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Author Topic: Dive system weights and sizes  (Read 1995 times)

terry1956

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Dive system weights and sizes
« on: April 06, 2021, 05:08:02 pm »

Hi out there. I purchased a sub that needed some tlc.  And I need some help with the dive system. The lower hull is split into sections. The front section holds an air pump. This feeds two small paint cylinders in the second section, air being taken from that  section and compressed in the cylinders. There are two wires from the air pump, pos and neg. my question is what is the best way to control this pump. And from where do I purchase the bits needed within the uk. Thanks.
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HMS Invisible

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Re: A bit of help needed
« Reply #1 on: April 06, 2021, 06:18:30 pm »

Hi out there. I purchased a sub that needed some tlc.  And I need some help with the dive system. The lower hull is split into sections. The front section holds an air pump. This feeds two small paint cylinders in the second section, air being taken from that  section and compressed in the cylinders. There are two wires from the air pump, pos and neg. my question is what is the best way to control this pump. And from where do I purchase the bits needed within the uk. Thanks.
The arrangement of the ballast is the way Wullie Guthrie built his Darnell sub. I can't rememember if he built a T or S class.Anyway,  his sub (U.S.E. pump, servo & microswitch ballast + AST pitch control on the dive servo + RC) worked and sailed well enough with nothing more, nothing less.
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terry1956

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Re: A bit of help needed
« Reply #2 on: April 06, 2021, 06:33:32 pm »

Thanks but how did he control the air pump. What parts and who sells them in detail.
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HMS Invisible

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Re: A bit of help needed
« Reply #3 on: April 06, 2021, 08:47:59 pm »

Please understand there have been so many "how to" books and other publications and you got a short, speculative reply amounting to:- you might have such and such, built by so and so, which uses this gear, based upon
two small paint cylinders
inside the ballast tank
air compression pump
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Fred Ellis

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Re: A bit of help needed
« Reply #4 on: April 07, 2021, 07:25:36 am »

Hi Terry 1956

Have a look at this site https://www.theassociationofmodelsubmariners.com it should have some of the answers that you are after.

Fred
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warspite

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Re: A bit of help needed
« Reply #5 on: April 07, 2021, 10:54:13 am »

Pictures also would help in identifying the parts you have :-))
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RST

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Re: A bit of help needed
« Reply #6 on: April 07, 2021, 11:17:27 am »

I found some info on Google by looking for "U.S.E. pump, Darnell submarine".


I just put "U.S.E. pump" in first and presumably that's not the original mechanism that was showing up -perhaps it is? Don't check on a works computer! LoL
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Subculture

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Re: A bit of help needed
« Reply #7 on: April 08, 2021, 09:56:39 am »

They were a little oscillating steam engine used by Darnell in the earlier days of the hobby. Rigged to an electric motor, they can move air about, but they tended to be rather leaky without modifications. Most replaced them with a small compressor culled from the small 12v tyre inflator units you can get in Halfords etc.

Nowadays there are excellent small diaphragm pumps available secondhand on ebay for a few pounds that work much better provided you don't want to pump higher pressures (e.g. over 5-10psi).

The main snag with compressed air is that it can be harder to control for fine trim, so most systems using it tend to have it as a binary system- full or empty, and use an additional trim tank for fine trimming that last 1% of variance owing to water conditions e.g. temperature and mineral content.

Where a single tank system is desired many now use water pumps as they allow fine control and are easier to check.

terry1956

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Re: A bit of help needed
« Reply #8 on: May 01, 2021, 05:47:52 pm »

Thanks chaps, all bar one whos sarcastic reply was about as useless as it gets. I have dumped  the whole compartment system in the hull and I am how in the process of making a WTC cylinder with water pump and bladder bag for the dive system. 
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terry1956

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Dive system weights and sizes
« Reply #9 on: May 07, 2021, 05:34:46 pm »

Hi chaps. I am part way making a WTC for my T class sub. I had the idea that I had worked out all the sums for weight and size etc. However it took me a bit longer then it should for me to complete the WTC end stops. So today was the first time I was able to test the cylinder in the bath. And it was here I run into some problems. Ok, here are some details the cylinder is 500mm long and the Inside dia is 86mm. I was going to use a bladder bag dive system. So I worked out that with water weighing 1 pound per litre that 2 litres should start the cylinder dive. Wrong, even with 2.5 litres the cylinder is still floating. So my big question is. What have I done wrong, over looked ? And how do I overcome the problem. Thanks Michael
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solidus

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Re: Dive system weights and sizes
« Reply #10 on: May 08, 2021, 01:09:48 am »

Maybe you need to measure the outside diameter of the cylinder which is likely to be 3mm thick. That probably gives a displacement of more than 3 liters of water.
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tsenecal

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Re: Dive system weights and sizes
« Reply #11 on: May 08, 2021, 06:36:51 am »

my math for a 92mm x 500mm gives a volume of 3324 CC's  or 3.3+ litres    pi * radius squared * length  or 4.6 x 4.6 x pi x 50...
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tsenecal

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Re: Dive system weights and sizes
« Reply #12 on: May 08, 2021, 06:38:41 am »

even doing the math with an 86mm inside diameter, which won't work, i get 2.9+ liters
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tsenecal

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Re: Dive system weights and sizes
« Reply #13 on: May 08, 2021, 06:58:06 am »

this is a very long winded reply...  but hang with me.

in reality, your whole concept is wrong.  a "real" model submarine would  have your WTC enclosed inside a scale representation of a real submarine (ie your t class submarine), with the area between the wtc and the outer skin being free flooding water, plus whatever lead you need to get that outside skin to "float" at its representational water line. then the bladder inside the wtc is filled with enough water to compensate for the outer skin's volume that is still above water...  when that amount of water is filled into the bladder, the outer skin's volume will be "removed" from the buoyancy... and the sub should sink.    you will never be able to fill the bladder with enough water to make the entire WTC submerge on its own.  the bladder would have to fill the entire volume of the WTC to compensate for the volume of the WTC.

follow my logic...  i have a 1/32 scale type II uboat that is 53" long, and 6" wide.  it is made of fiberglass about 1/16" thick.  the waterline of that scale hull is about 2" below the deck.  the ballast tank has to be large enough to displace the same volume of water as the entire fiberglass hull that sits above the waterline when the sub is surfaced, or slightly less than the equivalent to a sheet of fiberglass that would be 53" x 10" x 1/16" thick.  (2" sidewalls and 6" width) those numbers have nothing to do with the dimensions of the 18" x 4" tube that makes up the WTC.  my math puts that sheet of fiberglass at roughly 33 cubic inches, or 540 cc.  the true volume of it isn't really 33 cubic inches, since the hull narrows at the bow and stern (it isn't 6" wide for its entire 53" length), but the fact remains that the submarine dives with an engel 500ml piston tank.  lastly, no need to convert litres of water to lbs of weight.  a ml of water and a cc of water and a gram of water are the same.  if you need to fill a bladder with 540cc of water, you need to fill it with 540ml of water, or 540 grams of water.   metric really works well for ballast tanks

a properly sized ballast tank (in my case 500ml) will never hold enough water to get the bare WTC completely submerged on its own (in my case 3.34 litres).  that is not what the ballast tank is intended to do.
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tonyH

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Re: Dive system weights and sizes
« Reply #14 on: May 08, 2021, 11:17:37 am »

Volume is not the answer. The fundamental problem is that 1 litre of water weighs a kilo and not a pound.
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solidus

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Re: Dive system weights and sizes
« Reply #15 on: May 08, 2021, 05:41:50 pm »

Its important to calculate the external size of the WTC. The plastic tube with all its accessories  is much lighter than water and has positive buoyance, which means it enhances the floatation of your WTC.
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tonyH

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Re: Dive system weights and sizes
« Reply #16 on: May 08, 2021, 07:37:35 pm »

 

Quote from the first post posing the question.
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tonyH

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Re: Dive system weights and sizes
« Reply #17 on: May 08, 2021, 07:42:26 pm »

So I worked out that with water weighing 1 pound per litre that 2 litres should start the cylinder dive.

Agreed about the volume but if the weight per litre is wrong it mucks up the whole lot!
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Subculture

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Re: Dive system weights and sizes
« Reply #18 on: May 09, 2021, 12:38:13 pm »

https://www.britannica.com/science/Archimedes-principle

https://www.engineeringtoolbox.com/density-solids-d_1265.html

Most modellers get into trouble because they lack understanding of first principles with regards to material mass and displacement.

You have another issue to think about. Those old Darnell hulls were quite chunky lay-up's and a T-class has a lot of freeboard. If your system isn't aspirated and recirculates the air, the pressures are going to get quite high inside the cylinder if you want to get close to a scale waterline.

https://en.wikipedia.org/wiki/Boyle%27s_law

So make sure the shaft seals are up to it, or build a bigger tube, or choose an aspirated or partially aspirated system.

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Re: Dive system weights and sizes
« Reply #19 on: May 10, 2021, 11:11:12 pm »

 
Topics merged.

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terry1956

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Re: Dive system weights and sizes
« Reply #20 on: May 16, 2021, 12:15:02 pm »

Thanks chaps. More testing too be done, but as its pouring with rain I need to wait for a clear day to get the sub in the fish pond to test things out.
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Subculture

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Re: Dive system weights and sizes
« Reply #21 on: May 17, 2021, 10:35:36 am »

It's difficult to accurately calculate displacement for a fairly complex hull form like a T-class, which is hand laid in GRP, so also likely to have varying degrees of laminate thickness.

A very accurate and straightforward method, is to put your hull in a test tank with enough buoyancy (closed cell foam is ideal0 to trim it until it is just slightly negative. The use some more of the same foam to attached to the hull below the intended waterline, to bring to right trim. Whatever the volume of the foam, that is the volume of the tank you require.

If you want to be really picky, you can also offset the weight of the foam, which will be minimal. Most modellers add on a few percent to account for extra details, paint etc.
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