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Catalina/Capri 25/250 Sailor's Forums

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Vent Plug

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SailAway
1st Mate

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USA
46 Posts

Initially Posted - 07/21/2014 : 13:30:34

I've only sailed my C250 WB twice now and I was just wondering what would happen if I forgot to install the little vent plug located in the anchor locker after the ballast tank was full. I keep the boat on a trailer and launch every time I use it. I'm sure I'll forget to put that thing in one of these times.

Greg

Edited by - on

britinusa
Web Editor

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USA
5404 Posts

Response Posted - 07/21/2014 : 19:38:10

Not a lot.
We have found it unplugged after a 3 day trip out. Sometimes it happens.

Paul

Edited by - on

zebra50
Captain

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USA
408 Posts

Response Posted - 08/30/2014 : 16:34:52

Does any one know where to get a replacement plug. I don't remember what I did with mine.

Edited by - on

bear
Admiral

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USA
909 Posts

Response Posted - 08/30/2014 : 18:48:41

I haven't used one in 2-3 years, not even sure where it is.

Edited by - on

zeil
Master Marine Consultant

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Canada
1297 Posts

Response Posted - 08/30/2014 : 19:41:09

Never used one...

Edited by - on

Jack Schafer
Navigator

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USA
112 Posts

Response Posted - 08/30/2014 : 20:48:21

Just suppose,,, You forget to close the intake valve and not have the stopper plugged in, is it possible that the water would syphon out of the ballast tank while underway? Just a thought.

Edited by - on

TakeFive
Master Marine Consultant

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2269 Posts

Response Posted - 08/31/2014 : 06:36:49

quote:
Originally posted by Jack Schafer

Just suppose,,, You forget to close the intake valve and not have the stopper plugged in, is it possible that the water would syphon out of the ballast tank while underway? Just a thought.

IMO it would be more likely to leak in and sink the boat. No different from drilling a hole in the bottom of the boat.

The self bailing attachments on Sunfishes and similar boats always leak in and need to be plugged. The Bernoulli effect just isn't strong enough at sailing speeds.

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/31/2014 : 08:02:55

If I get this picture--vent tube from ballast tank to anchor locker well above the waterline, with a drain in the locker to shed rainwater and occasional green water over the bow... Even if there were enough pressure from the intake, caused let's say by the speed of the boat through the water (??), to force water up the tub, what do you assume happens next? I'd say it drains out like other water does.

So then why would there be a plug? I'm guessing it's to prevent rainwater or melting snow from draining into an empty ballast tank where it could freeze, or "stuff" in the locker from clogging the vent. But I never had a WB.

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/31/2014 : 08:20:00

...although here's another theory... With water ballast, it's critical that the tank be completely full if it has any water in it. A partially-full tank will destabilize rather than stabilize the boat, as the ballast moves to the low (wrong) side. If both the intake and the vent are open, when the boat heels enough to raise part of the tank above the level of the water on the hull, ballast water could drain from the tank, thereby causing the remaining ballast to actually increase the heeling. The intake valve is the first defense... Perhaps the vent plug is the secondary one.

Funny story: Back when the C-250 was a new model, I stepped aboard one at an in-water show. I was startled by its instability to my just moving around, and asked the rep about it. He explained that it was a WB model and he had only partially filled the tank because he wasn't going to be sailing it. I tried to explain the issue to him, but unsuccessfully. That boat probably rocked and swayed at the dock for the entire four day show.

Edited by - Stinkpotter on 08/31/2014 08:28:39

TakeFive
Master Marine Consultant

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2269 Posts

Response Posted - 08/31/2014 : 17:41:22

quote:
Originally posted by Stinkpotter

If I get this picture--vent tube from ballast tank to anchor locker well above the waterline, with a drain in the locker to shed rainwater and occasional green water over the bow... Even if there were enough pressure from the intake, caused let's say by the speed of the boat through the water (??), to force water up the tub, what do you assume happens next? I'd say it drains out like other water does.

Oops, I didn't realize that the vent would be above the waterline. That makes sense.

I still do not think that any water would suck out of the tank. If inlet and vent valves were left open, the level of water in the tube should be the same as the waterline. It is a requirement of the laws of physics (as stated by Archimedes) that the waterline will always be higher than the ballast tank (assuming the water in the tank is the same salinity as the water that the boat is floating in). There's a reason why they call a boat's weight "displacement." The boat's weight is boat+water ballast, so when it floats it will displace an amount of water equal to boat+water ballast. Therefore, the ballast tank will always be below the waterline, and the vent tube will contain water above the top of the ballast tank.

I don't know much about the water ballast system, so take my comments with a grain of salt (or salt water if you prefer). ;-)

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/31/2014 : 20:56:01

The principle here is that if the ballast tank is filled and sealed, it's contribution to the boat's center of mass (CM) will shift to the high side as the boat heels. Meanwhile, the boat's center of buoyancy (CB) will shift toward the low side as the hull pushes down to leeward, producing the "righting moment" from the ballast. A ballasted keel does the same as it is levered up toward the windward side, moving the CM to windward.

A partially filled tank will allow the center of mass to move to leeward, which is precisely what you and the designer don't want. The waterline is not a factor--it's totally a CM vs CB issue. For example, if you put a large log, which itself is buoyant, into the hull below the waterline on one side of the centerline, the boat will tip that way, right? Right. Water is more dense than that log. So water ballast affects the stability even more, without any regard to the outside waterline.

Sorry if I'm preaching to the choir, but I've had this discussion enough times to realize the physics is not intuitive--even including what turned into a nasty argument with a physics professor who claimed the waterline was the determining factor for the righting moment from water ballast. (Sorry, Prof, but........

)

Edited by - Stinkpotter on 09/01/2014 09:54:01

GaryB
Master Marine Consultant

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USA
4275 Posts

Response Posted - 09/01/2014 : 01:00:37

quote:
Originally posted by Stinkpotter

The principle here is that if the ballast tank is filled and sealed, it's contribution to the boat's center of mass (CM) will shift to the high side as the boat heels. Meanwhile, the boat's center of buoyancy (CB) will shift toward the low side as the hull pushes down to leeward, producing the "righting moment" from the ballast. A ballasted keel does the same as it is levered up toward the windward side, moving the CM to windward.

A partially tank will allow the center of mass to move to leeward, which is precisely what you and the designer don't want. The waterline is not a factor--it's totally a CM vs CB issue. For example, if you put a large log, which itself is buoyant, into the hull below the waterline on one side of the center line, the boat will tip that way, right? Right. Water is more dense than that log. So water ballast affects the stability even more, without any regard to the outside waterline.

Sorry if I'm preaching to the choir, but I've had this discussion enough times to realize the physics is not intuitive--even including what turned into a nasty argument with a physics professor who claimed the waterline was the determining factor for the righting moment from water ballast. (Sorry, Prof, but........ )

I actually think I understand what you are saying and it makes sense.

Saying the same thing in a different way:

If the tank (being below the center of roll axis) is partially full the water just sloshes to the low side and therefore the weight of the water adds mass to leeward just like adding rail meat to the leeward rail.

If the tank is full the water cannot slosh (it's trapped in the tank) and as the hull rolls to leeward (the water being trapped below the center of the roll axis) is swung out to windward and therefore adds to the righting moment just like rail meat on the windward rail.

All sailboats basically operate like a metronome. A metronome has a weight (ballast tank) suspended below the shaft (roll axis) with the lighter weight arm (mast) above the shaft.

The farther the weight on the metronome is moved below the shaft the more stabilizing effect it has when the metronome arm is moved away from vertical. The higher up you move the weight the less stabilizing effect the weight has and above a certain point it becomes de-stabilizing.

Applied to a water ballasted sailboat, if the ballast tank is not full and the water is allowed to slosh, it has the same effect as raising the weight on a metronome. The weight of the water remains at or near the center of the vertical plane as the boat heels and therefore has little to no righting moment making the boat less stable (shorter moment arm).

If the tank is empty it becomes de-stabilizing since the weight above the roll axis of the boat is near or above the weight below the roll axis (weight on the metronome all the way up near the shaft).

If the ballast tank is full it has the same effect as the weight on the metronome being lowered. Since the water is trapped in the full tank it cannot slosh and is forced to move farther away from the center of the vertical plane (longer moment arm). This causes a stabilizing effect as it swings farther out.

Probably didn't say all of this correctly but it's clear in my head. Probably should have stopped just before getting into the metronome discussion!

Edited by - GaryB on 09/01/2014 02:49:19

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 09/01/2014 : 10:07:17

quote:
Originally posted by TakeFive

...I still do not think that any water would suck out of the tank. If inlet and vent valves were left open, the level of water in the tube should be the same as the waterline. It is a requirement of the laws of physics (as stated by Archimedes) that the waterline will always be higher than the ballast tank (assuming the water in the tank is the same salinity as the water that the boat is floating in). There's a reason why they call a boat's weight "displacement." The boat's weight is boat+water ballast, so when it floats it will displace an amount of water equal to boat+water ballast. Therefore, the ballast tank will always be below the waterline, and the vent tube will contain water above the top of the ballast tank.

I agree with your conclusion, when the boat is sitting upright. But I'm suspicious the ballast tank could be partially above the waterline on the windward side when the boat is heeling. It's at that point that, if the intake and vent are left open, some water could be replaced by air and the ballast would effectively shift to leeward. I wouldn't suggest trying it...

Edited by - on

TakeFive
Master Marine Consultant

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2269 Posts

Response Posted - 09/01/2014 : 10:14:55

quote:
Originally posted by Stinkpotter
I agree with your conclusion, when the boat is sitting upright. But I'm suspicious the ballast tank could be partially above the waterline on the windward side when the boat is heeling. It's at that point that, if the intake and vent are left open, some water could be replaced by air and the ballast would effectively shift to leeward. I wouldn't suggest trying it...

Yes you are right. I had neglected the effect of heeling. That would cause some air to get into the ballast tank and would be problematic.

Time to get back to single handing my C250WK, which I'm doing right now.

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 09/01/2014 : 10:42:25

Ooooo... Texting while driving?!?

Edited by - Stinkpotter on 09/01/2014 10:42:54

TakeFive
Master Marine Consultant

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2269 Posts

Response Posted - 09/01/2014 : 10:50:10

quote:
Originally posted by Stinkpotter

Ooooo... Texting while driving?!?

Otto is my pilot.

Edited by - on

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