Add a hundred pounds of cork to your experiment, and it starts to make sense.

The more water something displaces, the more buoyancy it has. The more buoyant it is, the less it "weighs" under water. In this case, the order from most to least buoyant would be cork, then concrete, then lead.

 

concrete also has air trapped in it. So it is naturally lighter than lead which is very dense.

 

Yes, it is correct. It is all about density. Water has a density of 1g/cc. Anything that is less dense than that will float. Anything that is more dense will sink. If something sinks, it will weigh less than it does in air because water has a relatively high density and the relative density is important. The denser something is, the less water it displaces so it will weigh more.

 

It's all about volume versus weight under water. However; as ballast in a boat, 100# is 100# if it can be put in the same space, such as an enclosed keel. That isn't normally the case, because concrete takes up more space that lead, thus it usually cannot be placed as low down. Lead keeled boats often can carry more sail and point better than boats with concrete as ballast. However; boats with concrete ballast can have an easier motion than lead ballast boats. My old Cheoy Lee 31 has a very nice, easy motion. I never felt seasick, even in rough weather. I've been on other similar sixed sailboats with lead ballast, and was queasy when it got choppy.

 

Yes, it is correct. It's all about the density of the material. Some areas even specifically ban the use of concrete for moorings because its underwater weight is a /.55 factor...

For example if you wanted a submerged mooring weight of 3000 pounds (about right for a 27-32 foot boat), and it was made of concrete, you'd really need a 5450 pound block to start with...

 

Yes, it is correct. It's all about the density of the material. Some areas even specifically ban the use of concrete for moorings because its underwater weight is a /.55 factor....

Some areas? Maine, for instance?....where the only thing to use underwater is a big block of Maine granite? I'm surprised you guys aren't exporting the stuff.

 

This is like "what weights more 100 lb of feathers or 100 lbs of lead." Answer, they weight the same but the feathers (when contained) will float where the lead will sink. It's all about displacement. If a boat displaces 1000 lbs that is based on how much water it displaces at a fixed water density so the water line on the boat would be different in salt water vs fresh water.

So, 100 lbs of lead weighs the same as 100lbs of concrete. But the volume of the concrete would be greater and hence have a bit more bouyancy (wouldn't hold as good as an anchor).

 

Think of it this way:

1000 liters of sea water: apparent weight in air = 1025 kg; apparent weight in seawater = 0 kg
1000 liters of concrete: apparent weight in air = 2400 kg; apparent weight in seawater = 1475 kg
1000 liters of iron: apparent weight in air = 7874 kg; apparent weight in seawater = 6849 kg
1000 liters of lead: apparent weight in air = 11340 kg; apparent weight in seawater = 10315 kg

- apparent weight in seawater is (density*volume) - (seawater density*volume);
- numbers are approximate, depending on salinity and the concrete mix;
- apparent weight in air and apparent weight in a vacuum are virtually identical, as air has a density of only about 1.3 kg/1000 liters)

 

It's called specific gravity and all about weight for a given VOLUME , sort of a eureka moment. Since a cubic foot of ocean weighs 64 lbs (2 lbs of salt) a cubic foot of concrete weighs 64 lbs less in the water, If it's in the bilge it's all the same but your metacentric heights change but that's another story.

 

Specific gravity is the density of something relative to the density of a reference material (usually pure water). In this case it's a bit more convenient to reference density directly, since the frame of reference keeps changing.

 

Not the amount of surface area (that plays no role), but volume. Volume plays a role here. More volume = more displacement.

 

Former Fiziks instructor Frogwatch here:

Density of lead is roughly 12 gm/cm3
Density of concrete is roughly 2.5 gm/cm3

100 lb ~45 Kg =45000 gm For lead, this means that it occupies 3750 cm3 (divide the 45000 gm by 12 gm/cm3).
For concrete,100 lb occupies 18000 cm3. So, you can see that the same weight of concrete occupies 4.8 times the volume of lead (the lead would occupy a cube 15 cm on a side while the concrete would occupy a cube of 26 cm on a side)
Now, just like you used to do in basic physics, draw an up arrow and a down arrow. The down arrow should be labelled mg and the up arrow should be labelled Fb (buoyant force).
Buoyant force is equal to the weight of water displaced so for the case of lead, the total force is then 45000g-3750g =41250g. For concrete, it is 45000g-18000g = 27000g.
Thus, the 100 lb of lead "weighs" more than the 100 lb of concrete in water. However in both cases, the mass is the same 45 kg. The confusion arises from the definition of weight vs mass and that we commonly interchange weight and mass however THEY HAVE DIFFERENT UNITS. Weight is a unit of force that varies depending on the place it is measured whereas mass does not change from place to place (Newtonian physics here).

 

OK, I mis-remembered the density of lead as being 12 when it is really 11.3. However, the density of mercury is 13. or so. I once picked up a pint bottle of mercury and almost immediately dropped it. That thing was seriously heavy. I once had a brick sized block of depleted U as a door stop painted to look like a normal brick. Seriously heavy even compared to those of us familiar with lead bricks. Gold is seriously dense. Movies where the guy casually picks up a gold brick.............no way. Pure gold is so soft that it makes lead look like steel, you can scratch it just by thinking about it.
The ultimate ballast, Osmium. I just bought 10 gms of the stuff for optical coatings. Densest element, very hard, will not corrode, rarer but cheaper than gold

 

...
The ultimate ballast, Osmium. I just bought 10 gms of the stuff for optical coatings. Densest element, very hard, will not corrode, rarer but cheaper than gold

It's also extremely toxic, at least its oxides and salts are. (I used to have to deal with osmium compounds for electron microscopy work.)

 

There is a metal called Gallium that melts at room temp and it looks a lot like Mercury but it is non-toxic. Unfortunately, its density is only 5.9 (iron is roughly 8.9). However, water is only a density of 1. So...................
A combo water ballast/pumpable gallium ballast. A hollow reservoir is mounted on the to-rails on an extension to get it about a foot away from the boat. The one on the downhill side serves to provide buoyancy to that side whereas the other one is filled with Gallium that has been pumped there. Most ballast would be water ballast but the use of Gallium only serves to minimize the size of these reservoirs.

 

A combo water ballast/pumpable gallium ballast. A hollow reservoir is mounted on the to-rails on an extension to get it about a foot away from the boat. The one on the downhill side serves to provide buoyancy to that side whereas the other one is filled with Gallium that has been pumped there.

Or just get a fat kid to sit on the rail. Less to go wrong, easier to find another one if you break it. Less pollution if it leaks too.

 

Jeesh, now you tell me .... I'll just chop of that darn keel, eat more and sit on the rail. That way the pesky rocks close to shore can't jump out and play tag with me.

 

The weight of a body in water is equal to the items weight minus the water is displaces. So, if 100 lbs of lead is smaller and displaces 10 lbs of water, its weight is 90 lbs. If a 100 lb block of concrete which is larger, displaces 20 lbs of water, its weight is 80 lbs. In the same way, a boat "displaces" 6700 lbs, it will drop into the water until the amount of water that it pushes out of the way is 6700 lbs.

 

I think the fat kid needs more maintance... however, depending on the age of the kid, he might be as dense as lead

 

I think the fat kid needs more maintance...

True. The lead is pretty much a "set it and forget it" sort of thing; but you'll probably have to keep feeding the fat kid sandwiches and beer all day.

however, depending on the age of the kid, he might be as dense as lead

Even truer.

 

That's odd. I thought this thread was about concrete and lead.

Slow...
With the signature you are using, all of your posts belong in the "off topic" thread category. Your "Che Guevarra" type Obama signature also fell into the same category.
I suggest we all try not to incite anger in the rest of the forum members.

 

That settles that, thanks for the input everyone.


And Slow..
I agree with seaduction, your signature line is not the place for your political objectives.

 

Volume and displacement aside, in air 100# of lead and 100# of concrete both weigh the same 100#. The chunk of lead is much smaller. immersed in the water it ain't so.

["Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent."]

http://www.cement.org/basics/concretebasics_concretebasics.asp

Since concrete may be as much as 20% water, that "weight" comes off the top when measuring it with a scale when immersed in water. The guys at the marina were right.

Down

 

Volume and displacement aside, in air 100# of lead and 100# of concrete both weigh the same 100#. The chunk of lead is much smaller. immersed in the water it ain't so.

["Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent."]

Concrete Basics | Portland Cement Association (PCA)

Since concrete may be as much as 20% water, that "weight" comes off the top when measuring it with a scale when immersed in water. The guys at the marina were right.

Down

Actually, once the concrete cures, the water that was added into the original mix is mostly incorporated into the crystalline structure of the newly formed "rock". When Portland cement is manufactured the water is driven off, leaving an anhydrous dust (mostly calcium and silicon oxides). When water is added to this dust it re-hydrolyzes these oxides forming pretty complex crystalline lattice that varies a bit depending on the exact amount of water in the cure. The point is that the water is not "free water". It is chemically bonded to the rest of the concrete. So you can't really talk about it's density, as it has no real density per se.

 

Hang anything that sinks on a large spring scale suspended above water... as you lower it into the water the object will 'lose' weight on the scale. Regardless of what the object weighs, it will 'lose' as much weight as the equivalent volume of the water - ie as much as it displaces.

A 700 lb cu ft of lead would 'lose' 62.5 lbs approx.... as would a cu ft of concrete or anything else.

 

Okay, I'm cancelling my concrete order.

 

Here's one for the engineers.

Does an object really weigh less once submerged? That implies an elimination of mass. It is subject to an opposing force (buoyancy). Does an aircraft weigh less when airborne because of lift?

Having taught deep wreck diving when I was younger, I get the concept being discussed. Just a question of technical clarity on whether something actually weighs less. I don't think so. I think it's really a case of opposing forces that makes the object appear to weigh less.

 

Here's one for the engineers.

Does an object really weigh less once submerged? That implies an elimination of mass. It is subject to an opposing force (buoyancy). Does an aircraft weigh less when airborne because of lift?

Having taught deep wreck diving when I was younger, I get the concept being discussed. Just a question of technical clarity on whether something actually weighs less. I don't think so. I think it's really a case of opposing forces that makes the object appear to weigh less.

It all depends on what definition one is using: mass, weight, or apparent weight. Mass is a measure of the inertia of an object. Weight is the force an object exerts in a gravitational field (weight = mass x gravity; we talk about an object "weighing" so many kilograms, but in fact the proper units of weight are newtons). For a body at rest, apparent weight takes buoyancy into account. The buoyancy provided by air is small enough, and gravity is constant enough, that our operational use of apparent weight to estimate mass is sufficient for most applications. However, as this thread demonstrates, there are exceptions.

 

Cool. If weight = mass x gravity, then an object does not weigh less when buoyed, it just appears to.

 

right... mass never changes. Even in the Microgravity of earth orbit. You could get a ton of weight moving slowly in orbit by yourself (if you had something to push against or an EVA thruster pack) but it could also crush you very slowly if it trapped you between it and something.

 

you guys are failing to mention th e lead as a biohazard. will raise your lead levels in blood and cause learning curve disorders--and agression increases.

concrete doesnt do that. is easier to obtain as it isnt a biohazard substance and wil not create learning curve dysfunction.

just add stuff to it as did the taiwanese when they made many boats with concrete keels--they added steel, lead chunks, whatever was in the yard that could be added.

 

encapsulated in your keel... lead is not going to hurt you. Sanding it, touching raw lead, or cutting it without proper biohazard precautions would bring about lead poisoning.. and that can take years to get over.

Concrete ships are nothing new. The US was playing with them in WW1. There is the remains of one off of Cape May NJ. The Atlantis. It was mothballed after the war and towed to Cape May to make a ferry terminal. Unfortunately it broke loose in a storm before it could be sunk in place and was blown ashore at Cape May Point where it has slowly deteriorated and sunk deeper into the sand.

Almost 100 years after she was launched, there is not a lot left.. but a lot more than if she had been built of steel

 

...

Concrete ships are nothing new. The US was playing with them in WW1. There is the remains of one off of Cape May NJ. ...

There's also one in Capitola, CA (just south of Santa Cruz, CA).