Measuring pressure diffferences

Discussion in 'OnBoard Electronics & Controls' started by rwatson, Jul 1, 2013.

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TANSLSenior Member

Have you taken into account the elongation of the spring can not be directly proportional to the pressure, as you call it, exerted by the engine ?. The spring constant may change, not be constant, from a certain elongation.
Maybe you will need to draw previously, using a calibrated apparatus, the pressure vs elongation curve of the spring.
Maybe I'm wrong and I'm complicating what is simple.

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rwatsonSenior Member

Yes indeed - a very valid point. I cannot assume the rate of change will be linear at all.

I am going to have to individually measure the 'pressure' applied at each level.

While I think about it, I will need to establish an upper and lower possible range

From observed tests, I would imagine the range would be from Zero to no more than 100 grams per square centimetre.

If I can accurately detect the difference, I have to equate them to equivalent power at 1:5 scale.

http://www.allconversions.com/

Assuming the Propeller is say, a square centimeter in area,

maybe that provides a conversion of .1 Kilograms per square centimeter, to provide a Force measurement of 0.980665 newtons per square centimeter

If I can convert that Force Unit x the mass of the boat , does that give me the Power required measurement, which can be scaled up ?

I will need to get onto a science forum to verify the logic.

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TANSLSenior Member

I'm afraid I can not answer your questions because it is a very specific field, of which I am not an expert and therefore, I could say a lot of nonsense.
Only two points to note:
1. Force x mass of the boat does not give anything valid.
2. The extrapolation of the values ​​obtained with a model to the real boat is an operation involving many complicated factors and dimensionless numbers.

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cmckessonNaval Architect

A whoopi cushion

Being a fluids guy, I liked the original manometer-based idea. Maybe a balloon - something like a hot water bottle - with a clear plastic tube sticking straight up. Height of fluid in tube yields pressure in balloon. Contrive boat-to-balloon-to-motor interface such that areas are known and remain constant, say two identical 2" square blocks.

Force on blocks must be in equilibrium with pressure in balloon, et voila monsieur.

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rwatsonSenior Member

Thanks for those observations.

Now that I see the points, it would have to be Force x Mass x Velocity

edit: Note to self - Not velocity, Acceleration ?

You are right about extrapolating to the real world.

I am content with 'indicative' results, so that I have a fair confidence I don't build a hull that is a real underperformer.

I would hate to be in a project where I was getting a hull designed in a totally radical shape - that would be scary. Luckily, I am in a fairly well traveled area.

I have also had supporting calculations done ( attached )

Thanks for the input Tansl

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• Results.jpg
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rwatsonSenior Member

This still intrigues me too.

The actual mechanism eludes me though, both its placement and the data capture method.

Maybe what I should be thinking of is instead of a spring in the tilt control with indicators, is should push a head of liquid up a vertical pipe, or even pull a column up ?

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• PressureSensorColumn.jpg
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TANSLSenior Member

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rwatsonSenior Member

Yes, I tried one out for other purposes, but I couldn't get reliable results. The hull angle through waves seemed to have bad effects

My Science mates have been checking the formulae
http://www.sciencechatforum.com/viewtopic.php?p=267397#p267397

I had to draw up better diagram for them,

So far
Force times speed = power

Say the force is 1 Newton, and the speed attained with that force is 1 meter/second.

Then the power is what you get by multiplying the two: 1 Newton meter per second = 1 Joule per second
= 1 Watt.

1 joule per second = 0.00134102209 horsepower
http://www.aqua-calc.com/what-is/power/joule-per-second

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BMcFSenior Member

In a somewhat similar fashion, all of our testing with outboard-powered models is done with pancake load cells reacting motor thrust. We always calibrate the measurement end-to-end by pulling on the lower unit, prop shaft centerline, with a fully certified-calibrated tension load cell.

The outboards are not mounted directly to the transom though..we mount them on a hinged plate with composite bearings in the hinge. In the pics below are front and rear shots of a 4-engine mounting and thrust measurement setup for a 37-foot experimental craft we were working on. Each of those tubular extensions is for the reaction link from the engine plate to connect to a load cell inside the transom of the boat.

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rwatsonSenior Member

Yes - but what do you use on 1:5 scale setups ? - manufacturer and brands appreciated.

Its good to get a bit of verification of the process - with a hinged motor system.

Very interesting post Mr B

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rwatsonSenior Member

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• PowerFormuale.jpg
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BMcFSenior Member

That WAS a 1:5 model..more or less. Heck, we built and operated a 1:20 free-running remote-controlled model for ONR that was 57-feet LOA.

We use exclusively Strainsert load measurement devices. Here is a pic of a smaller pancake thrust cell that is used on one of our outboard motor hinge brackets that supports engines in the 70-115HP range (the mounts in the earlier pic support up to 300HP each, for comparison. Much smaller ones are available; we've used them on little baby Yamaha 2.5 HP outboards even.

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rwatsonSenior Member

A bit too big for a 10 kilo model unfortunately

Putting this link in here for a 50kg RC Model Load Sensor
https://www.sparkfun.com/products/10245

"For anyone trying to build a somewhat precise scale with this sensors please DO NOT use them. Their accuracy and repeatability is very poor"

Extra Data
p = F ÷ a. pressure = Force ÷ area.
F = m x a. Force = mass x acceleration.

http://www.gcsescience.com/peq.htm

So to use Pressure to calculate Force, I would need to know the acceleration rate,
Not looking hopefull :=(

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rwatsonSenior Member

What are you waffling on about strain gauges for ?

You should be able to tell me how to do the Power calculations I need ?

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BMcFSenior Member

?? No you don't...pressure x area = force. Force/area = pressure

We've used pressure tension setups during bollard and towed-model tests many times. A hydraulic cylinder and a pressure gauge and there you are. We towed a 13.5-meter model (it was 1:4 scale at that size) with a rig like that.

You mentioned a teeny tiny model though..10 kg?? We probably carry more than 10 kg of beer on board some of our scale models....like this 1:4 scale model.

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