Using surface air to blow the ballast tank

[cstranc]

On one of Mylo's posts he mentioned blowing the ballast tank by coming close to the surface and then pulling surface air to flush the ballast tank.



You could bring the surface air in from a vent in the sail, or from an extended snorkel.

I was planning on using compressed air (120psi) to blow my ballast tank. I think I will get about 4 complete discharge & fill cycles before I am out of pressure.

But if I used the compressed air to get the sail showing, then took in surface air I could be out there as long as I had batteries...

Does anyone have experience with this?

[raalst]

No experience...

But think it is/was

No experience...

But think it is/was used in the UK in the old days.

I'm also planning on using something like it,
since the planned on-board video forces me to have the
periscope above water anyway.

combined with a small bit of reserve bouyancy it
might be a safe way to go. as always, a few drops of
extra bilge water might sink you...

[raalst]

No experience...

But think it is/was

No experience...

But think it is/was used in the UK in the old days.

I'm also planning on using something like it,
since the planned on-board video forces me to have the
periscope above water anyway.

combined with a small bit of reserve bouyancy it
might be a safe way to go.

As always, a few drops of extra bilge
water might sink you...

[Guest]

Seen this system used in

Seen this system used in a couple of boats, although most people pump water into a sealed tank instead of a vented one.

On that subject it's worth noting that most folk incorporating a vented tank system tend to use a water pump rather than an air pump.

Typically a gear based pump is used, as they can operate in either direction. However another system I have observed used two centrifugal pumps, switched independently for pumping in and out.

The pumps used are the cheap centrifugal faucet type pumps which you can pick up for a few pounds on ebay. Reich make high quality versions, but there are many (cheaper) alternatives which work well. The advantage of these pumps, is that they are high volume low pressure types. Geared pumps can pump higher pressure, but generally pump low volume.

As Ronald has already highlighted, a disadvantage with this system is the low reserve buoyancy.

This is why many model submariners in the UK have switched to using a sealed tank- air is compressed inside the ballast tank as the water is pumped in, and this acts as a built in failsafe.
The disadvantage with this method is you require a larger tank, as the pumps can typically only fill the tank to about 70% capacity. You also have an exposed water surface, therefore you must baffle the tank to prevent sloshing reducing the metacentric height, which in most boats will be small already..

Andy

[mylo]

Chris,

Yep, that's the idea,

Chris,

Yep, that's the idea, using your limited on board compressed air reserve to get the sail to break surface only (or just 'drive' near the surface using dive planes), then fire up the pump to purge the ballast tank, getting the sub to surface trim. As well, your compressed air is for emergency blow too.....I guess that's the failsafe. The concept though is to have the ability to purge the ballast tanks pretty much an unlimited amount of times in any given sailing, reduce the onboard air reserve tank size ...... or, I suppose, get more mileage out of a bigger tank.

Caswell Inc. recently introduced some sort of micro air pump for this very application. Might want to check that out, the space savings is considerable, the thing is not big, making this system quite feasible in smaller subs. (there, I said the 's' word).

Mylo

[anonymous]

Hi Chris,

Just for completeness, there

Hi Chris,

Just for completeness, there is another vented pump tank system which can even bring the sub up from deep. (Without air compressors etc.)

It uses the difference in density/viscosity between air and water.

The vent tube is a capillary (a syringe needle , say).

You can pump water out of the tank more quickly than water will come in through the capillary vent tube. So you pull a vacuum in the tank, any air expands (It must require some air in the tank?) and your sub surfaces.

In air the capillary vent tube works just like a normal vent tube.

Now, I have never tried it myself but I have seen it working on several subs. It sounds a little fiddly to me.

(I use the sealed pressure tank approach with a gear pump to pump in and pump out. (With baffled tanks as Andy says.))

Best of luck with your experimentrs and I look forward to another excellent write up in the magazine in due course?!

Davy

[cstranc]

Ohh no. Too much excellent

Ohh no. Too much excellent information.

Ronald,
I'm building a Russian sub so there is lot's of reserve buoyancy. Some times I think that half the sub has to get above water level... That is one of the reasons for the large ballast tank, and corresponding air reservoirs.

Or were you warning me that a few drops of water in an air pump could lead to a swimming after your sub event.

Or am I not following the reserve buoyancy point at all?

Andy,
I just happen to have a few gear pumps and I have been seeing what they can do. I liked gear pumps because they will handle water. In fact I do not like the idea that a little water getting to the pump will cause it to stop working. So I was actually planning on pumping a mix of water and air through the pump into the ballast tank.



The gear pump (small fuel pump from local hobby store) does not push the volume I am looking for. You are right a cetrifical pump would have better characteristics.

Not sure I like that sealed tank approach. I tried one of those. Probably standard construction: ABS tube + Plexiglas ends, baffles inside. I glued and screwed the end plates on. When it was under pressure I got the 70% fill, and I would stare at the bend in the end caps waiting for them to give way... Maybe I'm a nervous sailor, but I never tried to use it over deep water...

Also you need a good check valve for that setup and I could not get a small valve that would allow a good flow rate.

I believe in free flooding ballast tanks now (but I am pretty in-experienced).

Mylo,
Thanks for the pointer. It's these right?

http://www.caswellplating.com/models/pumps.html

Ok, I'm dying to ask this question. So my model is getting more and more of these motors in it. The drive motor and side thrusters will all have caps on them to reduce electrical noise. If you were putting a little pump like this in would you put the caps on it too?

Davy,
This is too cool to be ignored. So if you change the tank so that it is not quite so free flooding then as you pump out water it will pull in the air. You could even take the airline and route it below the bottom of the tank to create a "reverse siphon" that way you would not need a valve to prevent air leaking out...

Just need to see how you can restrict the water from entering when you want to blow the tank, but allow it to enter when you want to dive...

Thanks everyone. This is why I love this hobby so much. So many things to think about. And so many really great people.

[mylo]

Chris,

Yes, those are the

Chris,

Yes, those are the pumps.

Mylo

[JWLaRue]

The system that you are

The system that you are describing has been used quite successfully for a number of years...particularly in the SubTech Albacore and Marlin kits. Skip Asay designed the ballast parts along the same lines you have been describing. The parts need to build that ballast system can be purchased individually either through SubTech (i.e. Pandan Models) in the U.K. or for those in North America through Mike's SubWorks.

-Jeff

[kazzer]

SNORT

Yes, great idea.

Merriman and I have developed this SNORT system.

Use this pump

as it will suck water and air.

Use the switch SUBES1

to operate the pump from #5 receiver position (or whatever you have spare).

If you coat the pump motor with

http://www.caswellplating.com/aids/plastidip.html

you can place the pump in the WET part of the hull. Simply run the intake pipe into the SNORT (Schnorkel) and run the output pipe directly into the ballast tank. Of course, if you have all that bladder crud in your boat, it won't work!

All you have to be careful about is the power wire from the WTC to the external pump is well sealed when it exits your cylinder.

If this pump sucks in water, it will just pump it slowly into the ballast tank, and so won't alter the displacement any.

See also http://www.moonrakers.com/submarines/snort.pdf

Trials so far show this system to be extremely effective. If you run at periscope depth all day, you'll hardly use any gas. I think you could even do a dynamic dive from periscope depth, allowing the boat to surface back to periscope depth using no gas.

Certainly blows RCABS out of the water!

Enjoy!

[fbradasc]

Hi All,
for my Trumpeter Kilo

Hi All,
for my Trumpeter Kilo conversion I plan to use a similar system, which uses a siphon instead a valve to avoid the air to escape from the ballast tank when the boat is surfaced:



The pressure in the ballast tank is always the same of the external pressure and can have any shape, not limited to the cylinder one.

[cstranc]

I am begining to wish

I am begining to wish I had asked this question about 2 weeks ago. Time to undo some of the changes to my ballast tank.

That is the simplest & most elegant implementation I have seen yet.

The only thing it is missing is the "emergency blow" feature where you can use propell to blow the tank if you go too deep and lose signal.

Chris

[kazzer]

The only thing it is

The only thing it is missing is the "emergency blow" feature where you can use propel to blow the tank if you go too deep and lose signal.

Chris

No its not!

Your failsafe should already be rigged to blow the tank. If you rig your failsafe to operate the normal blow signal as well, you are all set to go.

It is a separate operation to this.

[raalst]

I fail to see what

I fail to see what the function/advantage of the syphon is,
can you please explain ?

[JWLaRue]

The only thing it is

The only thing it is missing is the "emergency blow" feature where you can use propell to blow the tank if you go too deep and lose signal.

Well, the system that Skip developed for the Albacore includes an emergency blow that uses a small Propel charge to empty the ballast tank......

-Jeff