Electric Vent?

[hoaglandr]

But an empty, vent-closed ballast tank offers no more flotation than an empty, vent-open ballast tank, the boat doesn't float higher in the water.

Agreed!

Replacing the air with water, below the waterline doesn't make the boat sit any lower in the water because the ballast is the same density as the surrounding water.

This is where we disagree. The thing is - air is less dense than water - and a tank full of air will ride higher than a tank full of water. Makes no difference if it's sealed or not. The reason that floaded Mac sits so low in the water is because all of the air was displaced by water.

There is a diffence between filling a below-the-waterline ballast tank, and filling an above-the-waterline-hull.

This may be an extreme example but the point is the same. The more air that is displaced by water the lower the boat will sit in the water. It makes no difference if the water is in the ballast tank or the bilge.

Russell

[Divecoz]

I believe any weight added to the boat changes how deep the boat rides.. Why wouldn't it? IN the ballast or above the ballast tank the hull disperses the weight......of the boat and its contents..

[Jim Bunnell]

Replacing the air with water, below the waterline doesn't make the boat sit any lower in the water because the ballast is the same density as the surrounding water. No positive nor negative bouyancy is achieved.

The water in the tank is the same density as the water outside, agreed - but when the tank is empty, sealed and forced below the waterline by the weight of the boat above the waterline, the air generates a lifting bouyancy. Try pushing a balloon underwater - you can feel the resistance and lifting force. Your water bottle has so little weight above the waterline that the change is probably negligible. Your body, on the other hand, is a significant load on the inflatable. Doesn't your mac float closer to the factory waterline with ballast in than out? Mine does. How else would reducing ballast lift a boat off a grounding?

[Divecoz]

Thanks Jim B . I agree . I watched my boot stripe while I pumped out water from my ballast.. It was my only means to see if the pump would raise......the boat off a sand bar..I wasn't on a sand bar that had been earlier I was tied to my brothers dock / boat lift / seawall ?

[K9Kampers]

This is where we disagree. The thing is - air is less dense than water - and a tank full of air will ride higher than a tank full of water. Makes no difference if it's sealed or not.

Agreed, and the same tank half filled with water will not float any lower in the water than the point where the inner and outer waterlines are equal, ie; the half (water) filled tank won't sink to 3/4.

The reason that floaded Mac sits so low in the water is because all of the air was displaced by water.

In the flooded boat, the air in the boat was displaced by water. The air in the cabin wasn't providing any floatation to the vessel, because it wasn't trapped / contained, it didn't have a positive force against the water. The boat sat lower in the water because the amount of water in it overcame the positive bouyancy potential of the positive flotation objects in (foam & trapped air) and of the boat (hull shape).

This may be an extreme example but the point is the same. The more air that is displaced by water the lower the boat will sit in the water. It makes no difference if the water is in the ballast tank or the bilge.
Russell

Displacing ambient air is not the same as overcoming positive bouancy.
A sealed ballast tank filled with air has the potential to provide positive bouyancy, but by itself is not making the boat ride higher in the water!

[K9Kampers]

How else would reducing ballast lift a boat off a grounding?

Because you are reducing the ballast, not because you are adding air. It's not semantics. There is a difference.

[hoaglandr]

Let me give this one more shot! I apologize if I seem argumentative but this discussion has caused me to think a little bit, which is always a good thing!

The waterline of the boat is determined by it's displacement. That is, the amount of water displaced is equal in weight to the weight of the boat. The heavier the boat the more water it displaces. When the weight of the boat equals the weight of the water it displaces, it floats - we have reached a state of equilibrium. A rock on the other hand, weighs more than the water it displaces. That's why it doesn't float.

When we add water ballast to a boat, that water is no longer being displaced. The boat has not changed weight, but to make up for the loss of displacement, the boat has to sink a little farther down in the water (displacing more water as it does so) to again reach that point of equilibrium.

I hope on these points we can all reach agreement.

Russell

[Divecoz]

I am not thinking the tank provides buoyancy ... ( I ) am referencing the hulls ability to displace a set amount of weight . Maybe that is where the confusion is coming from?
If I am understanding what you saying? You feel If I am standing in a row boat and I measure the free board and I now fill that boat with water.. calculated to be less in volume than what is available not to exceed the existing free board, that the boat will not sink deeper into the water? I dont agree..
Take that same row boat tie it to a sea wall or a pier and fill it with water what will happpen?

[K9Kampers]

f#$%@&##*g computers!!
I just got back to spend 30 minutes typing a response only to have this POS freeze up before sending!! AARRGGGHHHHH

I will rejoin this pleasant discussion after I return from tonite's engagement...

[Divecoz]

Hahahahahaha BTDT TOO hahahaha ..........
""after I return from tonite's engagement"" Me Too or tomorrow AFTER the honey do list is finished..

f#$%@&##*g computers!!
I just got back to spend 30 minutes typing a response only to have this POS freeze up before sending!! AARRGGGHHHHH

I will rejoin this pleasant discussion after I return from tonite's engagement...

[vizwhiz]

Wow this is fun!!

Okay, K9, no offense, but the ballast does make a difference, and this quote by Jim

The water in the tank is the same density as the water outside, agreed - but when the tank is empty, sealed and forced below the waterline by the weight of the boat above the waterline, the air generates a lifting bouyancy.

is a well-worded description of what is happening. Tank empty with no seal on vent is the same as with a seal, you are correct there. That is because they are rigid, and continue to displace the water with or without a cap.

The inflatable dinghy isn't a good example because it collapses and does not displace it's full potential volume (make the chambers out of solid hard plastic tubes like your water bottle, and then the dinghy is a good example) - the Mac does not collapse - so the first part of the explanation by hoaglandr is pretty good

The waterline of the boat is determined by it's displacement. That is, the amount of water displaced is equal in weight to the weight of the boat. The heavier the boat the more water it displaces. When the weight of the boat equals the weight of the water it displaces, it floats - we have reached a state of equilibrium. A rock on the other hand, weighs more than the water it displaces. That's why it doesn't float.

The part I disagree with on hoaglandr's comment is the effect of the ballast...it DOES increase the weight of the boat because it is inside the non-collapsing, rigid hull. Pretend the ballast tanks are sitting up on a set of saw-horses in the cabin instead of at the bottom - their vertical location in the rigid hull of the boat (whether above or below waterline) doesn't change the physics, only whether they are inside or outside the rigid hull - that water still presses down on the hull to hold it "up" against gravity. The water in the ballast tanks represents weight, whether the tanks are at the bottom of the boat, chest high, or laying up on the deck. That added weight results in the rigid hull of the boat having to displace more water to reach that equilibrium (as noted above), an amount equal to the amount of weight in the ballast tanks, and sinking into the water further does that. The point is that your boat walls are rigid, so the potential displacement doesn't change when you add water to the ballast tanks, the weight of the boat does.

That said, the effect of the weight in the ballast tanks IS the same when the boat is sitting straight as it is when the boat is heeling...the water in the ballast tanks weighs the same and acts on the boat the same, whether the ballast tanks are located below the waterline at the moment the boat is upright, or above the waterline when the boat heels. That's a different topic.

I think some of the confusion had to do with the pressure to push the water out of the valve - which is entirely different than the discussions above - and the difference that makes IS based on the difference between the water level in the ballast tanks and the water level outside...because they are connected through the open valve and they will try to equalize levels (which is really equalizing pressures)...until you put more pressure in the ballast tanks...then the pressures equalize again by water-plus-extra-surface-pressure = water-plus-atmospheric-pressure...may have to draw this one.

[K9Kampers]

Yes, a fun discussion indeed!

I don’t doubt any of our abilities to understand and express the concepts that allow our boats to float and how to change the situation when they are not floating right. Contrary to my earlier mention of semantics, it’s most likely that we are expressing the same thing in different ways. I am not discounting anyone’s personal observations of the issues, as they are as real as my own observations. Furthermore, I allow that my interpretation of said observations may be incorrect, and that one has to consider both sides of a discussion to reach the truth.

My initial stance on the ‘full’ ballast tank description is based on my observation of where the ballast tank in my boat is in relation to the water level outside the boat. Visual reference of how high the boat sits in the water relative to the sink drain thru-hull relative to the top of the ballast tank under the cabin floor. I reference the top of the ballast tank relative to the hull waterline as a means to help illustrate points that follow. My observation approximates the topmost level of the ballast tank to be at or slightly below the hull waterline, but not above it (on the ).

From that starting point, I visualize a ‘full’ ballast tank having no effect on causing the hull to sit lower in the water, as equilibrium is met, (contrary to opposing observations). Further, to attempt to support the opposing observation, I visualize the top of the tank needing to be higher than the hull waterline, to thus allow more ballast to make the boat ride lower in the water, when equilibrium is met. Right or wrong, that’s how I visualize it.

Looking back, my entry to this discussion was about the point of whether or not the (equal density) ballast makes the boat sit lower in the water. My position evolved to counter the statement made about an empty ballast tank offering a positive buoyancy effect on the boat.
I counter again…it is less water in the boat, NOT more air in the boat that makes it float higher (not sink). The air in the cabin & in the ballast tank, vent open or closed, is in a static state, not a positive flotation state. Just as filling the cabin with hundreds of air filled balloons would not allow the hull to sit higher in the water. For an empty ballast tank to provide a positive buoyancy effect, the water would need to be in the boat, above and around said tank.

Looking back at the original idea being explored…emptying the ballast tank to float the boat off a grounding. I submit that a boat with a ballast tank level with the hull waterline…no; a boat with a ballast tank level taller than the hull waterline…yes. Yet the former, not the latter, best describes the MacX. That said, I defer to anyone who has actually done it in practice.

Cheers,
Andrew

[K9Kampers]

I tried air and maybe it was just wrong pump etc. for the job. Then I tried a scuba tank and that didn't work too good either..Two people it would have worked a lot ........better, but I didn't have help. When I got off and got back to my brothers , he was the one who said: Your by yourself. Everything needs to work with one guy doing it.. Lets hook up the raw water pump to drain the ballast. My pump , pumps about 4 gallons a minute .W.M. PAR-Max 4gpm 12V Kit.
About 15 minutes will give me about 60 gallons or about 480 lbs. It lifts my boat several inches .. One man no muss no fuss except to set two anchors and then winch her to open water... AIR.. I think you need to set up for it before you need it.. Your going to need some way to hold the hose in the tank and it would be nice to have a valve you can turn off and keep the pressure where you have it , so to speak... that would give you a chance to run up and close the Inlet..J-IMHO

I have used a low-pressure inflator pump to pump out the ballast on Beija-Flor, my old - it works pretty well.

To my thinking, it would be impossible to evacuate the tank thru the gate valve to a safe empty point by air pressure. As soon as the water level was down to the height of the gate valve, the air would escape over the top of the water at the valve, or fart intermittently and to great aggravation. The remaining ballast water would be left at an unsafe partial level. In an emergency, you could put a bilge pump hose down the ballast vent, with no further modification except to having a longer hose to reach the bottom of the tank from the vent hole access, and a hose out the fore-hatch for the output.

Here's my idea for a simple (manual) solution...
An inflatable dinghy foot pump connected to a ball valve that threads, when needed, to a bushing pre-installed at the ballast tank vent hole, as per Rick's low pressure pump on his Mac 19. To counter the water backflowing thru the transom gate valve and farting intermittently as per ALX, install a suitable sized duckbill valve, when needed / remove when finished.
Is this plausible as a low-tech solution?

[Rick Westlake]

Replacing the air with water, below the waterline doesn't make the boat sit any lower in the water because the ballast is the same density as the surrounding water. No positive nor negative bouyancy is achieved.

Try this example...

I just did this in my sink with a waterbottle. Empty bottle with & without cap floats the same level in the dirty water. Dirty water scumline indicates waterline. Draw marker at scumline to illustrate waterline. Fill bottle to with water to drawn waterline, then cap. Ballasted vessel (bottle) floats at same waterline as it did with air only, with & without cap. Observed conclusion: vessel floatation doesn't change with or without ballast.

Try your example with a heavy glass bottle instead of a near-weightless plastic bottle. That way you can see if the weight of the bottle makes a difference. I believe you'll observe that it does.

Now how about trying it with your boat? Put your boat in the water, with the ballast empty and transom fill-valve closed to keep the water out of the ballast tank. Measure and mark the actual waterline, on the transom by the fill-valve. Now open the valve and fill the ballast tank. I'm pretty sure you'll find that the "ballast-empty" mark will be close to 1 3/4 inches underwater when the ballast is full.

Admittedly, I haven't tried this myself - yet. But I just did some very rough computations of the area of the hull at the waterline, figuring it to be about 12 square meters (or 120,000 square centimeters). The 1150-lb ballast comes to about 525 kg, or 525,000 cubic centimeters of water. 525000/120000=4.375 ... that is to say, the ballast is equivalent to filling an area the size of the Mac's hull at the waterline with 4.375 cm of water.

[Rick Westlake]

I looked over that "bottle experiment" and realized that not everybody has an old-fashioned heavy glass milk bottle to try this out. So I did it for myself:

Here's the empty bottle and a jar of water to float it. Left to its own devices, in a sink full of water, the bottle tipped over on its side, deep enough that water flowed in the neck and quickly sank the bottle. The top of the jar is small enough to hold the bottle upright, big enough to let the bottle slide down until it reached its displacement equilibrium. (Incidentally, I put the bottle into the jar before I took this photo. You can see the water level in the jar - that is how much water the bottle forced out.) In the right-hand image, you can see the empty bottle sank to the "Shake Before Using" label.


To continue K9 Kamper's bottle experiment, I filled the bottle with water, up to that "Shake Before Using" label. When I put the bottle back in the jar, it sank much deeper - not because it was being "pulled down by the water"; rather, it was being pushed down by the weight of the bottle itself. The added water "filled the flotation-volume of the bottle," so it had to sink deeper to reach its displacement equilibrium. Incidentally, the water in the jar was pushed out by the displacement of the bottle, so the jar's water-level is right up to the rim of the jar.

The ballast tank of your Mac is in the very bottom of the hull. When it is empty it is occupied by air, so its volume is added to the flotation of the hull - but the weight of the boat forces the hull deep enough that its "empty volume" is partially below the waterline. When you fill the ballast tank, its volume is no longer helping to float the boat, so the hull is pushed down deeper - the same as my milk bottle. To simulate the same action, I might have put an empty test-tube into the neck of the bottle (simulating the ballast tank) and seen how deep it floated; then I might fill the test-tube, still inside the bottle, and seen how much deeper the bottle floated. I tried that with a small graduate measure that fit into the neck of the bottle; but as the measure's spout kept it from fitting down in the bottom of the bottle, I decided it "wouldn't quite look right" and would call my claim into question...