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Author Topic: Salt Water Darnell U37  (Read 54882 times)

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Re: Salt Water Darnell U37
« Reply #250 on: October 14, 2016, 08:38:36 pm »

I haven't tried the 12 volt units, only the type I linked to. They all work on similar principles and are very good value.

unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #251 on: October 16, 2016, 08:09:57 pm »

Today I played about with the model. However, first things first. Upon turning on the model, nothing happened....no lights, no twitching, nothing. Did the flood damage of a couple of weeks ago cause a problem only becoming apparent now?
Turns out the only thing to fail was a two quid on/off switch. Once bodged with a rat tail file, all sprung into life..result.
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #252 on: October 16, 2016, 08:14:04 pm »

Now on to serious stuff, ballast tanks. Following this weeks online discussions, I have decided to fit a vented system, i.e. one that pumps water in to a tank and vents the air above the water, to atmosphere. This is arranged via a thin pipe run up to a high point on the model.
Loosing the high pressure system should sort out problems encountered so far. Options, options, options...
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #253 on: October 16, 2016, 08:21:54 pm »

The first picture shows the high pressure system with the two saddle tanks, which reduced the pressure on the acrylic tank, allowing more water into it than would be the case otherwise.
The second picture shows the same tank without the saddles. This would be vented to air instead, solving the pressure related issues.
Picture three is a PVC tank, slightly smaller diameter, but better assembled than my old tube. This would lower the height of the tank in the model slightly, for better or for worse.
The fourth picture shows a 32oz Ripmax fuel tank, which I happened to have, and which by some quirk of nature, fits between the existing plumbing nicely. Also, being florescent yellow it looks like an old school Darnell with the hatch on. Bright colours visible through the free flood holes!


So, I have three options, today I just wanted to test everything with a vented system, trialling how small a vent pipe I could use etc. This is where the trouble started...
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unbuiltnautilus

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Re: Salt Water Darnell U37
« Reply #254 on: October 16, 2016, 08:34:54 pm »

Previously, with the high pressure system, I was having a problem with the pump when venting the tank. It was drawing air from within the main pressure hull and dumping that into the lake, along with the ballast water. This was resulting in a vacuum being created inside the RC compartment. This, in turn, encouraged water to find its way into the model through all the seals. I assumed this had something to do with the pressures involved when venting the tank. No, it still does it, with no pressure in the tank, and with the sub open to atmosphere, I could see bubbles being pumped into my pot of water as the sub pumped itself dry. Nothing to do with the system being sealed up, as it is now under no pressure whatsoever.
So, I swapped the pipes over on the pump, 'in' becoming 'out' and vice versa. Now the air bubbles were only visible INSIDE the ballast tank as the model filled with water. It turns out this only happens in one direction of rotation of the pump..most puzzling. So as a pump in/pressure out system, with the pump running in its 'good' direction, this unit is fine. However, pumping both in and out, a problem is occurring. With the new plan for a vented system, whatever I do, air escapes, one way or the other.
As a pressurised system with the pump only running one way, Sheerline style, all would be good. If I want to pump up and pump down however, this pump is problematic. Whatever happens, it's got to come out. I will strip it down and have a proper look at it, while considering options..


Most annoying >:-o




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Re: Salt Water Darnell U37
« Reply #255 on: October 16, 2016, 09:11:05 pm »

Previously, with the high pressure system, I was having a problem with the pump when venting the tank. It was drawing air from within the main pressure hull and dumping that into the lake, along with the ballast water. This was resulting in a vacuum being created inside the RC compartment. This, in turn, encouraged water to find its way into the model through all the seals. I assumed this had something to do with the pressures involved when venting the tank. No, it still does it, with no pressure in the tank, and with the sub open to atmosphere, I could see bubbles being pumped into my pot of water as the sub pumped itself dry. Nothing to do with the system being sealed up, as it is now under no pressure whatsoever.
So, I swapped the pipes over on the pump, 'in' becoming 'out' and vice versa. Now the air bubbles were only visible INSIDE the ballast tank as the model filled with water. It turns out this only happens in one direction of rotation of the pump..most puzzling. So as a pump in/pressure out system, with the pump running in its 'good' direction, this unit is fine. However, pumping both in and out, a problem is occurring. With the new plan for a vented system, whatever I do, air escapes, one way or the other.
As a pressurised system with the pump only running one way, Sheerline style, all would be good. If I want to pump up and pump down however, this pump is problematic. Whatever happens, it's got to come out. I will strip it down and have a proper look at it, while considering options..


Most annoying >:-o
me made um vacuum prevention doodah and no understand all dat!

The tank choice may come down to material. It is easy to bond to pvc, acrylic but not polypropylene.
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Re: Salt Water Darnell U37
« Reply #256 on: October 16, 2016, 10:11:40 pm »

Have you got a leak in the seals on the pump I wonder. Sounds very peculiar to me. A gear pump shouldn't really care which way it pumps.

One thing to bear in mind, is that the water is likely to be colder than the air, so the air inside the compartments will pull a slight vacuum, so that's a potential source of leaks. I test small enclosures for leaks by putting them in hot and cold water.
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