.

I have a digital tachometer but it requires contact with the rotation shaft.  On the bench at home it's easy - just drop the rudder off the boat and apply the tacho onto the propeller boss.  Hence my reading of 4500 RPM at the bench.

I have toyed with trying to make some sort of non-contact tacho (as several of you have suggested magnetic, optical, what-have-you) but the real problem is how to transmit data from Ship to Shore.

Geoff
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Seaspray

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Re: Propeller speed - In the water
« Reply #12 on: August 24, 2008, 02:12:20 PM »

Not knowing much about it I would start at with the car modellers as they use counters ( possible transponders) on their cars for counting the laps each car has done. If that system can be converted to count revs and send to a receiver it would solve the problem.

Basically you need a rev counter that can transmit the data to a receiver over a given distance. There will be something out there that does this.

The more current you give a motor the faster it will rev till it has peak on it's max revs. The motor has a max efficiency current ,thats when the motor is running at its peak performance. (sweat)

Stall current is when a  load bias is too much for the motor to rev ( bigger the load the greater the current  )
Martin
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Proteus

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Re: Propeller speed - In the water
« Reply #13 on: August 24, 2008, 03:03:59 PM »

cars use a transponder that transmits a low power coded  signal that when driven over a loop in the track records it.so no good. but there are plenty of 433MHz units that will do it, also some of the 2.4 ghz units have a facility to transmit back to TX

http://www.spektrumrc.com/Articles/Article.aspx?ArticleID=1536


Proteus
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GG

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Re: Propeller speed - In the water
« Reply #14 on: August 24, 2008, 04:56:49 PM »

Geoff,
          Ii's proportional but an in an inverse fashion, i.e. as the rotational speed increases then the current falls in a linear fashion.

Still waiting to hear why anyone would worry about the difference (between the static and moving rotational speeds) save for very fast models.
GlynnG
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geoff p

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Re: Propeller speed - In the water
« Reply #15 on: August 25, 2008, 10:54:18 AM »

GG, I stand corrected on the current (amperage) issue. Thanks.

I don't have a 'very fast boat', I actually have a tug but I have been making my own propellers - nice, shiny, brass ones are not available to buy here, in Taiwan - and have been getting curious about their effect on shaft speed.

For instance, I have a 65mm dia, rather coarse pitch jobby that pushes the boat very well.  I also have an 80mm dia, fine pitch prop. that pushes the same boat very well, but the latter sends up something of a rooster-tail at full power.

Now, if the smaller one is trying to push a small column of water very fast (coarse pitch) and the larger one is trying to push a much larger (80^2 * PI/4 vs. 65^2 * Pi/4) column at a slower speed due to its finer pitch, which one allows the motor to turn nearer to its optimum speed?  I want to measure it.

Proteus, I have written to the Spektrumrc site you offered, it could be just what I'm looking for.  Thanks.

Geoff
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GG

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Re: Propeller speed - In the water
« Reply #16 on: August 25, 2008, 11:22:57 AM »

Geoff,
       There's an article on the very subject of prop sizes (pitch and diameter) and rotational speed awaiting publication by "Model Boats".  Using the concept of momentum its not too difficult to understand how these variables interact.  Perhaps the effect of diameter, due to the squared term, is the one that catches most people out.

One thought is can you sail the model at full speed with a digital ammeter on board and see what the current is?  This would show any differences in motor speed.  Another idea is there any flowing water you can safely sail in?  This ought to reduce the relative speed and make observation of the ammeter easier.

I'm looking forward to hearing your results.

GlynnG
P.S.  do not hold your breath waiting for the article to be published!
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Seaspray

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Re: Propeller speed - In the water
« Reply #17 on: August 25, 2008, 02:31:27 PM »

Part of working out what a prop does when its in the water is the amount of water that it moves. Wheather this is per one revolution at a certain speed ..can't remember

Thats getting too technical for me. I just try and try till in comes good


Martin
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geoff p

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Re: Propeller speed - In the water
« Reply #18 on: August 26, 2008, 02:18:04 PM »

Funnily enough, Glyn, just today I was watching some flowing water and thinking much the same.  I don't know if I'll have the courage to try it in this particular stream, though, it's going at over a metre per second.  So I would have to tether the boat for when I power-down or wave it Goodbye  ::) And it's a long way down from the bank  :(

For now, I'll just continue as Seaspray does - suck it and see.

Geoff
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Seaspray

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Re: Propeller speed - In the water
« Reply #19 on: August 27, 2008, 07:13:58 AM »

Yep suck it and see.

Takes a while but boy do you gain a lot of experience good and bad.
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Timo2

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Re: Propeller speed - In the water
« Reply #20 on: September 06, 2008, 12:59:21 PM »

Hi All

  Is this what you looking for ::)

       http://www.spektrumrc.com/Products/Default.aspx?ProdID=SPM1305

     Add its rpm sensing to give you a handle on your motor speed plus Simply mount the temperature sensor to your motor, heatsink, or battery, plus Volt meter

   Timo2 O0
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geoff p

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Re: Propeller speed - In the water
« Reply #21 on: September 06, 2008, 01:40:45 PM »

Hi Timo,
I contacted Messrs Spektrum a couple of weeks ago and they passed me on to vendors (primarily because I don't live in the States) but without answering my questions.  The vendors, of course, could not expand on Spektrum's info or lack of it.

It seems their modules will only work when connected with their own 2.4Ghz transmitter and 6 or 99 function gear.  It becomes rather OTT both in facilities and cost so I will not pursue that route any further.

Currently I am scratching around an idea to collect the shaft-signals by data-logging, perhaps onto an MP3 player/recorder - when I can figure how to do it  ::) - then analyse the data at home.  Not nearly so much fun as getting the ship-to-shore connection but less expensive, I hope.

Geoff
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Timo2

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Re: Propeller speed - In the water
« Reply #22 on: September 06, 2008, 02:24:01 PM »

Hi Geoff

    It is all money  ::),  But new 2.4Ghz unit   DX 5e at about 60.00 look good  , it mite fit the SPM1305 .

    PS. you could hire it out by the hour ?  ::)

    Timo2
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Seaspray

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Re: Propeller speed - In the water
« Reply #23 on: September 06, 2008, 03:45:08 PM »

I am just waiting to see how the 2.4 Ghz settles down in the market place.

Don't quote me

But I was lead to believe that it was designed for R/C/ cars.

Also it range wasn't that good (I heard) and with our ariel partly below the waterline and water between us and the boat which may affect the signal. I was wondering if its suitable for us boaters, especially the multi racers

Martin

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Subculture

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Re: Propeller speed - In the water
« Reply #24 on: September 13, 2008, 12:06:39 PM »

An Eagletree elogger is the device for you. It can measure RPM using either a magnetic/hall effect sensor, or an optical sensor. The former will not throw the shaft out of balance, as it uses four small neodymium magnets spaced at 90 degree intervals, which you can mount on a small disk that attaches to your propshaft.

It can also measure Voltage, Current, temperature, even GPS if that's your bag. If you get the powerpanel too, you can measure all this on your bench, without using a computer.

Andy
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