100 lb of Lead Vs 100 lb of concrete - SailNet Community
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post #1 of 36 Old 04-20-2013 Thread Starter
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100 lb of Lead Vs 100 lb of concrete

I was in the marina yesterday and heard something from the staff that had me perplexed.
He said that under water lead weighs more than concrete if they are of the same weight.

Now; after some checking it appears he may be right. As a ball of lead would have less "displacement" than a similar weight of concrete that would be much larger size. Does the amount of surface area on a sunken mass play a factor in its level of buoyancy?

Is this correct?
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Re: 100 lb of Lead Vs 100 lb of concrete

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.
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post #3 of 36 Old 04-20-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

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

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Re: 100 lb of Lead Vs 100 lb of concrete

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.
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Re: 100 lb of Lead Vs 100 lb of concrete

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.



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Re: 100 lb of Lead Vs 100 lb of concrete

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Originally Posted by benesailor View Post
I was in the marina yesterday and heard something from the staff that had me perplexed.
He said that under water lead weighs more than concrete if they are of the same weight.

Now; after some checking it appears he may be right. As a ball of lead would have less "displacement" than a similar weight of concrete that would be much larger size. Does the amount of surface area on a sunken mass play a factor in its level of buoyancy?

Is this correct?
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...

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Re: 100 lb of Lead Vs 100 lb of concrete

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).
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Re: 100 lb of Lead Vs 100 lb of concrete

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)

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Re: 100 lb of Lead Vs 100 lb of concrete

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.
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Re: 100 lb of Lead Vs 100 lb of concrete

Quote:
Originally Posted by benesailor View Post
I was in the marina yesterday and heard something from the staff that had me perplexed.
He said that under water lead weighs more than concrete if they are of the same weight.

Now; after some checking it appears he may be right. As a ball of lead would have less "displacement" than a similar weight of concrete that would be much larger size. Does the amount of surface area on a sunken mass play a factor in its level of buoyancy?

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


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