Troubleshooting a bubble (bubbles) trapped in Ballast Tank after Fully Valve Venting

[corsaire]

Hi fellow submariners,

Any ideas as to how to address this inconvenience? It hasn't happened to me yet, but I've read that a trapped air
bubble or bubbles in the ballast tank could precipitate an unwilling (out of control) dive to crushing depths.

I've thought of opening up more the vent valve to allow for more air escaping as much as possible w/o compromising leaking.

Any feedback will be appreciated,

Corsaire

[Ralph --- SSBN 598]

Placement of vent in tank to minimize trapped air.
Usually the boat will take a bow down attitude during a dive.
Placing the vent at the rear of the tank's highest point will help the air leave the tank.
Some tanks will have a vent at both the front and rear of the tank.

Trimming can help stop those crushing to the bottom out of control dives.
Trimming the boat in the fully flooded tank with a very small portion of the boat's mast and or sail will help.
Keeping a 1/4" of the sail above the surface when the tank is completely void of air will help with depth keeping.
With only 1/4" above the surface, just a very small amount of forward motion will cause the boat to dive.

Depending on ballast system, you can trim the boat heavy for a completely static dive but your transmitter will need to be able to give feed back as to the position of the dive system.
I do not have one but have seen transmitters with digital displays that showed a percentage of dive system position.
Example: Reading on display says 96% and the boat would dive to 1/4" of sail above surface.
another 1% and the boat would go to top of masts above surface.
at 98%+ the boat would be heavy and be headed for the bottom without forward motion to counter act this dive.
Then a reading of 5% would be full surface at design waterline.
0% would give an above waterline attitude.

What this does, is it gives the operator the ability to travel to other ponds that are at different than the home pond and not have to retrim the boat with adding or removing foam.
Just put the boat in the water.
Observe dive and surface to clean the tank and then observe the waterline percentage and then the 1/4" percentage and you are good to go.

Those of us who don't have a transmitter with this display will have to use other means to adjust trim if you run at more than one pond.

I use an adjustable tube in the tank to vary the surfaced trim.
I have several small and different blocks of foam in my tool box to correct the submerged trim. Not always accurate.
Have an idea on an adjustable submerged trim adjustment device.
Unlike the transmitter percentage control, mine are manual.

Another item to tank about is cleaning your tank well to remove all oils.
Production lubricants and personal hand oils.
The oils on the tank walls will hold on to air bubbles.
Cleaning will reduce this clinging on effect.
Some tanks are enclosed and it is hard to get in there.
Try diluted liquid dish soap.
Pour a little in the tank and slosh it around.
Rinse until water is clear and no bubbles are created when shaken.

[southern or]

Interesting, how does one clean a closed piston tank? If not for this reason, I would still like to do it after every run thanks to contamination/introduction of foreign life.

[corsaire]

Just to add more info, this is one Meriman's WTC with a fixed ballast tank size. I've noticed too that after the first vent to let in water into the tank, subsequent vents only attain asort of partial venting, as if water can't get into the tank no more preventing the model from reaching correct surface trimming.

Could it be that the vent valve is not opening enough and a bubble gets trapped in it? I've tried to blow off water retained in the vent hole on top of the WTC achieving mixed
results.

[Guest]

How much of an air bubble are we talking here? Doubtful it displaces much, and as with this type of ballast system you should trim for slight positive buoyancy with a full tank anyway. The fact that the bubble is very small and will compress a bit as you dive deeper is not really going to affect diddley squat, especially bearing in mind that water tends to get denser once you get below a few feet.

Also worth mentioning that who goes below a few feet anyway. The deepest I've had a sub is about 10 feet, and that was just an academic exercise to see how well the seals held up, usually I limit operations to about 2-5 feet assuming the water permits that depth, which the majority of the time it doesn't.

[JWLaRue]

....especially bearing in mind that water tends to get denser once you get below a few feet.

For all practical purposes and at the depths we run our boats down to, water is incompressible. I think what you meant to convey is that the water pressure increases?

-Jeff

[corsaire]

Basically my problem boils down to this:

Vent valve doesn't seem to vent all the air out by letting water in 100%, it does it 100% only on the first vent, subsequent vents for some reason are not as successful as the first vent.
On the first vent around after intial sub placement in the water, the boat submerges positively letting just about 1/2 inch of the conning tower visible above the water (fully flooded tank, or at least almost 100%), but after blowing ballast and trying to vent again, and many vents after that it only positively submerges letting almost all of the conning tower above the water.
I wonder what's happening here, what it doesn't submerges like the first time with only 1/2 inch sail above water? Does it make sense?

[Guest]

Nope. What I meant was that water varies in temperature not by much, but enough to make your boat a little more buoyant than it was at the surface. That's in the Summer months. In winter months it can go the other way, and the water temperature can be higher lower down than near the surface.

If it's a gas ballast system, don't forget that each time you blow the tank, the gas tank will lose a bit of fluid, and therefore the boat will lighten a bit.

[corsaire]

If it's a gas ballast system, don't forget that each time you blow the tank, the gas tank will lose a bit of fluid, and therefore the boat will lighten a bit.

This anomaly has been replicated by the boat even in bathtub testings, with or w/o on-board gas bottle filled. I still wonder what gives. My best guess is air bubble trapped
at the vent valve small orifice, perhaps a drop of oil might solve this problem?

[Ralph --- SSBN 598]

I am not sure oil is the answer.
Oil will try to repel the air as it gets close to the opening making it hard for the air to get past it.
Just as wax on a car causes water to bead up, the oil will do the same.

I am wondering what the clearance of the valve seal to the tank distance is when fully open
If the distance is small the air will trap itself between the two surfaces.
More distance will help.
If the vent hole is less than 1/8" the air bubbles will cross the gap and become a plug of sorts.
At first the pressure would force the air through the hole but as the air supply becomes less there is less pressure to get through the hole trapping the last little bit.
=============
Many years ago while racing sailboats, the idea was to get the water as close to the hull as possible.
Wax was used only on surfaces used to walk on by the crew.
All under water surfaces where cleaned using liquid cleaner to remove all oil and scum from the water.
Once a month, the bottom of the boat was wet sanded with 600 or 800 wet and dry sand paper or if it was a 3 day race weekend, the bottom was cleaned every morning and a quick wipe with the 800 sand paper.

You can test this your self.
Take a small toy boat and clean and sand the bottom.
Put in tank of water and use a electric fan to blow the boat across the tank.
Measure the time.
Then wax the bottom of the boat and repeat the test.
The wax boat will be noticeably slower even in a short distance.

[scott t]

How about experiment with liquid dish soap (since you are thinking of using oil) to ease that gas bubble problem.

Scott T

[corsaire]

I can only try and experiment indeed. In the pic, you'll see the old version of the vent orifice, Dave became aware of this deficiency a long while ago and presently he's supplying the orifice with a piece of brass tubing through it cut to size (width of orifice) which is what I have now. It's working much better than the cast orifice only version. I'd say the orifice (tubing in it) is about 1/8" OD. I'll try to re-adjust lowered the screw under the rubber piece to allow for a bit more opening when venting. I can surely try a dish wash drop and see what happens. Are you suggesting to use wax in the orifice?

[Ralph --- SSBN 598]

A thought.
Was the cast piece cleaned (washed) to remove all the mold parting wax or spray from the part?

No wax or oil.

[corsaire]

I never did, never thought of that, hmmm I'll wash it next time.

[JWLaRue]

I've found that it's less the seasonal temperature when running a boat and more the variation in specific gravity depending on the quality of the water where the boat is run. Of course, YMMV.

-Jeff