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water ballast tank

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Navy1
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USA
126 Posts

Initially Posted - 07/17/2009 : 19:37:09

Several days ago when I took "Navy Too" out of the water to take her to a new watering hole, I used a rather heavy duty 12V air compressor to try and force the water out of the ballast tank before pulling her from the water onto the trailer. I had read in the forum that it didn't take a high output compressor to do the job. I was dissapointed to find out after I got her onto the trailer, out of the water, that I hadn't emptied any water out of the ballast tank. What did I do wrong? Roger

Roger Richter

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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 07/17/2009 : 19:44:22

This may be a stupid question, but was the valve open?

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Navy1
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126 Posts

Response Posted - 07/18/2009 : 09:51:18

David, Yes, I made sure the valve was open, and before I shut off the air compressor I closed the valve. The air compressor I used has a 130 pse rating on it. Roger

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bear
Admiral

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USA
909 Posts

Response Posted - 07/18/2009 : 12:04:51

You should never close the drain valve with air still being forced in. 130 P.S.I. sounds way way too much pressure to me. I use the output hose from a hand held vacuum cleaner. Did the water come out as you left the water, also I presume you used the vent line in the front of the anchor locker. Your lucky you didn't blow a hole in the ballast tank. When you figure all this out, the boat should be rising in the water as the water comes out and after about 10 minutes or so bubbles should be seen bubbling up from the side of the boat. I would presume if the drain valve worked properly to let the water in it should have popped down to let it out. Some times you have to push down on the threaded rod to unseat it.

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Navy1
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USA
126 Posts

Response Posted - 07/19/2009 : 08:59:24

Bear, Yes, you made good point, but I had covered all of them, so I am still confused why the water wasn't forced out. Yes, I always have to push down on the lever under the step to unseat the valve. And I watched both sides and the back of the boat for the telltale bubles, but they never came. And after I pulled the boat out of the water and opened the drain again, water poured out for about as long as it ever did. It just doesn't make sense to me why the water wasn't forced out. There is something screwey there I don't know about. Roger

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John Russell
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3444 Posts

Response Posted - 07/19/2009 : 09:12:34

Only one of two things could have happened. You either were losing pressure somewhere or pressure was building but the valve was stuck closed. Did you notice an increase in pressure at the pump end? The only logical explanation to the pressure loss is a hole in the system somewhere. (How's that for overstating the obvious

) Ever have any unexplained water in the bilge after filling the tank?

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britinusa
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5404 Posts

Response Posted - 07/19/2009 : 09:18:42

I'm not an engineer!

Our 12v air pump is used to inflate air beds and water ballast tanks... very low pressure, a lot of volume.

Isn't an air compressor the other way around? ie. high pressure, low volume!

But of course there may be a really simple reason.. .could the hose from the anchor locker to the water ballast tank be disconnected (blown apart)?

Paul

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Navy1
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126 Posts

Response Posted - 07/19/2009 : 09:22:46

John, No extra water in bilge after filling tank. I agree, I must be losing pressure somewhere, that has to be the answer. I'll also have to check if I had an increase in pressure at the pump, because I have to admit I didn't watch the pressure gauge. Thanks for the feedback guys! Roger

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Navy1
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126 Posts

Response Posted - 07/19/2009 : 19:03:35

Problem solved guys. Due to recommendation I check compressor psi I went out and pluged it into my car and got only a measly 10psi. Then I disconnected the long coiled air hose and got so much pressure I couldn't stop it with my finger. Must be some kind of obstruction in the coiled airline hose or connector. Thanks again for the ideas. p.s. The compressor was brand new and not a cheapo, so it should have been o.k., but wasn't. Roger

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delliottg
Former Mainsheet C250 Tech Editor

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4479 Posts

Response Posted - 07/19/2009 : 19:40:20

Roger,
If you're using an air compressor, you're probably headed for disaster now that you've figured out what was wrong. Whatever the blockage was may well have saved your ballast tank. As Paul said, you want high volumes of air at low pressure. Think of your ballast tank as a very large balloon. Just a few PSI of pressure could burst your balloon due to the enormous increase in pressure over the surface area of your tank.

I did some rough calculations knowing that the tank holds about 120 gallons. I calculated a sphere that holds 120 gallons has a surface area of roughly 20k square inches. I know that the ballast tank isn't a sphere, so my calculations aren't completely applicable, but you can see where with such a large surface area, even a small increase in pressure could have catastrophic effects.

My recommendation would be to use a low pressure high volume air pump like for an air bed instead of a compressor.

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zeil
Master Marine Consultant

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Canada
1314 Posts

Response Posted - 07/19/2009 : 22:09:14

Does anyone know if the ballast tank has internal partitions or is divided in interconnected compartments

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delliottg
Former Mainsheet C250 Tech Editor

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4479 Posts

Response Posted - 07/20/2009 : 09:48:28

I was thinking about how to best illustrate how this could damage your boat and I remembered the Mythbuster's episode on explosive decompression. Here's a link to [url="http://www.youtube.com/watch?v=V9sYXqO5L0k"]part of the episode[/url] on Youtube. The discussion about pressure happens at about 3:30, and the demonstration takes place about 7:30 in the video. It's worth watching the whole thing simply because it's the Mythbusters, but at 6 PSI, they blew out their jury rigged plywood panels simply due to the large surface area of each window. They go in and reinforce them, and blow them out again.

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essen48183
Navigator

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USA
143 Posts

Response Posted - 07/20/2009 : 10:31:54

If you just blew into the vent hose with your mouth, the pressure inside the tank would be greater than the atmospheric pressure pushing down on the surface of the water all around you. Water would leave the tank to equalize the pressure, the boat rises, and you could blow another preath in and the same would happen over and over again until the tank was empty... it would take a very long time and might not be possible because of the pauses to take a breath in... anyway.... (this is the reverse of drafting water by creating a vacuum... when you suck a straw... you suck enough air pressure out of the straw and the atmospheric pressure pushes down on the top of the drink and pushes it up the straw.)

It is not a round/cylindrical high pressure tank it is a low pressure with flat sides tank... not appropriate to add more than a tiny bit extra pressure. The mattress pump is only able to push a low pressure... like your lungs... but it does it without stopping and prevents you from seeing stars by the time the mattress is full (ballast tank is empty)... but does not put enough pressure to rupture/deform/stress your hull.

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britinusa
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5404 Posts

Response Posted - 07/20/2009 : 16:42:05

David Great Post (Mythbusters)!

6psi and those windows blew out! Wow!

I wonder what the pressure is on our Coleman battery powered air pump is?

Did a bit of research on google but didn't find any definitive yet.

Great topic.

Paul

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delliottg
Former Mainsheet C250 Tech Editor

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4479 Posts

Response Posted - 07/20/2009 : 17:21:57

Looks like it might be as low as 1 PSI judging from this [url="http://www.freepatentsonline.com/4644597.html"]patent listing[/url] I found. I know our Coleman mattress has a relief valve for exactly this purpose.

That wouldn't surprise me, I think my Avon is only inflated to 3 PSI for the tubes, and about 10 PSI on the inflatable keel. Our pump is two stage and you have to switch between the LP & HP valves to inflate the tubes and keel respectively.

Edited by - delliottg on 07/20/2009 17:23:37

bill bosworth
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USA
172 Posts

Response Posted - 07/24/2009 : 07:00:56

Roger;
I always use a large bellows foot pump, designed to blow up an inflatable raft, It takes about 7 minutes to blow he ballast. Even when only using this foot pump, I'm careful to make sure the bottom valve is open, I don't want to open any ballast tank seams. I have had to repair one leaking ballast tank seam, don't want to do any more.

Bill, c250wb #134 Serendipity
on beautiful Kerr Lake, NC

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Navy1
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USA
126 Posts

Response Posted - 08/07/2009 : 09:38:25

Hi Guys, Sorry I haven't been around to reply to your many good answers. I have been away sailing in the Apostlde Islands. Great time on the water! Anyhow, I will be sure and use low pressure air pumps next year when I put "Navy Too" back in the water. She is stored early because 5 of us are chartering a 43' Beneteau in the Apostles this month and then my wife and I are going to the Northwest in our travel trailer for a month. Cheers

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britinusa
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5404 Posts

Response Posted - 08/07/2009 : 16:43:09

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Navy1
 Hi Guys, Sorry I haven't been around to reply to your many good answers. I have been away sailing in the Apostlde Islands. Great time on the water! Anyhow, I will be sure and use low pressure air pumps next year when I put "Navy Too" back in the water. She is stored early because 5 of us are chartering a 43' Beneteau in the Apostles this month and then my wife and I are going to the Northwest in our travel trailer for a month. Cheers
<hr height="1" noshade id="quote"></blockquote id="quote">

Must be tough! but I guess someone has to do it!

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Navy1
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USA
126 Posts

Response Posted - 08/09/2009 : 10:29:52

Paul. I see you have crossed quills on your posts. Were you a Yeoman in the Navy? I was. Roger

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Cavitating
1st Mate

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USA
47 Posts

Response Posted - 08/25/2009 : 16:12:10

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by delliottg
 I was thinking about how to best illustrate how this could damage your boat and I remembered the Mythbuster's episode on explosive decompression. Here's a link to [url="http://www.youtube.com/watch?v=V9sYXqO5L0k"]part of the episode[/url] on Youtube. The discussion about pressure happens at about 3:30, and the demonstration takes place about 7:30 in the video. It's worth watching the whole thing simply because it's the Mythbusters, but at 6 PSI, they blew out their jury rigged plywood panels simply due to the large surface area of each window. They go in and reinforce them, and blow them out again.
<hr height="1" noshade id="quote"></blockquote id="quote">

Very nice Myth Buster's experiment but they failed to take in consideration the suction powe of an airplane flying at several hundred miles per hour.

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britinusa
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5404 Posts

Response Posted - 08/25/2009 : 16:18:33

Roger,
No, the quills are the assoc secretary burgee.

My other burgee is a spider on a web. I don't use it much as it has a bug in it

Paul

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