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Author Topic: X9 ...  (Read 10327 times)

Subculture

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Re: X9 ...
« Reply #25 on: July 11, 2015, 06:38:25 PM »

Think I can hear crickets.....

U-33

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Re: X9 ...
« Reply #26 on: July 11, 2015, 06:45:40 PM »

Damn...I thought it was minds ticking over I could hear.
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Rich

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salmon

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Re: X9 ...
« Reply #27 on: July 11, 2015, 07:06:49 PM »

Smarty pants, I just read your questions.
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salmon

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Re: X9 ...
« Reply #28 on: July 11, 2015, 08:17:47 PM »

it would involve quite a lot of work to make videos showing these things, and it would still only show one example.I think perhaps I should ask some questions of those asking the questions, to test current level of knowledge.1. What empirical method can we use to find out the ballast tank volume required for any model submarine, assuming we have an adequately sized test tank to put it in?/quote]
Well, I have never done this my self, so all my answers are based on what I have read. So, the empirical method would to use Archimedes method of weighing it in water and air and work out the displacement. I have also read just weighing the above waterline structure. Finding it's density compared to water ( like styrene is close to water so what it weighs is what you would need to displace).
2. A WTC is 8cm in diameter and 20cm long. What is its displacement in
/quote]
I will assume you are referring to cylinder, 1 Liter

3. How much weight in kilograms can that cylinder support?
/quote]
1 Kilogram

4. What is the average density of fresh water, and by how much does it vary in density in percentage terms?
/quote]
I should remember this fromdiving, but alas I do not. So my guess is 10%. Temperature and dissolved particles can effect this, so it may be higher.

5. If I compress air from a 500ml ballast tank into a 1000ml reservoir, what will be the pressure in bar or psi.

O.k so continuing my guessing, it doubles with every half, so 2 bar?
O.k. Did I really mess this up or am I close?
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salmon

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Re: X9 ...
« Reply #29 on: July 11, 2015, 08:18:12 PM »

My brain hurts
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Re: X9 ...
« Reply #30 on: July 11, 2015, 09:30:26 PM »

Okay. So question one- correct, although it can be tricky to accurately weigh an item like a model submarine in water and air.

The best method is to trim the boat to neutral buoyancy or just slightly negative, then use polystyrene foam to bring it to the correct waterline. Very simple, but very accurate. Skip Asay published this method many years ago, although I believe it was credited to Dave Copeland.

Answers to two and three, correct. Converting volume to weight is so much easier when working with the metric system.

Answer four- not quite. Water actually varies very little, just 1% depending on temperature and mineral content.

Question five. You are compressing 500ml into 1000ml, pressure will need to increase by 50% above atmospheric about 0.5 bar or 7 psi (rounded down to simple figures).



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Re: X9 ...
« Reply #31 on: July 11, 2015, 10:03:12 PM »

Regarding question five, how did I arrive at that answer?

Well Boyle's law says that our initial pressure multiplied by our initial volume should equal our final pressure multiplied by our final volume. Now we knew three of those answers.

Initial pressure is normal atmospheric pressure, which tends to be about 1 bar- this varies a bit depending on your location, but for example we'll leave it that figure.

Our initial volume is 1500ml, adding both the ballast tank and the reservoir, so 1x1500= 1500.

Our final volume is 1000ml, so in order to have that reach the 1500 magic number, then we get a pressure figure of 1.5 bar. of course on a pressure gauge that would read 0.5 bar, because our gauge will be at atmospheric pressure like the rest of the kit.

Hope that makes sense.

All these questions were asked for a purpose. If you are building a WTC, then you really should know how much it will displace, and how much weight it will support.

If you are designing a ballast system, you might want to know how much extra to add on if you want to be able to trim your boat for different water conditions.

If the ballast system recirculates air, then you will want to know what pressure it will reach, to see if your pump can make pressure.

You can of course use solely empirical methods, but a few minutes spent with a pocket calculator can save hours and wasted material.

salmon

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Re: X9 ...
« Reply #32 on: July 12, 2015, 12:18:05 AM »

So, Andy, are you going to do a three part video? Oh please oh please oh please (said in my best whiny voice). I like your piston video and think this would be a great starting place for many, seasoned and beginners. Show the math and show the practical ways to use the information.


Skip Asay is one of the greats in the hobby. The foam will relate to the volume needed for the cylinder, I remember reading that.


I have not dealt with using a pressure tank, either by bag or water, but I will someday. So, understanding this aspect better will help. I am surprised by 1% variance in water, I would have bet it was more (And I am not a gambling type). I need to look that up just to retrain my mind.


Thank you for the testing and challenge. I can only learn more when I do not know what I do not know.

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U-33

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Re: X9 ...
« Reply #33 on: July 12, 2015, 11:06:26 AM »

And I haven't got a clue what you're both on about...this is a perfect illustration of why the dinosaurs died out.  :embarrassed:
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Rich

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Re: X9 ...
« Reply #34 on: July 12, 2015, 02:07:03 PM »

salmon

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Re: X9 ...
« Reply #35 on: July 12, 2015, 04:15:36 PM »

Before you posted that last link, I went looking.
I found this sight, [size=78%]http://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html[/size]
From this site it would appear a 5% variance, but most of this information goes way over my head. No matter what the answer is, it is not 10%. Thank you for the exercise!
Now back to that video......
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Re: X9 ...
« Reply #36 on: July 12, 2015, 05:18:51 PM »

I should have added within the type of temperatures you will operate a sub at your local puddle.

salmon

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Re: X9 ...
« Reply #37 on: July 13, 2015, 03:19:07 AM »

Richard, you better post something about your sub or I will keep harassing Andy to make a video!
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U-33

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Re: X9 ...
« Reply #38 on: July 13, 2015, 12:25:49 PM »

Tom, I know Andy would have kittens over the way I go about building, trimming and ballasting my boats. It's much easier with a box to start with, but once the box or the cylinder is installed in the boat, it's a case of putting it in the bath...if it sinks I either remove a piece of lead, or add foam.. If it floats too high, I add lead...and that's about it really.


I don't understand all the technicalities of volume, displacement, water temperatures, etc...if the boat floats when I want it to and sinks when I want it to, and is level on or under the water, that's good enough for me. If it comes back up when I want it to...then that's a bonus.





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Rich

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~~~~~~~  "Motorflotes need love too...."  ~~~~~~~

MotorFlote build log : http://www.modelboatmayhem.co.uk/forum/index.php/topic,15222.0.html

unbuiltnautilus

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Re: X9 ...
« Reply #39 on: July 13, 2015, 12:48:47 PM »

Truly a Zen Master of the principles of 'suck it and see'! I salute your approach to technology and envy you..."Respect to the Man... :-))
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Re: X9 ...
« Reply #40 on: July 13, 2015, 12:52:20 PM »

Skip, I've been called a lot of things over the years, but never before have I been called a Zen Master...I thank you for that from the heart of my bottom.   ;D
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Rich

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~~~~~~~  "Motorflotes need love too...."  ~~~~~~~

MotorFlote build log : http://www.modelboatmayhem.co.uk/forum/index.php/topic,15222.0.html

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Re: X9 ...
« Reply #41 on: July 13, 2015, 12:52:36 PM »

As I said in an earlier post, nothing wrong with taking an empircal approach. 

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Re: X9 ...
« Reply #42 on: July 13, 2015, 11:10:50 PM »

i will have to agree with andy that using metric to convert between mass and volume is so much easier than using imperial units... especially when i buy the majority of my gadgets and toys from the germans.
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Re: X9 ...
« Reply #43 on: July 15, 2015, 07:33:56 AM »

What did I learn from this ??? well for starters I need a bit of foam in my type VIIc to bring it back to its correct water line the instruction contained in those well written lines were so simple to understand thanks  for taking the time to put that out here Andy.

Mark.
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