Glug, glug, glug

[Barnacle Jim]

Ahoy,

My knowledge of nautical theory for the most part leaves me shipwrecked.
Yet, a friend of mine (a far better sailor than I) offered some confusing comments about water ballast.
I must admit that I share some of that confusion.

After all, how can water ballast really work? By that I mean, water weighs the same whether inside a MacGregor hull or outside the hull.
His confusing comments and my own confusion got me to thinking about water ballast and such.

Water ballast must work. For one thing, large cruise ships use water ballast, and sometimes releasing the ballast at the wrong port, leaving a trail of scum and debris.
No, I do not think the MacGregor boats do that. Most MacGregor skippers run a cleaner ship than that.

And yet when it comes to ballast, whether lead, or water, or concrete, or steel maybe the focus should not be so much on weight as on displacement.
If displacement is right, a boat can be made of concrete and still float. Some boats, in fact, are concrete boats.
Yet, we all know that concrete sinks. Modern ships are made of steel, and we all know that steel sinks in water.
Even so, because of displacement ratio, the ship floats.

I rather suspect that something similar happens with the water ballast in a MacGregor.
Admittedly, water weighs the same whether inside or outside the boat.
And yet, a pound is a pound. A pound of lead weighs the same as a pound of water, or a pound of feathers.

I may be wrong, but the next time someone asks, "How can water ballast possible work?" I will talk about displacement rather than weight.
Maybe that will make more sense.

Let's go back to the notion of shipwrecked again.
We happen to find a bottle. We write our note, place the note into the bottle, and then, stuff the bottle opening with a cork.
Of course, the bottle will float, and if we are fortunate, some unknown soul will read our note, and we may be rescued after all.

Fill that same bottle with water, and throw the bottle into the sea . . .
Glug, glug, glug -- the bottle promptly sinks.

Yet the water within the bottle weighs the same as the water outside the bottle.
It's not weight; it's displacement and ratio.

So, maybe the next time someone asks how does water ballast work, I can easily say,
"Fill this bottle with water and throw it overboard and see if it floats or sinks."

One of the remarkable features of the MacGregor design is that we can empty the ballast even while the boat is on the water.
We can get the lead out, and really move.

[kurz]

it is very easy: The waterballast works as soon as the boat heels. Then the water ballast tank gravity comes out of the water an and gives gravity to heel back.
Becouse of this a MacGregor heels at the beginning more than a leaded keel boat.
This is the easy general explanation. To describe the physics in all details may be very complecated.

[Mac26Mpaul]

Yeah, like Kurz said...

And that is why water ballasted boats are more tender, and have kind of a big dinghy feel. But once the boat heels to a certain angle, it hardens up fine. If one wants the benifits of a boat that can do all the Mac can, then they need to accept a boat that is a bit more tender. Without the water ballast, we dont have a reasonably light to trailer 26 foot boat that can also, at the opening of a valve, be turned into a reasonably capable, and cheap to run, fast motor cruiser.

Some sailers seem to think water ballast is pure evil but there are enough water ballasted sailboats about now for us to know it works

Each to their own, but this currently beer ballasted sailer, loves his water ballasted boat lots

[Catigale]

The water ballast works even when it is under the waterline, it does not have to be lifted up to magically turn on....

Consider two boats with 8 pounds of ballast total. One has a gallon (8 pounds) of water in a tank attached to the bottom of the hull, the other an 8pound block of lead.

Because the water is contained in a tank, the boats have exactly the same righting moment even though the tank is "below" the waterline...

The tenderness of Macs isn't due the ballast content or buoyancy, but the location of the ballast itself.

[jbousquin]

What Catigale said... It is all about the location of the ballast, and not the composition of the ballast itself. Think about the old sailing ships, which would "shift" ballast in their hulls to balance out the boat. It's about where you put the weight, not the material that has the weight itself. Because, as Jim rightly pointed out in the beginning of this post, a pound of feathers weighs the same as a pound of lead.

When I was first researching the MacGregor before buying my , this link helped me understand water ballast a lot. The diagrams with the arrows at the point of the righting moment are especially illustrative.

http://kobernus.com/hunter260/water_bal ... aper1.html

Cheers,

Joe

[kevinnem]

I think the first thing that people are held up on is the fact that they think the water outside the hull has an effect on the ballast. It doesn't.

consider the following thought experiment - take the mac out of the water , and set it on a ring of rollers such that it can roll back and forth - with a sail up. When the wind blows - the boat will tilt in the way we expect. Given the same wind, the only difference between ballast in and water ballast out is the degree that the boat will tilt. That is to say that the degree to which the boat tilts depends on the weight of the counter weight(ballast).

<insert joke here> Q: what weights more 100lbs of feathers, or 100 lbs of steel? A: they both weight the same silly <end joke here>

so you might say to me, "I follow you, but you took it out of the water! that changes everything!" But really does it. I mean, as long as the weight/ballast (what ever it is) is on the "bottom" of the boat, it will cause a force to "level off" the boat where it is forced off the bottom(up the side) by the wind.

In fact if we were to fill the tank with gasoline, it would still work , because the gas in the ballast tanks in the bottom 1/2 of the boat still weights less then the "air " in the top 1/2 of the boat the the boat will sit upright, with the weight pulling the weight to the lowest point. Why did I mention gas? Well Gas weights less then water! So IF the outside water did have an effect on the ballast within, then the boat would flip upside down! As a side note, because the total boat weight would be less, the boat would sit higher in the water in this case.

Put more directly, you need to separate out the aspects that cause the whole boat to to sit higher or lower in the water, and the ideas of what makes the boat "twist" to an upright position.

The benefit of the "keel" boat, mostly bulb keel, is that the FARTHER/deeper the weight is from the center the stronger this leveling force is. It is a bigger lever arm. Now in a power boat this would be annoying, because it would create all the drag and stuff - but we need to have a board like that anyway to stop us form being pushed sideways, so it is not a big deal to just make it heavy.

[Mac26Mpaul]

Well, must admit, I'v learnt something here today

[mastreb]

The benefit of the "keel" boat, mostly bulb keel, is that the FARTHER/deeper the weight is from the center the stronger this leveling force is. It is a bigger lever arm. Now in a power boat this would be annoying, because it would create all the drag and stuff - but we need to have a board like that anyway to stop us form being pushed sideways, so it is not a big deal to just make it heavy.

It's a little bit more complicated than this. Everything said thus far about weight being weight irrespective of what it's made of is correct for a boat where the ballast is located >inside< the rotating body of the hull. This works because the total weight of the boat is less than the total weight of the displaced water (so it's buoyant) and the ballast at the bottom causes the buoyant body to rotate such that the heaviest part of it is down. If the boat was filled with water, a water ballast would have no righting effect.

However, for ballast located below the hull in a non-buoyant body (in a bulb keel) the weight of the ballast must be greater than the weight of the displaced water. If a bulb were (for example) filled with water, it would not exert downward righting force. It would exert exactly no force other than the minor weight of the structure. If the bulb were filled for example with air, it would be buoyant and would cause upward, capsizing force. This is why a bulb keel would still have to be made of lead in order to function.

[Hamin' X]

Look at it this way: The water ballast is not in the water, it is in the boat. Take a scale and place it on the cabin sole. Place a gollon container on it and zero the scale. Fill it with water and see what it weighs. I'll bet it is 8 Lbs.

~Rich

[Catigale]

If a bulb were (for example) filled with water, it would not exert downward righting force. It would exert exactly no force other than the minor weight of the structure

If the bulb is filled with water, the righting moment is still the mass of the bulb structure plus the water inside. Just like the ballast tank, only the mass of the material inside is important, and not its composition

There are so many good gedanken experiments with this involving gedunking things, I'm gedoonkt....

[kevinnem]

The benefit of the "keel" boat, mostly bulb keel, is that the FARTHER/deeper the weight is from the center the stronger this leveling force is. It is a bigger lever arm. Now in a power boat this would be annoying, because it would create all the drag and stuff - but we need to have a board like that anyway to stop us form being pushed sideways, so it is not a big deal to just make it heavy.

It's a little bit more complicated than this. Everything said thus far about weight being weight irrespective of what it's made of is correct for a boat where the ballast is located >inside< the rotating body of the hull. This works because the total weight of the boat is less than the total weight of the displaced water (so it's buoyant) and the ballast at the bottom causes the buoyant body to rotate such that the heaviest part of it is down. If the boat was filled with water, a water ballast would have no righting effect.

However, for ballast located below the hull in a non-buoyant body (in a bulb keel) the weight of the ballast must be greater than the weight of the displaced water. If a bulb were (for example) filled with water, it would not exert downward righting force. It would exert exactly no force other than the minor weight of the structure. If the bulb were filled for example with air, it would be buoyant and would cause upward, capsizing force. This is why a bulb keel would still have to be made of lead in order to function.

Okay, fair enough, I was trying to simplify things. I might also be useful to think of the basllest of our boats as "making a hole of air in the water" rather then push weight up the side.

[mastreb]

If the bulb is filled with water, the righting moment is still the mass of the bulb structure plus the water inside. Just like the ballast tank, only the mass of the material inside is important, and not its composition

There are so many good gedanken experiments with this involving gedunking things, I'm gedoonkt....

Disclaimer: I said this is more complicated, NOT that it's in any way useful or important

Righting moment is torque. Torque is created when you have an offset force which causes a system to rotate. In boats, the offset force is a mass which is pulled by gravity to the lowest point in the system. When the boat is at rest, there is no torque, when the boat is heeled, there is torque proportional to the lever-arm and mass of the ballast.

It is the difference in displacement between a mass and whatever it is that the mass displaces which creates the offset mass and therefore the righting torque, NOT the simple total weight.

Inside the hull, this differential displacement is available because the hull is filled with air. The water ballast is displaces air which weighs very little, and therefore we have an offset weight that can rotate the system. We tend to forget that we are "displacing air" because it weighs so little. Inside the boat, the weight can be anything that has a high enough "air displacement" to create an offset weight to provide righting torque.

Consider that feathers do not have enough air displacement to create an offset weight. 1300 lbs. of feathers inside your cabin would likely fill it to capacity, and therefore not create the necessary offset of mass to create a torque lever arm. So it's not true that all weight is created equal, just most weight that we would reasonably consider. Most things have substantially higher mass than air (just not feathers) for us not to bother thinking about this at all.

So to create an offset weight, we have the (ballast mass) - (displaced mass) = righting torque. With air, the displaced mass is negligible and we fail to think of it.

Below the boat, you must displace water, which weighs considerably more. So we have the weight of the bulb assembly + the weight of the ballast water - weight of the displaced water. Because the ballast water and displaced water weight the same, they cancel out and we have the now negligible weight of the bulb assembly alone for ballast.

Consider this thought experiment: If you fill a plastic fuel tank with gasoline, it will float even though it weighs say 20lbs. If you put that gasoline filled bulb at the bottom of your boat, it will not provide righting moment because it displaces less than the water--it's buoyant. If your boat heels, the buoyancy of the gasoline filled bulb will tend to capsize the boat, not right it--irrespective of the 20 lb. weight, because the entire bulb must weigh more than the displaced water to have any effect.

Now having said that, it is the very failure to understand torque and displacement that causes the bias against water ballast amongst keel boat sailors who presume that righting moment is caused by "lead" and not by mass displacement.

Anyway, I've wandered far off into esoteric engineering land, just don't bother trying to make a water ballasted keel bulb and you'll be fine.

[Uncle Jim]

And yet, a pound is a pound. A pound of lead weighs the same as a pound of water, or a pound of feathers.

Just because no one esle has said it yet...
A pound is not always a pound as in the old gag.. what weighs more a pound of feathers or a pound of gold?
the answer of course is the pound of feathers

[Don T]

Hello Fellow Mac'ers,
Here is how I look at it. What rights the boat is the different positions of the "center of gravity" vs "the center of buoyancy." As the boat heels, the center of gravity rotates off center. The greater the distance between these forces the greater the lever or arm. The reason water ballast is more tender than lead is because of density. More mass moves off center for any degree of heel with lead. The reason Mac classic 26's stiffen earlier is because the ballast tank is long, low & skinny and along the centerline. The tanks in the X & M boats are taller & wider so the center of gravity is closer (read higher) to the center of buoyancy. The reason they changed it was for head room. That 6' of standing head room on the X & M boat meant the tank had to be moved. You stand on the tank in the classic so a pop top is required.

Here's a drawing I did:
http://www.macgregorsailors.com/modt/in ... ?view=1234

Hope that helps explain it.

[Don T]

And yet, a pound is a pound. A pound of lead weighs the same as a pound of water, or a pound of feathers.

Just because no one esle has said it yet...
A pound is not always a pound as in the old gag.. what weighs more a pound of feathers or a pound of gold?
the answer of course is the pound of feathers

Unless you are talking politically then the gold far out weighs the feathers.