Overfilling the ballast tank?

[Neo]

Hi Gents,

The 26M manual says we should fill the ballast tank to 1" below the vent hole ... But ignoring the potential for water splashing out of the hole, wouldn't it be better to fill that tank up as much as possible? ... Because when heeling any air in the tank would only move to the high side and thus reduce the counter balancing weight/effect of the ballast?

What's your thoughts on this?


All the best.
Neo

[Jimmyt]

Hi Gents,

The 26M manual says we should fill the ballast tank to 1" below the vent hole ... But ignoring the potential for water splashing out of the hole, wouldn't it be better to fill that tank up as much as possible? ... Because when heeling any air in the tank would only move to the high side and thus reduce the counter balancing weight/effect of the ballast?

What's your thoughts on this?


All the best.
Neo

Shame on me for assuming filling the ballast tank meant actually filling the ballast tank. i have always filled it until it's splashing in the funnel. So far, no worm holes have transported me to a parallel universe, (as far as I know).

I guess it's possible that thermal expansion could crack the tank if you don't leave a little room for growth. Never really thought about it until you posted this.

[Neo]

Nice one Jimmy.

My vent hole is actually piped (via a high loop) out to my anchor locker so thermal expansion would not be an issue for me....and I guess the brass/rubber plug would also blow out well before the tank was under too much stress.

Having said all this I'm now wondering... is the Vent Hole's higher than the cabin floor (roof of the tank) or at the same level?... is it on a raised section of the tank?

All the best.
Neo

[Jimmyt]

...and I guess the brass/rubber plug would also blow out well before the tank was under too much stress.

Having said all this I'm now wondering... is the Vent Hole's higher than the cabin floor (roof of the tank) or at the same level?... is it on a raised section of the tank?

The tank is tested at the factory to somewhere around 2-3 psi as I recall. I think the expanding rubber plug I use would go beyond that easily. Thermal expansion is no joke. I can't imagine another reason for leaving air in the ballast tank.

The vent hole is on a raised portion.

[Neo]

Hmmm ... the port does look raised in that diagram.

[Cougar]

Water expansion is only 0.2 percent at 10 degrees rise of temperature. I don't think it'll damage the ballast tank. IMO what they meant in the manual is to fill the ballast tank to at least 1" below vent level.

[Jimmyt]

Water expansion is only 0.2 percent at 10 degrees rise of temperature. I don't think it'll damage the ballast tank. IMO what they meant in the manual is to fill the ballast tank to at least 1" below vent level.

While I can envision several scenarios where the water would be subjected to much larger temperature swings, you are absolutely correct regarding the intent of the instructions.

The manual says NO MORE THAN 1 inch below the hole. The language above that clearly says to fill it completely. So, Roger wasn't worried about thermal expansion.

[Tomfoolery]

I fill mine until it won't fill any more, then I close the valve and put in the plug.

If I forget to move some boat junk from the vee berth after trailering to a destination, it will fill the cup, but that's on me because it's way bow-heavy. Loaded normally, the water is high enough to feel with a finger, but not high enough to splash out (usually).

And FWIW, I don't think the expanding rubber vent plug or the stern valve are water/air tight enough to allow it to build pressure if it wanted to. They're kind of old, but still functional enough. Like the skipper.

[Jimmyt]

I fill mine until it won't fill any more, then I close the valve and put in the plug.

If I forget to move some boat junk from the vee berth after trailering to a destination, it will fill the cup, but that's on me because it's way bow-heavy. Loaded normally, the water is high enough to feel with a finger, but not high enough to splash out (usually).

And FWIW, I don't think the expanding rubber vent plug or the stern valve are water/air tight enough to allow it to build pressure if it wanted to. They're kind of old, but still functional enough. Like the skipper.

Maybe so. Mine are 2013 vintage and still seal pretty well - unlike the skipper (in my case).

Even a small volume variation in a confined incompressible fluid can cause a pretty hairy pressure increase (which you know better than me, Tom - just running through a thought process). See illustration.



Fortunately, the ballast tank will flex more than a rigid container, somewhat canceling out the effect for the first few degrees... And if the valve and plug won't hold over 2-3 psi, you've got pressure relief; so no worries.

Some people work crossword puzzles. Other people evaluate doomsday scenarios. Different strokes...

[Tomfoolery]

That curve assumes an infinitely rigid container. Water is compressible, like (presumably) all liquids, and increasing the temperature within a fixed volume definitely will increase the pressure (same as a lump of steel not allowed to change size/shape). It's actually something that has to be taken into account in a lot of hydraulic system designs.

But a ballast tank is a big, mostly flat container and as such has very little structural rigidity against pressure changes. A very small amount of flex will make a relatively large increase in volume. Since the ballast water comes from the stuff the boat is floating in, and the stuff the boat is floating in doesn't change temperature much, neither will the ballast water. But if it did, I have little doubt that anything bad would happen, between the flexy surfaces and the less-than-perfect seals.

If my math is correct, a 10°F increase in ballast water temperature will increase the volume of 1400 lb (168 gallons) of ballast by about 3 cups. FRP also expands with temperature increases, but I'm not even going to attempt to calculate the volume increase, in large part due to the weird shape of the tank which would have to be accurately defined. Yikes!

In other words, I'm not worried about it.

[Democritus]

Water is compressible, like (presumably) all liquids, and increasing the temperature within a fixed volume definitely will increase the pressure (same as a lump of steel not allowed to change size/shape).

Typically liquids are not compressible, except under extreme conditions.

[Tomfoolery]

Typically liquids are not compressible, except under extreme conditions.

Water is not compressible in any relevant way in terms of the ballast tank, but the chart posted above assumes constant volume. So the pressure shoots up as the temperature goes up, since the water has nowhere to go, and the top of that curve is at 8700 psi.

Fluid bulk modulus is something I have had to deal with on fast response (like 400 Hz) hydraulic systems on test machinery, where the forces the hydraulics are imposing are very tightly controlled. Read all about it here. But most common (read: slow response) hydraulics, especially manually controlled, don't matter as much. Think excavators and crane hoist motor systems, though some of them I've designed go to 6000 psi.

https://www.hydraulicspneumatics.com/te ... -important

[Jimmyt]

Water is not compressible in any relevant way in terms of the ballast tank, but the chart posted above assumes constant volume. So the pressure shoots up as the temperature goes up, since the water has nowhere to go, and the top of that curve is at 8700 psi.

Fluid bulk modulus is something I have had to deal with on fast response (like 400 Hz) hydraulic systems on test machinery, where the forces the hydraulics are imposing are very tightly controlled. Read all about it here. But most common (read: slow response) hydraulics, especially manually controlled, don't matter as much. Think excavators and crane hoist motor systems, though some of them I've designed go to 6000 psi.

https://www.hydraulicspneumatics.com/te ... -important

Agree that the chart is for rigid vessels and piping. Just included for those who don't think water volume thermal expansion exists and has consequences in certain cases.

I did include a bit of a hint at your big flat flexible fiberglass vessel comment:

Fortunately, the ballast tank will flex more than a rigid container, somewhat canceling out the effect for the first few degrees... And if the valve and plug won't hold over 2-3 psi, you've got pressure relief; so no worries.

Now, if you filled it in a 60 degree lake and beached it on a scorching hot day...

Where I sail, everything is hot, water, air, and crew...

[Tomfoolery]

I did include a bit of a hint at your big flat flexible fiberglass vessel comment:

Fortunately, the ballast tank will flex more than a rigid container, somewhat canceling out the effect for the first few degrees... And if the valve and plug won't hold over 2-3 psi, you've got pressure relief; so no worries.

So you did. I should spend more time reading and less time thinking about how I'm going to respond to a post before I've actually read the whole thing.

[Jimmyt]

Nothing wrong with a bit of spirited discussion. Besides, I agree with you.

I'm used to a lot more name calling, and being told I'm too stupid to even understand the explanation... lots of years designing and troubleshooting construction for the military and federal government projects.

So, next time could you lead off with some disparaging remarks about my ancestry? It would remind me of days gone by.